[libcxxabi] copy back std::string_view patches from LLVM

[nickdesaulniers]

Summary

I made a series of changes to LLVM's demangle in:

• D148348
• D148353
• D148363
• D148375

and so did Fangrui in 3ece37b3fa2c and Ashay in D149061.

I didn't notice the banner about there being two copies of this in tree
and was modifying the downstream versions. Copy these changes back to
the upstream version. Oops!

[nickdesaulniers]

libcxxabi/src/demangle/ItaniumDemangle.h

20

From the build bots:

/home/libcxx-builder/.buildkite-agent/builds/google-libcxx-builder-libcxx-precommit-ci-group-sh1r-1/llvm-project/libcxx-ci/libcxxabi/src/demangle/ItaniumDemangle.h:20:10: fatal error: 'StringViewExtras.h' file not found
#include "StringViewExtras.h"
         ^~~~~~~~~~~~~~~~~~~~

I need to copy back this file, too.

[nickdesaulniers]

libcxxabi/src/demangle/ItaniumDemangle.h

20

I did that in the parent commit https://reviews.llvm.org/D148556. Perhaps the build bot didn't test both patches together in series?

FWICT, I marked the parent revision at 1:43pm, and harbormaster marked the build tests complete at 2:16pm, but I can't tell when harbormaster started the test. Perhaps I just need a rebase in order to rekick harbormaster now that the relationships between patches in the series is properly expressed in phab?

[phosek]

LGTM

[nickdesaulniers]

TODO(ndesaulniers): D148546 has more changes to appease msvc's impl of std::string_view, those need to be recopied in this patch.

[nickdesaulniers]

• rebase on final version of D148546, copy back formatting and msvc changes

[nickdesaulniers]

libcxxabi/src/demangle/ItaniumDemangle.h

2489

wrong namespace for this file

[nickdesaulniers]

libcxxabi/src/demangle/ItaniumDemangle.h

1851

also, std::string_view::substr may throw std::out_of_range which can't/shouldn't be used in libcxxabi (results in incredibly spooky linkage failures that took me longer than I'd like to admit to track down). That should be fixed first downstream, then this change rebased on that (with those changes recopied).

[nickdesaulniers]

libcxxabi/src/demangle/ItaniumDemangle.h

1851

https://reviews.llvm.org/D148959

[nickdesaulniers]

Also needs a rebase on D149061.

[ldionne]

LGTM once the blocker is resolved, assuming this makes the libcxxabi copy equivalent to the llvm copy. Also, please make sure you rebase onto main and re-upload the patch to get a green CI run before you merge this. Thanks!

[nickdesaulniers]

• rebase

[MaskRay]

Are the bots green now?

[nickdesaulniers]

• fixup cxa_demangle.cpp. I had this later in my local patch series, but as presubmit pointed out, this needs to in atomically now with the rest of these changes, not later.

[nickdesaulniers]

In D148566#4354662, @MaskRay wrote:

Are the bots green now?

No. :(
https://reviews.llvm.org/harbormaster/unit/view/7358113/
They spotted a change I had locally in a child patch (to libcxxabi/src/cxa_demangle.cpp) that needs to go in this commit. Should be fixed up now, but will wait again for presubmit bots. Change should be uncontroversial and limited to that file:
https://reviews.llvm.org/D148566?vs=523556&id=523921

[nickdesaulniers]

Hmm...some unittests are failing to link...

/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o: in function `std::__1::basic_string_view<char, std::__1::char_traits<char> >::basic_string_view[abi:v170000](char const*, unsigned long)':
unittest_demangle.pass.cpp:(.text._ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEEC2B7v170000EPKcm[_ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEEC2B7v170000EPKcm]+0x6d): undefined reference to `abort_message'
/usr/bin/ld: unittest_demangle.pass.cpp:(.text._ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEEC2B7v170000EPKcm[_ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEEC2B7v170000EPKcm]+0xca): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o: in function `unsigned long std::__1::__char_traits_length_checked[abi:v170000]<std::__1::char_traits<char> >(std::__1::char_traits<char>::char_type const*)':
unittest_demangle.pass.cpp:(.text._ZNSt3__128__char_traits_length_checkedB7v170000INS_11char_traitsIcEEEEmPKNT_9char_typeE[_ZNSt3__128__char_traits_length_checkedB7v170000INS_11char_traitsIcEEEEmPKNT_9char_typeE]+0x40): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o: in function `std::__1::basic_string_view<char, std::__1::char_traits<char> >::remove_prefix[abi:v170000](unsigned long)':
unittest_demangle.pass.cpp:(.text._ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEE13remove_prefixB7v170000Em[_ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEE13remove_prefixB7v170000Em]+0x5e): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o: in function `std::__1::basic_string_view<char, std::__1::char_traits<char> >::front[abi:v170000]() const':
unittest_demangle.pass.cpp:(.text._ZNKSt3__117basic_string_viewIcNS_11char_traitsIcEEE5frontB7v170000Ev[_ZNKSt3__117basic_string_viewIcNS_11char_traitsIcEEE5frontB7v170000Ev]+0x51): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o:unittest_demangle.pass.cpp:(.text._ZNKSt3__117basic_string_viewIcNS_11char_traitsIcEEEixB7v170000Em[_ZNKSt3__117basic_string_viewIcNS_11char_traitsIcEEEixB7v170000Em]+0x5e): more undefined references to `abort_message' follow
/usr/bin/ld: /home/libcxx-builder/.buildkite-agent/builds/google-libcxx-builder-3bf078479ae8-1/llvm-project/libcxx-ci/build/generic-cxx2b/test/Output/unittest_demangle.pass.cpp.dir/t.tmp.exe: hidden symbol `abort_message' isn't defined
/usr/bin/ld: final link failed: bad value

related to https://reviews.llvm.org/D149092 (or https://reviews.llvm.org/D150825)

[nickdesaulniers]

In D148566#4361245, @nickdesaulniers wrote:

Hmm...some unittests are failing to link...

/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o: in function `std::__1::basic_string_view<char, std::__1::char_traits<char> >::basic_string_view[abi:v170000](char const*, unsigned long)':
unittest_demangle.pass.cpp:(.text._ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEEC2B7v170000EPKcm[_ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEEC2B7v170000EPKcm]+0x6d): undefined reference to `abort_message'
/usr/bin/ld: unittest_demangle.pass.cpp:(.text._ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEEC2B7v170000EPKcm[_ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEEC2B7v170000EPKcm]+0xca): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o: in function `unsigned long std::__1::__char_traits_length_checked[abi:v170000]<std::__1::char_traits<char> >(std::__1::char_traits<char>::char_type const*)':
unittest_demangle.pass.cpp:(.text._ZNSt3__128__char_traits_length_checkedB7v170000INS_11char_traitsIcEEEEmPKNT_9char_typeE[_ZNSt3__128__char_traits_length_checkedB7v170000INS_11char_traitsIcEEEEmPKNT_9char_typeE]+0x40): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o: in function `std::__1::basic_string_view<char, std::__1::char_traits<char> >::remove_prefix[abi:v170000](unsigned long)':
unittest_demangle.pass.cpp:(.text._ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEE13remove_prefixB7v170000Em[_ZNSt3__117basic_string_viewIcNS_11char_traitsIcEEE13remove_prefixB7v170000Em]+0x5e): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o: in function `std::__1::basic_string_view<char, std::__1::char_traits<char> >::front[abi:v170000]() const':
unittest_demangle.pass.cpp:(.text._ZNKSt3__117basic_string_viewIcNS_11char_traitsIcEEE5frontB7v170000Ev[_ZNKSt3__117basic_string_viewIcNS_11char_traitsIcEEE5frontB7v170000Ev]+0x51): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-xl26yrf1/unittest_demangle-74952e.o:unittest_demangle.pass.cpp:(.text._ZNKSt3__117basic_string_viewIcNS_11char_traitsIcEEEixB7v170000Em[_ZNKSt3__117basic_string_viewIcNS_11char_traitsIcEEEixB7v170000Em]+0x5e): more undefined references to `abort_message' follow
/usr/bin/ld: /home/libcxx-builder/.buildkite-agent/builds/google-libcxx-builder-3bf078479ae8-1/llvm-project/libcxx-ci/build/generic-cxx2b/test/Output/unittest_demangle.pass.cpp.dir/t.tmp.exe: hidden symbol `abort_message' isn't defined
/usr/bin/ld: final link failed: bad value

related to https://reviews.llvm.org/D149092 (or https://reviews.llvm.org/D150825)

I can build DemangleTests just fine, though I do set -DLLVM_ENABLE_LLD=ON to link with LLD rather than BFD. Let me try BFD.

