[lldb] Enable Rust v0 symbol demangling

[asm]

Summary

Rust's v0 name mangling scheme [1] is easy to disambiguate from other
name mangling schemes because symbols always start with _R. The llvm
Demangle library supports demangling the Rust v0 scheme. Use it to
demangle Rust symbols.

Added unit tests that check simple symbols. Ran LLDB built with this
patch to debug some Rust programs compiled with the v0 name mangling
scheme. Confirmed symbol names were demangled as expected.

Note: enabling the new name mangling scheme requires a nightly
toolchain:

$ cat main.rs
fn main() {
    println!("Hello world!");
}
$ $(rustup which --toolchain nightly rustc) -Z symbol-mangling-version=v0 main.rs -g
$ /home/asm/hacking/llvm/build/bin/lldb ./main --one-line 'b main.rs:2'
(lldb) target create "./main"
Current executable set to '/home/asm/hacking/llvm/rust/main' (x86_64).
(lldb) b main.rs:2
Breakpoint 1: where = main`main::main + 4 at main.rs:2:5, address = 0x00000000000076a4
(lldb) r
Process 948449 launched: '/home/asm/hacking/llvm/rust/main' (x86_64)
warning: (x86_64) /lib64/libgcc_s.so.1 No LZMA support found for reading .gnu_debugdata section
Process 948449 stopped
* thread #1, name = 'main', stop reason = breakpoint 1.1
    frame #0: 0x000055555555b6a4 main`main::main at main.rs:2:5
   1    fn main() {
-> 2        println!("Hello world!");
   3    }
(lldb) bt
error: need to add support for DW_TAG_base_type '()' encoded with DW_ATE = 0x7, bit_size = 0
* thread #1, name = 'main', stop reason = breakpoint 1.1
  * frame #0: 0x000055555555b6a4 main`main::main at main.rs:2:5
    frame #1: 0x000055555555b78b main`<fn() as core::ops::function::FnOnce<()>>::call_once((null)=(main`main::main at main.rs:1), (null)=<unavailable>) at function.rs:227:5
    frame #2: 0x000055555555b66e main`std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>(f=(main`main::main at main.rs:1)) at backtrace.rs:125:18
    frame #3: 0x000055555555b851 main`std::rt::lang_start::<()>::{closure#0} at rt.rs:49:18
    frame #4: 0x000055555556c9f9 main`std::rt::lang_start_internal::hc51399759a90501a [inlined] core::ops::function::impls::_$LT$impl$u20$core..ops..function..FnOnce$LT$A$GT$$u20$for$u20$$RF$F$GT$::call_once::h04259e4a34d07c2f at function.rs:259:13
    frame #5: 0x000055555556c9f2 main`std::rt::lang_start_internal::hc51399759a90501a [inlined] std::panicking::try::do_call::hb8da45704d5cfbbf at panicking.rs:401:40
    frame #6: 0x000055555556c9f2 main`std::rt::lang_start_internal::hc51399759a90501a [inlined] std::panicking::try::h4beadc19a78fec52 at panicking.rs:365:19
    frame #7: 0x000055555556c9f2 main`std::rt::lang_start_internal::hc51399759a90501a [inlined] std::panic::catch_unwind::hc58016cd36ba81a4 at panic.rs:433:14
    frame #8: 0x000055555556c9f2 main`std::rt::lang_start_internal::hc51399759a90501a at rt.rs:34:21
    frame #9: 0x000055555555b830 main`std::rt::lang_start::<()>(main=(main`main::main at main.rs:1), argc=1, argv=0x00007fffffffcb18) at rt.rs:48:5
    frame #10: 0x000055555555b6fc main`main + 28
    frame #11: 0x00007ffff73f2493 libc.so.6`__libc_start_main + 243
    frame #12: 0x000055555555b59e main`_start + 46
(lldb)

[1]: https://github.com/rust-lang/rust/issues/60705

[teemperor]

This seems reasonable to me, but I'll leave this open for a while in case someone that knows more about Rust mangling shows up. Otherwise I'll accept this next week.

lldb/source/Core/Mangled.cpp

213

Please use LLDB_LOG in new code: LLDB_LOG(log, "demangled rustv0: {0} -> \"{1}\"", M, demangled_cstr); (same in the line below).

(The code above just didn't get updated yet)

lldb/unittests/Core/MangledTest.cpp

72

Could you do me a favour and change your test functions to LLDB's code style, so mangled_name as a variable name instead of MangledName and so on.

I'm