Strange, setting -DLLVM_USE_LINKER=ld (and hacking out LINKER_SUPPORTS_COLOR_DIAGNOSTICS from the cmake rules):

$ cd llvm/build; ninja DemangleTests; cd -
$ llvm-readelf -p .comment llvm/build/unittests/Demangle/DemangleTests
String dump of section '.comment':
[     0] GCC: (Debian 12.2.0-14) 12.2.0
[    1f] Android (10006735, +pgo, +bolt, +lto, -mlgo, based on r487747b) clang version 17.0.1 (https://android.googlesource.com/toolchain/llvm-project d9f89f4d16663d5012e5c09495f3b30ece3d2362)

(Shows that I was able to link via ld.bfd successfully) I wonder what else could be going wrong for this particular presubmit? _LIBCPP_VERBOSE_ABORT is not set AFAICT.

[nickdesaulniers]

In D148566#4362390, @nickdesaulniers wrote:

I can build DemangleTests just fine, though I do set -DLLVM_ENABLE_LLD=ON to link with LLD rather than BFD. Let me try BFD.

(Shows that I was able to link via ld.bfd successfully) I wonder what else could be going wrong for this particular presubmit? _LIBCPP_VERBOSE_ABORT is not set AFAICT.

Ah, it's check-cxxabi ninja target that's failing to build (w/ gcc and bfd). I can repro now. Digging (this seems related to _LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS).

[nickdesaulniers]

In D148566#4362478, @nickdesaulniers wrote:

In D148566#4362390, @nickdesaulniers wrote:

I can build DemangleTests just fine, though I do set -DLLVM_ENABLE_LLD=ON to link with LLD rather than BFD. Let me try BFD.

(Shows that I was able to link via ld.bfd successfully) I wonder what else could be going wrong for this particular presubmit? _LIBCPP_VERBOSE_ABORT is not set AFAICT.

Ah, it's check-cxxabi ninja target that's failing to build (w/ gcc and bfd). I can repro now. Digging (this seems related to _LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS).

https://reviews.llvm.org/D151160

[nickdesaulniers]

• rebase onto D151160

[nickdesaulniers]

• rebase onto D151260

[nickdesaulniers]

• one more rebase to rekick presubmit tests

[ldionne]

Please land when the tests are green. Rebasing should take care of that.

[nickdesaulniers]

• rebase again for presubmit CI

[nickdesaulniers]

libcxxabi/src/demangle/ItaniumDemangle.h

3723

egads...libcxx/utils/ci/run-buildbot generic-debug-mode is failing because:

llvm::StringView::end and std::string_view::end have different semantics! llvm::StringView::end returns the last element, std::string_view::end returns 1 past the end element!

Let me fix that first downstream, then rebase this on top again.

[nickdesaulniers]

• rebase onto D151760, copy back Ashay's changes from D149061

libcxxabi/src/cxa_demangle.cpp

 #include "__cxxabi_config.h"
 #include <cassert>
 #include <cctype>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
 #include <functional>
 #include <numeric>
+#include <string_view>
 #include <utility>
 
 using namespace itanium_demangle;
 
 constexpr const char *itanium_demangle::FloatData<float>::spec;
 constexpr const char *itanium_demangle::FloatData<double>::spec;
 constexpr const char *itanium_demangle::FloatData<long double>::spec;
 
struct DumpVisitor {
   template<typename ...Ts> static bool anyWantNewline(Ts ...Vs) {
     for (bool B : {wantsNewline(Vs)...})
       if (B)
         return true;
     return false;
   }
 
   void printStr(const char *S) { fprintf(stderr, "%s", S); }
-  void print(StringView SV) {
-    fprintf(stderr, "\"%.*s\"", (int)SV.size(), SV.begin());
+  void print(std::string_view SV) {
+    fprintf(stderr, "\"%.*s\"", (int)SV.size(), &*SV.begin());
   }
   void print(const Node *N) {
     if (N)
       N->visit(std::ref(*this));
     else
       printStr("<null>");
   }
   void print(NodeArray A) {

libcxxabi/src/demangle/ItaniumDemangle.h

 // cp-to-llvm.sh to update the copy.  See README.txt for more details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef DEMANGLE_ITANIUMDEMANGLE_H
 #define DEMANGLE_ITANIUMDEMANGLE_H
 
 #include "DemangleConfig.h"
-#include "StringView.h"
+#include "StringViewExtras.h"
 #include "Utility.h"
 #include <__cxxabi_config.h>
 #include <algorithm>
 #include <cassert>
 #include <cctype>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
 #include <limits>
 #include <new>
+#include <string_view>
 #include <type_traits>
 #include <utility>
 
 #ifdef _LIBCXXABI_COMPILER_CLANG
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunused-template"
 #endif
 
public:
   virtual void printLeft(OutputBuffer &) const = 0;
 
   // Print the "right". This distinction is necessary to represent C++ types
   // that appear on the RHS of their subtype, such as arrays or functions.
   // Since most types don't have such a component, provide a default
   // implementation.
   virtual void printRight(OutputBuffer &) const {}
 
-  virtual StringView getBaseName() const { return StringView(); }
+  virtual std::string_view getBaseName() const { return {}; }
 
   // Silence compiler warnings, this dtor will never be called.
   virtual ~Node() = default;
 
 #ifndef NDEBUG
   DEMANGLE_DUMP_METHOD void dump() const;
 #endif
 };
struct NodeArrayNode : Node {
 
   template<typename Fn> void match(Fn F) const { F(Array); }
 
   void printLeft(OutputBuffer &OB) const override { Array.printWithComma(OB); }
 };
 
 class DotSuffix final : public Node {
   const Node *Prefix;
-  const StringView Suffix;
+  const std::string_view Suffix;
 
 public:
-  DotSuffix(const Node *Prefix_, StringView Suffix_)
+  DotSuffix(const Node *Prefix_, std::string_view Suffix_)
       : Node(KDotSuffix), Prefix(Prefix_), Suffix(Suffix_) {}
 
   template<typename Fn> void match(Fn F) const { F(Prefix, Suffix); }
 
   void printLeft(OutputBuffer &OB) const override {
     Prefix->print(OB);
     OB += " (";
     OB += Suffix;
     OB += ")";
   }
 };
 
 class VendorExtQualType final : public Node {
   const Node *Ty;
-  StringView Ext;
+  std::string_view Ext;
   const Node *TA;
 
 public:
-  VendorExtQualType(const Node *Ty_, StringView Ext_, const Node *TA_)
+  VendorExtQualType(const Node *Ty_, std::string_view Ext_, const Node *TA_)
       : Node(KVendorExtQualType), Ty(Ty_), Ext(Ext_), TA(TA_) {}
 
   const Node *getTy() const { return Ty; }
-  StringView getExt() const { return Ext; }
+  std::string_view getExt() const { return Ext; }
   const Node *getTA() const { return TA; }
 
   template <typename Fn> void match(Fn F) const { F(Ty, Ext, TA); }
 
   void printLeft(OutputBuffer &OB) const override {
     Ty->print(OB);
     OB += " ";
     OB += Ext;
public:
   void printLeft(OutputBuffer &OB) const override {
     OB += "operator ";
     Ty->print(OB);
   }
 };
 
 class PostfixQualifiedType final : public Node {
   const Node *Ty;
-  const StringView Postfix;
+  const std::string_view Postfix;
 
 public:
-  PostfixQualifiedType(const Node *Ty_, StringView Postfix_)
+  PostfixQualifiedType(const Node *Ty_, std::string_view Postfix_)
       : Node(KPostfixQualifiedType), Ty(Ty_), Postfix(Postfix_) {}
 
   template<typename Fn> void match(Fn F) const { F(Ty, Postfix); }
 
   void printLeft(OutputBuffer &OB) const override {
     Ty->printLeft(OB);
     OB += Postfix;
   }
 };
 
 class NameType final : public Node {
-  const StringView Name;
+  const std::string_view Name;
 
 public:
-  NameType(StringView Name_) : Node(KNameType), Name(Name_) {}
+  NameType(std::string_view Name_) : Node(KNameType), Name(Name_) {}
 
   template<typename Fn> void match(Fn F) const { F(Name); }
 
-  StringView getName() const { return Name; }
-  StringView getBaseName() const override { return Name; }
+  std::string_view getName() const { return Name; }
+  std::string_view getBaseName() const override { return Name; }
 
   void printLeft(OutputBuffer &OB) const override { OB += Name; }
 };
 
 class BitIntType final : public Node {
   const Node *Size;
   bool Signed;
 
void printLeft(OutputBuffer &OB) const override {
     OB += "_BitInt";
     OB.printOpen();
     Size->printAsOperand(OB);
     OB.printClose();
   }
 };
 
 class ElaboratedTypeSpefType : public Node {
-  StringView Kind;
+  std::string_view Kind;
   Node *Child;
 public:
-  ElaboratedTypeSpefType(StringView Kind_, Node *Child_)
+  ElaboratedTypeSpefType(std::string_view Kind_, Node *Child_)
       : Node(KElaboratedTypeSpefType), Kind(Kind_), Child(Child_) {}
 
   template<typename Fn> void match(Fn F) const { F(Kind, Child); }
 
   void printLeft(OutputBuffer &OB) const override {
     OB += Kind;
     OB += ' ';
     Child->print(OB);
   }
 };
 
 struct AbiTagAttr : Node {
   Node *Base;
-  StringView Tag;
+  std::string_view Tag;
 
-  AbiTagAttr(Node* Base_, StringView Tag_)
-      : Node(KAbiTagAttr, Base_->RHSComponentCache,
-             Base_->ArrayCache, Base_->FunctionCache),
+  AbiTagAttr(Node *Base_, std::string_view Tag_)
+      : Node(KAbiTagAttr, Base_->RHSComponentCache, Base_->ArrayCache,
+             Base_->FunctionCache),
         Base(Base_), Tag(Tag_) {}
 
   template<typename Fn> void match(Fn F) const { F(Base, Tag); }
 
-  StringView getBaseName() const override { return Base->getBaseName(); }
+  std::string_view getBaseName() const override { return Base->getBaseName(); }
 
   void printLeft(OutputBuffer &OB) const override {
     Base->printLeft(OB);
     OB += "[abi:";
     OB += Tag;
     OB += "]";
   }
 };
void printLeft(OutputBuffer &OB) const override {
     OB += " [enable_if:";
     Conditions.printWithComma(OB);
     OB += ']';
   }
 };
 
 class ObjCProtoName : public Node {
   const Node *Ty;
-  StringView Protocol;
+  std::string_view Protocol;
 
   friend class PointerType;
 
 public:
-  ObjCProtoName(const Node *Ty_, StringView Protocol_)
+  ObjCProtoName(const Node *Ty_, std::string_view Protocol_)
       : Node(KObjCProtoName), Ty(Ty_), Protocol(Protocol_) {}
 
   template<typename Fn> void match(Fn F) const { F(Ty, Protocol); }
 
   bool isObjCObject() const {
     return Ty->getKind() == KNameType &&
            static_cast<const NameType *>(Ty)->getName() == "objc_object";
   }
public:
 
   void printLeft(OutputBuffer &OB) const override {
     OB += "operator\"\" ";
     OpName->print(OB);
   }
 };
 
 class SpecialName final : public Node {
-  const StringView Special;
+  const std::string_view Special;
   const Node *Child;
 
 public:
-  SpecialName(StringView Special_, const Node *Child_)
+  SpecialName(std::string_view Special_, const Node *Child_)
       : Node(KSpecialName), Special(Special_), Child(Child_) {}
 
   template<typename Fn> void match(Fn F) const { F(Special, Child); }
 
   void printLeft(OutputBuffer &OB) const override {
     OB += Special;
     Child->print(OB);
   }
struct NestedName : Node {
   Node *Qual;
   Node *Name;
 
   NestedName(Node *Qual_, Node *Name_)
       : Node(KNestedName), Qual(Qual_), Name(Name_) {}
 
   template<typename Fn> void match(Fn F) const { F(Qual, Name); }
 
-  StringView getBaseName() const override { return Name->getBaseName(); }
+  std::string_view getBaseName() const override { return Name->getBaseName(); }
 
   void printLeft(OutputBuffer &OB) const override {
     Qual->print(OB);
     OB += "::";
     Name->print(OB);
   }
 };
 
struct ModuleEntity : Node {
   ModuleName *Module;
   Node *Name;
 
   ModuleEntity(ModuleName *Module_, Node *Name_)
       : Node(KModuleEntity), Module(Module_), Name(Name_) {}
 
   template <typename Fn> void match(Fn F) const { F(Module, Name); }
 
-  StringView getBaseName() const override { return Name->getBaseName(); }
+  std::string_view getBaseName() const override { return Name->getBaseName(); }
 
   void printLeft(OutputBuffer &OB) const override {
     Name->print(OB);
     OB += '@';
     Module->print(OB);
   }
 };
 
class QualifiedName final : public Node {
   const Node *Name;
 
 public:
   QualifiedName(const Node *Qualifier_, const Node *Name_)
       : Node(KQualifiedName), Qualifier(Qualifier_), Name(Name_) {}
 
   template<typename Fn> void match(Fn F) const { F(Qualifier, Name); }
 
-  StringView getBaseName() const override { return Name->getBaseName(); }
+  std::string_view getBaseName() const override { return Name->getBaseName(); }
 
   void printLeft(OutputBuffer &OB) const override {
     Qualifier->print(OB);
     OB += "::";
     Name->print(OB);
   }
 };
 
struct NameWithTemplateArgs : Node {
   Node *Name;
   Node *TemplateArgs;
 
   NameWithTemplateArgs(Node *Name_, Node *TemplateArgs_)
       : Node(KNameWithTemplateArgs), Name(Name_), TemplateArgs(TemplateArgs_) {}
 
   template<typename Fn> void match(Fn F) const { F(Name, TemplateArgs); }
 
-  StringView getBaseName() const override { return Name->getBaseName(); }
+  std::string_view getBaseName() const override { return Name->getBaseName(); }
 
   void printLeft(OutputBuffer &OB) const override {
     Name->print(OB);
     TemplateArgs->print(OB);
   }
 };
 
 class GlobalQualifiedName final : public Node {
   Node *Child;
 
 public:
   GlobalQualifiedName(Node* Child_)
       : Node(KGlobalQualifiedName), Child(Child_) {}
 
   template<typename Fn> void match(Fn F) const { F(Child); }
 
-  StringView getBaseName() const override { return Child->getBaseName(); }
+  std::string_view getBaseName() const override { return Child->getBaseName(); }
 
   void printLeft(OutputBuffer &OB) const override {
     OB += "::";
     Child->print(OB);
   }
 };
 
 enum class SpecialSubKind {
public:
 
   template<typename Fn> void match(Fn F) const { F(SSK); }
 
 protected:
   bool isInstantiation() const {
     return unsigned(SSK) >= unsigned(SpecialSubKind::string);
   }
 
-  StringView getBaseName() const override {
+  std::string_view getBaseName() const override {
     switch (SSK) {
     case SpecialSubKind::allocator:
-      return StringView("allocator");
+      return {"allocator"};
     case SpecialSubKind::basic_string:
-      return StringView("basic_string");
+      return {"basic_string"};
     case SpecialSubKind::string:
-      return StringView("basic_string");
+      return {"basic_string"};
     case SpecialSubKind::istream:
-      return StringView("basic_istream");
+      return {"basic_istream"};
     case SpecialSubKind::ostream:
-      return StringView("basic_ostream");
+      return {"basic_ostream"};
     case SpecialSubKind::iostream:
-      return StringView("basic_iostream");
+      return {"basic_iostream"};
     }
     DEMANGLE_UNREACHABLE;
   }
 
 private:
   void printLeft(OutputBuffer &OB) const override {
     OB << "std::" << getBaseName();
     if (isInstantiation()) {
       OB << "<char, std::char_traits<char>";
       if (SSK == SpecialSubKind::string)
         OB << ", std::allocator<char>";
       OB << ">";
     }
   }
 };
 
 class SpecialSubstitution final : public ExpandedSpecialSubstitution {
 public:
   SpecialSubstitution(SpecialSubKind SSK_)
       : ExpandedSpecialSubstitution(SSK_, KSpecialSubstitution) {}
 
   template<typename Fn> void match(Fn F) const { F(SSK); }
 
-  StringView getBaseName() const override {
-    auto SV = ExpandedSpecialSubstitution::getBaseName ();
+  std::string_view getBaseName() const override {
+    std::string_view SV = ExpandedSpecialSubstitution::getBaseName();
     if (isInstantiation()) {
       // The instantiations are typedefs that drop the "basic_" prefix.
-      assert(SV.startsWith("basic_"));
-      SV = SV.dropFront(sizeof("basic_") - 1);
+      assert(starts_with(SV, "basic_"));
+      SV.remove_prefix(sizeof("basic_") - 1);
     }
     return SV;
   }
 
   void printLeft(OutputBuffer &OB) const override {
     OB << "std::" << getBaseName();
   }
 };
public:
 
   void printLeft(OutputBuffer &OB) const override {
     OB += "~";
     Base->printLeft(OB);
   }
 };
 
 class UnnamedTypeName : public Node {
-  const StringView Count;
+  const std::string_view Count;
 
 public:
-  UnnamedTypeName(StringView Count_) : Node(KUnnamedTypeName), Count(Count_) {}
+  UnnamedTypeName(std::string_view Count_)
+      : Node(KUnnamedTypeName), Count(Count_) {}
 
   template<typename Fn> void match(Fn F) const { F(Count); }
 
   void printLeft(OutputBuffer &OB) const override {
     OB += "'unnamed";
     OB += Count;
     OB += "\'";
   }
 };
 
 class ClosureTypeName : public Node {
   NodeArray TemplateParams;
   NodeArray Params;
-  StringView Count;
+  std::string_view Count;
 
 public:
   ClosureTypeName(NodeArray TemplateParams_, NodeArray Params_,
-                  StringView Count_)
+                  std::string_view Count_)
       : Node(KClosureTypeName), TemplateParams(TemplateParams_),
         Params(Params_), Count(Count_) {}
 
   template<typename Fn> void match(Fn F) const {
     F(TemplateParams, Params, Count);
   }
 
   void printDeclarator(OutputBuffer &OB) const {
void printLeft(OutputBuffer &OB) const override {
     OB.printClose(']');
   }
 };
 
 // -- Expression Nodes --
 
 class BinaryExpr : public Node {
   const Node *LHS;
-  const StringView InfixOperator;
+  const std::string_view InfixOperator;
   const Node *RHS;
 
 public:
-  BinaryExpr(const Node *LHS_, StringView InfixOperator_, const Node *RHS_,
-             Prec Prec_)
+  BinaryExpr(const Node *LHS_, std::string_view InfixOperator_,
+             const Node *RHS_, Prec Prec_)
       : Node(KBinaryExpr, Prec_), LHS(LHS_), InfixOperator(InfixOperator_),
         RHS(RHS_) {}
 
   template <typename Fn> void match(Fn F) const {
     F(LHS, InfixOperator, RHS, getPrecedence());
   }
 
   void printLeft(OutputBuffer &OB) const override {
void printLeft(OutputBuffer &OB) const override {
     OB.printOpen('[');
     Op2->printAsOperand(OB);
     OB.printClose(']');
   }
 };
 
 class PostfixExpr : public Node {
   const Node *Child;
-  const StringView Operator;
+  const std::string_view Operator;
 
 public:
-  PostfixExpr(const Node *Child_, StringView Operator_, Prec Prec_)
+  PostfixExpr(const Node *Child_, std::string_view Operator_, Prec Prec_)
       : Node(KPostfixExpr, Prec_), Child(Child_), Operator(Operator_) {}
 
   template <typename Fn> void match(Fn F) const {
     F(Child, Operator, getPrecedence());
   }
 
   void printLeft(OutputBuffer &OB) const override {
     Child->printAsOperand(OB, getPrecedence(), true);
void printLeft(OutputBuffer &OB) const override {
     Then->printAsOperand(OB);
     OB += " : ";
     Else->printAsOperand(OB, Prec::Assign, true);
   }
 };
 
 class MemberExpr : public Node {
   const Node *LHS;
-  const StringView Kind;
+  const std::string_view Kind;
   const Node *RHS;
 
 public:
-  MemberExpr(const Node *LHS_, StringView Kind_, const Node *RHS_, Prec Prec_)
+  MemberExpr(const Node *LHS_, std::string_view Kind_, const Node *RHS_,
+             Prec Prec_)
       : Node(KMemberExpr, Prec_), LHS(LHS_), Kind(Kind_), RHS(RHS_) {}
 
   template <typename Fn> void match(Fn F) const {
     F(LHS, Kind, RHS, getPrecedence());
   }
 
   void printLeft(OutputBuffer &OB) const override {
     LHS->printAsOperand(OB, getPrecedence(), true);
     OB += Kind;
     RHS->printAsOperand(OB, getPrecedence(), false);
   }
 };
 
 class SubobjectExpr : public Node {
   const Node *Type;
   const Node *SubExpr;
-  StringView Offset;
+  std::string_view Offset;
   NodeArray UnionSelectors;
   bool OnePastTheEnd;
 
 public:
-  SubobjectExpr(const Node *Type_, const Node *SubExpr_, StringView Offset_,
-                NodeArray UnionSelectors_, bool OnePastTheEnd_)
+  SubobjectExpr(const Node *Type_, const Node *SubExpr_,
+                std::string_view Offset_, NodeArray UnionSelectors_,
+                bool OnePastTheEnd_)
       : Node(KSubobjectExpr), Type(Type_), SubExpr(SubExpr_), Offset(Offset_),
         UnionSelectors(UnionSelectors_), OnePastTheEnd(OnePastTheEnd_) {}
 
   template<typename Fn> void match(Fn F) const {
     F(Type, SubExpr, Offset, UnionSelectors, OnePastTheEnd);
   }
 
   void printLeft(OutputBuffer &OB) const override {
     SubExpr->print(OB);
     OB += ".<";
     Type->print(OB);
     OB += " at offset ";
     if (Offset.empty()) {
       OB += "0";
     } else if (Offset[0] == 'n') {
       OB += "-";
-      OB += Offset.dropFront();
+      OB += std::string_view(Offset.data() + 1, Offset.size() - 1);
     } else {
       OB += Offset;
     }
     OB += ">";
   }
 };
 
 class EnclosingExpr : public Node {
-  const StringView Prefix;
+  const std::string_view Prefix;
   const Node *Infix;
-  const StringView Postfix;
+  const std::string_view Postfix;
 
 public:
-  EnclosingExpr(StringView Prefix_, const Node *Infix_,
+  EnclosingExpr(std::string_view Prefix_, const Node *Infix_,
                 Prec Prec_ = Prec::Primary)
       : Node(KEnclosingExpr, Prec_), Prefix(Prefix_), Infix(Infix_) {}
 
   template <typename Fn> void match(Fn F) const {
     F(Prefix, Infix, getPrecedence());
   }
 
   void printLeft(OutputBuffer &OB) const override {
     OB += Prefix;
     OB.printOpen();
     Infix->print(OB);
     OB.printClose();
     OB += Postfix;
   }
 };
 
 class CastExpr : public Node {
   // cast_kind<to>(from)
-  const StringView CastKind;
+  const std::string_view CastKind;
   const Node *To;
   const Node *From;
 
 public:
-  CastExpr(StringView CastKind_, const Node *To_, const Node *From_, Prec Prec_)
+  CastExpr(std::string_view CastKind_, const Node *To_, const Node *From_,
+           Prec Prec_)
       : Node(KCastExpr, Prec_), CastKind(CastKind_), To(To_), From(From_) {}
 
   template <typename Fn> void match(Fn F) const {
     F(CastKind, To, From, getPrecedence());
   }
 
   void printLeft(OutputBuffer &OB) const override {
     OB += CastKind;
void printLeft(OutputBuffer &OB) const override {
     if (IsArray)
       OB += "[]";
     OB += ' ';
     Op->print(OB);
   }
 };
 
 class PrefixExpr : public Node {
-  StringView Prefix;
+  std::string_view Prefix;
   Node *Child;
 
 public:
-  PrefixExpr(StringView Prefix_, Node *Child_, Prec Prec_)
+  PrefixExpr(std::string_view Prefix_, Node *Child_, Prec Prec_)
       : Node(KPrefixExpr, Prec_), Prefix(Prefix_), Child(Child_) {}
 
   template <typename Fn> void match(Fn F) const {
     F(Prefix, Child, getPrecedence());
   }
 
   void printLeft(OutputBuffer &OB) const override {
     OB += Prefix;
     Child->printAsOperand(OB, getPrecedence());
   }
 };
 
 class FunctionParam : public Node {
-  StringView Number;
+  std::string_view Number;
 
 public:
-  FunctionParam(StringView Number_) : Node(KFunctionParam), Number(Number_) {}
+  FunctionParam(std::string_view Number_)
+      : Node(KFunctionParam), Number(Number_) {}
 
   template<typename Fn> void match(Fn F) const { F(Number); }
 
   void printLeft(OutputBuffer &OB) const override {
     OB += "fp";
     OB += Number;
   }
 };
void printLeft(OutputBuffer &OB) const override {
     Expressions.printWithComma(OB);
     OB.printClose();
   }
 };
 
 class PointerToMemberConversionExpr : public Node {
   const Node *Type;
   const Node *SubExpr;
-  StringView Offset;
+  std::string_view Offset;
 
 public:
   PointerToMemberConversionExpr(const Node *Type_, const Node *SubExpr_,
-                                StringView Offset_, Prec Prec_)
+                                std::string_view Offset_, Prec Prec_)
       : Node(KPointerToMemberConversionExpr, Prec_), Type(Type_),
         SubExpr(SubExpr_), Offset(Offset_) {}
 
   template <typename Fn> void match(Fn F) const {
     F(Type, SubExpr, Offset, getPrecedence());
   }
 
   void printLeft(OutputBuffer &OB) const override {
void printLeft(OutputBuffer &OB) const override {
     if (Init->getKind() != KBracedExpr && Init->getKind() != KBracedRangeExpr)
       OB += " = ";
     Init->print(OB);
   }
 };
 
 class FoldExpr : public Node {
   const Node *Pack, *Init;
-  StringView OperatorName;
+  std::string_view OperatorName;
   bool IsLeftFold;
 
 public:
-  FoldExpr(bool IsLeftFold_, StringView OperatorName_, const Node *Pack_,
+  FoldExpr(bool IsLeftFold_, std::string_view OperatorName_, const Node *Pack_,
            const Node *Init_)
       : Node(KFoldExpr), Pack(Pack_), Init(Init_), OperatorName(OperatorName_),
         IsLeftFold(IsLeftFold_) {}
 
   template<typename Fn> void match(Fn F) const {
     F(IsLeftFold, OperatorName, Pack, Init);
   }
 
class BoolExpr : public Node {
   bool Value;
 
 public:
   BoolExpr(bool Value_) : Node(KBoolExpr), Value(Value_) {}
 
   template<typename Fn> void match(Fn F) const { F(Value); }
 
   void printLeft(OutputBuffer &OB) const override {
-    OB += Value ? StringView("true") : StringView("false");
+    OB += Value ? std::string_view("true") : std::string_view("false");
   }
 };
 
 class StringLiteral : public Node {
   const Node *Type;
 
 public:
   StringLiteral(const Node *Type_) : Node(KStringLiteral), Type(Type_) {}
if (Type->getKind() == KClosureTypeName)
       static_cast<const ClosureTypeName *>(Type)->printDeclarator(OB);
     OB += "{...}";
   }
 };
 
 class EnumLiteral : public Node {
   // ty(integer)
   const Node *Ty;
-  StringView Integer;
+  std::string_view Integer;
 
 public:
-  EnumLiteral(const Node *Ty_, StringView Integer_)
+  EnumLiteral(const Node *Ty_, std::string_view Integer_)
       : Node(KEnumLiteral), Ty(Ty_), Integer(Integer_) {}
 
   template<typename Fn> void match(Fn F) const { F(Ty, Integer); }
 
   void printLeft(OutputBuffer &OB) const override {
     OB.printOpen();
     Ty->print(OB);
     OB.printClose();
 
     if (Integer[0] == 'n')
-      OB << "-" << Integer.dropFront(1);
+      OB << '-' << std::string_view(Integer.data() + 1, Integer.size() - 1);
     else
       OB << Integer;
   }
 };
 
 class IntegerLiteral : public Node {
-  StringView Type;
-  StringView Value;
+  std::string_view Type;
+  std::string_view Value;
 
 public:
-  IntegerLiteral(StringView Type_, StringView Value_)
+  IntegerLiteral(std::string_view Type_, std::string_view Value_)
       : Node(KIntegerLiteral), Type(Type_), Value(Value_) {}
 
   template<typename Fn> void match(Fn F) const { F(Type, Value); }
 
   void printLeft(OutputBuffer &OB) const override {
     if (Type.size() > 3) {
       OB.printOpen();
       OB += Type;
       OB.printClose();
     }
 
-    if (Value[0] == 'n') {
-      OB += '-';
-      OB += Value.dropFront(1);
-    } else
+    if (Value[0] == 'n')
+      OB << '-' << std::string_view(Value.data() + 1, Value.size() - 1);
+    else
       OB += Value;
 
     if (Type.size() <= 3)
       OB += Type;
   }
 };
 
 template <class Float> struct FloatData;
 
 namespace float_literal_impl {
 constexpr Node::Kind getFloatLiteralKind(float *) {
   return Node::KFloatLiteral;
 }
 constexpr Node::Kind getFloatLiteralKind(double *) {
   return Node::KDoubleLiteral;
 }
 constexpr Node::Kind getFloatLiteralKind(long double *) {
   return Node::KLongDoubleLiteral;
 }
 }
 
 template <class Float> class FloatLiteralImpl : public Node {
-  const StringView Contents;
+  const std::string_view Contents;
 
   static constexpr Kind KindForClass =
       float_literal_impl::getFloatLiteralKind((Float *)nullptr);
 
 public:
-  FloatLiteralImpl(StringView Contents_)
+  FloatLiteralImpl(std::string_view Contents_)
       : Node(KindForClass), Contents(Contents_) {}
 
   template<typename Fn> void match(Fn F) const { F(Contents); }
 
   void printLeft(OutputBuffer &OB) const override {
-    const char *first = Contents.begin();
-    const char *last = Contents.end() + 1;
-
     const size_t N = FloatData<Float>::mangled_size;
-    if (static_cast<std::size_t>(last - first) > N) {
-      last = first + N;
+    if (Contents.size() >= N) {
       union {
         Float value;
         char buf[sizeof(Float)];
       };
-      const char *t = first;
+      const char *t = &*Contents.begin();
+      const char *last = t + N;
       char *e = buf;
       for (; t != last; ++t, ++e) {
         unsigned d1 = isdigit(*t) ? static_cast<unsigned>(*t - '0')
                                   : static_cast<unsigned>(*t - 'a' + 10);
         ++t;
         unsigned d0 = isdigit(*t) ? static_cast<unsigned>(*t - '0')
                                   : static_cast<unsigned>(*t - 'a' + 10);
         *e = static_cast<char>((d1 << 4) + d0);
       }
 #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
       std::reverse(buf, e);
 #endif
       char num[FloatData<Float>::max_demangled_size] = {0};
       int n = snprintf(num, sizeof(num), FloatData<Float>::spec, value);
-      OB += StringView(num, num + n);
+      OB += std::string_view(num, n);
     }
   }
 };
 
 using FloatLiteral = FloatLiteralImpl<float>;
 using DoubleLiteral = FloatLiteralImpl<double>;
 using LongDoubleLiteral = FloatLiteralImpl<long double>;
 
template <typename Derived, typename Alloc> struct AbstractManglingParser {
   NodeArray popTrailingNodeArray(size_t FromPosition) {
     assert(FromPosition <= Names.size());
     NodeArray res =
         makeNodeArray(Names.begin() + (long)FromPosition, Names.end());
     Names.dropBack(FromPosition);
     return res;
   }
 
-  bool consumeIf(StringView S) {
-    if (StringView(First, Last).startsWith(S)) {
+  bool consumeIf(std::string_view S) {
+    if (starts_with(std::string_view(First, Last - First), S)) {
       First += S.size();
       return true;
     }
     return false;
   }
 
   bool consumeIf(char C) {
     if (First != Last && *First == C) {
       ++First;
       return true;
     }
     return false;
   }
 
   char consume() { return First != Last ? *First++ : '\0'; }
 
   char look(unsigned Lookahead = 0) const {
     if (static_cast<size_t>(Last - First) <= Lookahead)
       return '\0';
     return First[Lookahead];
   }
 
   size_t numLeft() const { return static_cast<size_t>(Last - First); }
 
-  StringView parseNumber(bool AllowNegative = false);
+  std::string_view parseNumber(bool AllowNegative = false);
   Qualifiers parseCVQualifiers();
   bool parsePositiveInteger(size_t *Out);
-  StringView parseBareSourceName();
+  std::string_view parseBareSourceName();
 
   bool parseSeqId(size_t *Out);
   Node *parseSubstitution();
   Node *parseTemplateParam();
   Node *parseTemplateParamDecl();
   Node *parseTemplateArgs(bool TagTemplates = false);
   Node *parseTemplateArg();
 
   /// Parse the <expr> production.
   Node *parseExpr();
-  Node *parsePrefixExpr(StringView Kind, Node::Prec Prec);
-  Node *parseBinaryExpr(StringView Kind, Node::Prec Prec);
-  Node *parseIntegerLiteral(StringView Lit);
+  Node *parsePrefixExpr(std::string_view Kind, Node::Prec Prec);
+  Node *parseBinaryExpr(std::string_view Kind, Node::Prec Prec);
+  Node *parseIntegerLiteral(std::string_view Lit);
   Node *parseExprPrimary();
   template <class Float> Node *parseFloatingLiteral();
   Node *parseFunctionParam();
   Node *parseConversionExpr();
   Node *parseBracedExpr();
   Node *parseFoldExpr();
   Node *parsePointerToMemberConversionExpr(Node::Prec Prec);
   Node *parseSubobjectExpr();
bool operator<(const char *Peek) const {
       return Enc[0] < Peek[0] || (Enc[0] == Peek[0] && Enc[1] < Peek[1]);
     }
     bool operator==(const char *Peek) const {
       return Enc[0] == Peek[0] && Enc[1] == Peek[1];
     }
     bool operator!=(const char *Peek) const { return !this->operator==(Peek); }
 
   public:
-    StringView getSymbol() const {
-      StringView Res = Name;
+    std::string_view getSymbol() const {
+      std::string_view Res = Name;
       if (Kind < Unnameable) {
-        assert(Res.startsWith("operator") &&
+        assert(starts_with(Res, "operator") &&
                "operator name does not start with 'operator'");
-        Res = Res.dropFront(sizeof("operator") - 1);
-        Res.consumeFront(' ');
+        Res.remove_prefix(sizeof("operator") - 1);
+        if (starts_with(Res, ' '))
+          Res.remove_prefix(1);
       }
       return Res;
     }
-    StringView getName() const { return Name; }
+    std::string_view getName() const { return Name; }
     OIKind getKind() const { return Kind; }
     bool getFlag() const { return Flag; }
     Node::Prec getPrecedence() const { return Prec; }
   };
   static const OperatorInfo Ops[];
   static const size_t NumOps;
   const OperatorInfo *parseOperatorEncoding();
 

 Node *
 AbstractManglingParser<Derived, Alloc>::parseUnnamedTypeName(NameState *State) {
   // <template-params> refer to the innermost <template-args>. Clear out any
   // outer args that we may have inserted into TemplateParams.
   if (State != nullptr)
     TemplateParams.clear();
 
   if (consumeIf("Ut")) {
-    StringView Count = parseNumber();
+    std::string_view Count = parseNumber();
     if (!consumeIf('_'))
       return nullptr;
     return make<UnnamedTypeName>(Count);
   }
   if (consumeIf("Ul")) {
     ScopedOverride<size_t> SwapParams(ParsingLambdaParamsAtLevel,
                                       TemplateParams.size());
     ScopedTemplateParamList LambdaTemplateParams(this);
 
     size_t ParamsBegin = Names.size();
     while (look() == 'T' &&
-           StringView("yptn").find(look(1)) != StringView::npos) {
+           std::string_view("yptn").find(look(1)) != std::string_view::npos) {
       Node *T = parseTemplateParamDecl();
       if (!T)
         return nullptr;
       Names.push_back(T);
     }
     NodeArray TempParams = popTrailingNodeArray(ParamsBegin);
 
     // FIXME: If TempParams is empty and none of the function parameters
if (!consumeIf("vE")) {
         Node *P = getDerived().parseType();
         if (P == nullptr)
           return nullptr;
         Names.push_back(P);
       } while (!consumeIf('E'));
     }
     NodeArray Params = popTrailingNodeArray(ParamsBegin);
 
-    StringView Count = parseNumber();
+    std::string_view Count = parseNumber();
     if (!consumeIf('_'))
       return nullptr;
     return make<ClosureTypeName>(TempParams, Params, Count);
   }
   if (consumeIf("Ub")) {
     (void)parseNumber();
     if (!consumeIf('_'))
       return nullptr;
     return make<NameType>("'block-literal'");
   }
   return nullptr;
 }
 
 // <source-name> ::= <positive length number> <identifier>
 template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parseSourceName(NameState *) {
   size_t Length = 0;
   if (parsePositiveInteger(&Length))
     return nullptr;
   if (numLeft() < Length || Length == 0)
     return nullptr;
-  StringView Name(First, First + Length);
+  std::string_view Name(First, Length);
   First += Length;
-  if (Name.startsWith("_GLOBAL__N"))
+  if (starts_with(Name, "_GLOBAL__N"))
     return make<NameType>("(anonymous namespace)");
   return make<NameType>(Name);
 }
 
 // Operator encodings
 template <typename Derived, typename Alloc>
 const typename AbstractManglingParser<
     Derived, Alloc>::OperatorInfo AbstractManglingParser<Derived,
Node *AbstractManglingParser<Derived, Alloc>::parseUnresolvedName(bool Global) {
   return make<QualifiedName>(SoFar, Base);
 }
 
 // <abi-tags> ::= <abi-tag> [<abi-tags>]
 // <abi-tag> ::= B <source-name>
 template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parseAbiTags(Node *N) {
   while (consumeIf('B')) {
-    StringView SN = parseBareSourceName();
+    std::string_view SN = parseBareSourceName();
     if (SN.empty())
       return nullptr;
     N = make<AbiTagAttr>(N, SN);
     if (!N)
       return nullptr;
   }
   return N;
 }
 
 // <number> ::= [n] <non-negative decimal integer>
 template <typename Alloc, typename Derived>
-StringView
+std::string_view
 AbstractManglingParser<Alloc, Derived>::parseNumber(bool AllowNegative) {
   const char *Tmp = First;
   if (AllowNegative)
     consumeIf('n');
   if (numLeft() == 0 || !std::isdigit(*First))
-    return StringView();
+    return std::string_view();
   while (numLeft() != 0 && std::isdigit(*First))
     ++First;
-  return StringView(Tmp, First);
+  return std::string_view(Tmp, First - Tmp);
 }
 
 // <positive length number> ::= [0-9]*
 template <typename Alloc, typename Derived>
 bool AbstractManglingParser<Alloc, Derived>::parsePositiveInteger(size_t *Out) {
   *Out = 0;
   if (look() < '0' || look() > '9')
     return true;
   while (look() >= '0' && look() <= '9') {
     *Out *= 10;
     *Out += static_cast<size_t>(consume() - '0');
   }
   return false;
 }
 
 template <typename Alloc, typename Derived>
-StringView AbstractManglingParser<Alloc, Derived>::parseBareSourceName() {
+std::string_view AbstractManglingParser<Alloc, Derived>::parseBareSourceName() {
   size_t Int = 0;
   if (parsePositiveInteger(&Int) || numLeft() < Int)
-    return StringView();
-  StringView R(First, First + Int);
+    return {};
+  std::string_view R(First, Int);
   First += Int;
   return R;
 }
 
 // <function-type> ::= [<CV-qualifiers>] [<exception-spec>] [Dx] F [Y] <bare-function-type> [<ref-qualifier>] E
 //
 // <exception-spec> ::= Do                # non-throwing exception-specification (e.g., noexcept, throw())
 //                  ::= DO <expression> E # computed (instantiation-dependent) noexcept

 }
 
 // <class-enum-type> ::= <name>     # non-dependent type name, dependent type name, or dependent typename-specifier
 //                   ::= Ts <name>  # dependent elaborated type specifier using 'struct' or 'class'
 //                   ::= Tu <name>  # dependent elaborated type specifier using 'union'
 //                   ::= Te <name>  # dependent elaborated type specifier using 'enum'
 template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parseClassEnumType() {
-  StringView ElabSpef;
+  std::string_view ElabSpef;
   if (consumeIf("Ts"))
     ElabSpef = "struct";
   else if (consumeIf("Tu"))
     ElabSpef = "union";
   else if (consumeIf("Te"))
     ElabSpef = "enum";
 
   Node *Name = getDerived().parseName();
   if (Name == nullptr)
     return nullptr;
 
   if (!ElabSpef.empty())
     return make<ElaboratedTypeSpefType>(ElabSpef, Name);
 
   return Name;
 }
 
 // <qualified-type>     ::= <qualifiers> <type>
 // <qualifiers> ::= <extended-qualifier>* <CV-qualifiers>
 // <extended-qualifier> ::= U <source-name> [<template-args>] # vendor extended type qualifier
 template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parseQualifiedType() {
   if (consumeIf('U')) {
-    StringView Qual = parseBareSourceName();
+    std::string_view Qual = parseBareSourceName();
     if (Qual.empty())
       return nullptr;
 
     // extension            ::= U <objc-name> <objc-type>  # objc-type<identifier>
-    if (Qual.startsWith("objcproto")) {
-      StringView ProtoSourceName = Qual.dropFront(std::strlen("objcproto"));
-      StringView Proto;
+    if (starts_with(Qual, "objcproto")) {
+      constexpr size_t Len = sizeof("objcproto") - 1;
+      std::string_view ProtoSourceName(Qual.data() + Len, Qual.size() - Len);
+      std::string_view Proto;
       {
-        ScopedOverride<const char *> SaveFirst(First, ProtoSourceName.begin()),
-            SaveLast(Last, ProtoSourceName.end());
+        ScopedOverride<const char *> SaveFirst(First, &*ProtoSourceName.begin()),
+            SaveLast(Last, &*ProtoSourceName.rbegin() + 1);
         Proto = parseBareSourceName();
       }
       if (Proto.empty())
         return nullptr;
       Node *Child = getDerived().parseQualifiedType();
       if (Child == nullptr)
         return nullptr;
       return make<ObjCProtoName>(Child, Proto);
Node *AbstractManglingParser<Derived, Alloc>::parseType() {
   //                ::= z    # ellipsis
   case 'z':
     ++First;
     return make<NameType>("...");
 
   // <builtin-type> ::= u <source-name>    # vendor extended type
   case 'u': {
     ++First;
-    StringView Res = parseBareSourceName();
+    std::string_view Res = parseBareSourceName();
     if (Res.empty())
       return nullptr;
     // Typically, <builtin-type>s are not considered substitution candidates,
     // but the exception to that exception is vendor extended types (Itanium C++
     // ABI 5.9.1).
     Result = make<NameType>(Res);
     break;
   }
Node *AbstractManglingParser<Derived, Alloc>::parseType() {
   // <builtin-type>s and <substitution>s have already bailed out, because they
   // don't get substitutions.
   if (Result != nullptr)
     Subs.push_back(Result);
   return Result;
 }
 
 template <typename Derived, typename Alloc>
-Node *AbstractManglingParser<Derived, Alloc>::parsePrefixExpr(StringView Kind,
-                                                              Node::Prec Prec) {
+Node *
+AbstractManglingParser<Derived, Alloc>::parsePrefixExpr(std::string_view Kind,
   Node *E = getDerived().parseExpr();
   if (E == nullptr)
     return nullptr;
   return make<PrefixExpr>(Kind, E, Prec);
 }
 
 template <typename Derived, typename Alloc>
-Node *AbstractManglingParser<Derived, Alloc>::parseBinaryExpr(StringView Kind,
-                                                              Node::Prec Prec) {
+Node *
+AbstractManglingParser<Derived, Alloc>::parseBinaryExpr(std::string_view Kind,
   Node *LHS = getDerived().parseExpr();
   if (LHS == nullptr)
     return nullptr;
   Node *RHS = getDerived().parseExpr();
   if (RHS == nullptr)
     return nullptr;
   return make<BinaryExpr>(LHS, Kind, RHS, Prec);
 }
 
 template <typename Derived, typename Alloc>
-Node *
-AbstractManglingParser<Derived, Alloc>::parseIntegerLiteral(StringView Lit) {
-  StringView Tmp = parseNumber(true);
+Node *AbstractManglingParser<Derived, Alloc>::parseIntegerLiteral(
+    std::string_view Lit) {
+  std::string_view Tmp = parseNumber(true);
   if (!Tmp.empty() && consumeIf('E'))
     return make<IntegerLiteral>(Lit, Tmp);
   return nullptr;
 }
 
 // <CV-Qualifiers> ::= [r] [V] [K]
 template <typename Alloc, typename Derived>
 Qualifiers AbstractManglingParser<Alloc, Derived>::parseCVQualifiers() {

 //                  ::= fL <L-1 non-negative number> p <top-level CV-Qualifiers> <parameter-2 non-negative number> _   # L > 0, second and later parameters
 //                  ::= fpT      # 'this' expression (not part of standard?)
 template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parseFunctionParam() {
   if (consumeIf("fpT"))
     return make<NameType>("this");
   if (consumeIf("fp")) {
     parseCVQualifiers();
-    StringView Num = parseNumber();
+    std::string_view Num = parseNumber();
     if (!consumeIf('_'))
       return nullptr;
     return make<FunctionParam>(Num);
   }
   if (consumeIf("fL")) {
     if (parseNumber().empty())
       return nullptr;
     if (!consumeIf('p'))
       return nullptr;
     parseCVQualifiers();
-    StringView Num = parseNumber();
+    std::string_view Num = parseNumber();
     if (!consumeIf('_'))
       return nullptr;
     return make<FunctionParam>(Num);
   }
   return nullptr;
 }
 
 // cv <type> <expression>                               # conversion with one argument
if (!T || !consumeIf('E'))
       return nullptr;
     return make<LambdaExpr>(T);
   }
   default: {
     // might be named type
     Node *T = getDerived().parseType();
     if (T == nullptr)
       return nullptr;
-    StringView N = parseNumber(/*AllowNegative=*/true);
+    std::string_view N = parseNumber(/*AllowNegative=*/true);
     if (N.empty())
       return nullptr;
     if (!consumeIf('E'))
       return nullptr;
     return make<EnumLiteral>(T, N);
   }
   }
 }

 AbstractManglingParser<Derived, Alloc>::parsePointerToMemberConversionExpr(
     Node::Prec Prec) {
   Node *Ty = getDerived().parseType();
   if (!Ty)
     return nullptr;
   Node *Expr = getDerived().parseExpr();
   if (!Expr)
     return nullptr;
-  StringView Offset = getDerived().parseNumber(true);
+  std::string_view Offset = getDerived().parseNumber(true);
   if (!consumeIf('E'))
     return nullptr;
   return make<PointerToMemberConversionExpr>(Ty, Expr, Offset, Prec);
 }
 
 // <expression> ::= so <referent type> <expr> [<offset number>] <union-selector>* [p] E
 // <union-selector> ::= _ [<number>]
 //
 // Not yet in the spec: https://github.com/itanium-cxx-abi/cxx-abi/issues/47
 template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parseSubobjectExpr() {
   Node *Ty = getDerived().parseType();
   if (!Ty)
     return nullptr;
   Node *Expr = getDerived().parseExpr();
   if (!Expr)
     return nullptr;
-  StringView Offset = getDerived().parseNumber(true);
+  std::string_view Offset = getDerived().parseNumber(true);
   size_t SelectorsBegin = Names.size();
   while (consumeIf('_')) {
     Node *Selector = make<NameType>(parseNumber());
     if (!Selector)
       return nullptr;
     Names.push_back(Selector);
   }
   bool OnePastTheEnd = consumeIf('p');

 };
 
 template <typename Alloc, typename Derived>
 template <class Float>
 Node *AbstractManglingParser<Alloc, Derived>::parseFloatingLiteral() {
   const size_t N = FloatData<Float>::mangled_size;
   if (numLeft() <= N)
     return nullptr;
-  StringView Data(First, First + N);
+  std::string_view Data(First, N);
   for (char C : Data)
     if (!std::isxdigit(C))
       return nullptr;
   First += N;
   if (!consumeIf('E'))
     return nullptr;
   return make<FloatLiteralImpl<Float>>(Data);
 }

 // extension      ::= ___Z <encoding> _block_invoke_<decimal-digit>+
 template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parse() {
   if (consumeIf("_Z") || consumeIf("__Z")) {
     Node *Encoding = getDerived().parseEncoding();
     if (Encoding == nullptr)
       return nullptr;
     if (look() == '.') {
-      Encoding = make<DotSuffix>(Encoding, StringView(First, Last));
+      Encoding =
+          make<DotSuffix>(Encoding, std::string_view(First, Last - First));
       First = Last;
     }
     if (numLeft() != 0)
       return nullptr;
     return Encoding;
   }
 
   if (consumeIf("___Z") || consumeIf("____Z")) {

libcxxabi/src/demangle/StringView.h

class StringView {
   const char *First;
   const char *Last;
 
 public:
   static const size_t npos = ~size_t(0);
 
   template <size_t N>
   StringView(const char (&Str)[N]) : First(Str), Last(Str + N - 1) {}
-  StringView(const char *First_, const char *Last_)
-      : First(First_), Last(Last_) {}
   StringView(const char *First_, size_t Len)
       : First(First_), Last(First_ + Len) {}
   StringView(const char *Str) : First(Str), Last(Str + std::strlen(Str)) {}
   StringView() : First(nullptr), Last(nullptr) {}
 
   StringView substr(size_t Pos, size_t Len = npos) const {
     assert(Pos <= size());
     if (Len > size() - Pos)
       Len = size() - Pos;
     return StringView(begin() + Pos, Len);
   }
 
   size_t find(char C, size_t From = 0) const {
     // Avoid calling memchr with nullptr.
     if (From < size()) {
       // Just forward to memchr, which is faster than a hand-rolled loop.
       if (const void *P = ::memchr(First + From, C, size() - From))
         return size_t(static_cast<const char *>(P) - First);
     }
     return npos;
   }
 
-  StringView dropFront(size_t N = 1) const {
-    if (N >= size())
-      N = size();
-    return StringView(First + N, Last);
-  }
-
-  StringView dropBack(size_t N = 1) const {
-    if (N >= size())
-      N = size();
-    return StringView(First, Last - N);
+  void remove_prefix(size_t N) {
+    assert(size() >= N);
+    First += N;
+  }
+  void remove_suffix(size_t N) {
+    assert(size() >= N);
+    Last -= N;
   }
 
   char front() const {
     assert(!empty());
     return *begin();
   }
 
   char back() const {
     assert(!empty());
     return *(end() - 1);
   }
 
-  char popFront() {
-    assert(!empty());
-    return *First++;
-  }
-
-  bool consumeFront(char C) {
-    if (!startsWith(C))
-      return false;
-    *this = dropFront(1);
-    return true;
-  }
-
-  bool consumeFront(StringView S) {
-    if (!startsWith(S))
-      return false;
-    *this = dropFront(S.size());
-    return true;
-  }
-
   bool startsWith(char C) const { return !empty() && *begin() == C; }
 
   bool startsWith(StringView Str) const {
     if (Str.size() > size())
       return false;
     return std::strncmp(Str.begin(), begin(), Str.size()) == 0;
   }
 

libcxxabi/src/demangle/Utility.h

 // is the original and the one in llvm is the copy.  Use cp-to-llvm.sh to update
 // the copy.  See README.txt for more details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef DEMANGLE_UTILITY_H
 #define DEMANGLE_UTILITY_H
 
-#include "StringView.h"
+#include "DemangleConfig.h"
+
 #include <array>
+#include <cassert>
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
 #include <exception>
 #include <limits>
+#include <string_view>
 
 DEMANGLE_NAMESPACE_BEGIN
 
 // Stream that AST nodes write their string representation into after the AST
 // has been parsed.
 class OutputBuffer {
   char *Buffer = nullptr;
   size_t CurrentPosition = 0;
do {
       *--TempPtr = char('0' + N % 10);
       N /= 10;
     } while (N);
 
     // Add negative sign.
     if (isNeg)
       *--TempPtr = '-';
 
-    return operator+=(StringView(TempPtr, Temp.data() + Temp.size()));
+    return operator+=(
+        std::string_view(TempPtr, Temp.data() + Temp.size() - TempPtr));
   }
 
 public:
   OutputBuffer(char *StartBuf, size_t Size)
       : Buffer(StartBuf), BufferCapacity(Size) {}
   OutputBuffer(char *StartBuf, size_t *SizePtr)
       : OutputBuffer(StartBuf, StartBuf ? *SizePtr : 0) {}
   OutputBuffer() = default;
   // Non-copyable
   OutputBuffer(const OutputBuffer &) = delete;
   OutputBuffer &operator=(const OutputBuffer &) = delete;
 
-  operator StringView() const { return StringView(Buffer, CurrentPosition); }
+  operator std::string_view() const {
+    return std::string_view(Buffer, CurrentPosition);
+  }
 
   /// If a ParameterPackExpansion (or similar type) is encountered, the offset
   /// into the pack that we're currently printing.
   unsigned CurrentPackIndex = std::numeric_limits<unsigned>::max();
   unsigned CurrentPackMax = std::numeric_limits<unsigned>::max();
 
   /// When zero, we're printing template args and '>' needs to be parenthesized.
   /// Use a counter so we can simply increment inside parentheses.
   unsigned GtIsGt = 1;
 
   bool isGtInsideTemplateArgs() const { return GtIsGt == 0; }
 
   void printOpen(char Open = '(') {
     GtIsGt++;
     *this += Open;
   }
   void printClose(char Close = ')') {
     GtIsGt--;
     *this += Close;
   }
 
-  OutputBuffer &operator+=(StringView R) {
+  OutputBuffer &operator+=(std::string_view R) {
     if (size_t Size = R.size()) {
       grow(Size);
-      std::memcpy(Buffer + CurrentPosition, R.begin(), Size);
+      std::memcpy(Buffer + CurrentPosition, &*R.begin(), Size);
       CurrentPosition += Size;
     }
     return *this;
   }
 
   OutputBuffer &operator+=(char C) {
     grow(1);
     Buffer[CurrentPosition++] = C;
     return *this;
   }
 
-  OutputBuffer &prepend(StringView R) {
+  OutputBuffer &prepend(std::string_view R) {
     size_t Size = R.size();
 
     grow(Size);
     std::memmove(Buffer + Size, Buffer, CurrentPosition);
-    std::memcpy(Buffer, R.begin(), Size);
+    std::memcpy(Buffer, &*R.begin(), Size);
     CurrentPosition += Size;
 
     return *this;
   }
 
-  OutputBuffer &operator<<(StringView R) { return (*this += R); }
+  OutputBuffer &operator<<(std::string_view R) { return (*this += R); }
 
   OutputBuffer &operator<<(char C) { return (*this += C); }
 
   OutputBuffer &operator<<(long long N) {
     return writeUnsigned(static_cast<unsigned long long>(std::abs(N)), N < 0);
   }
 
   OutputBuffer &operator<<(unsigned long long N) {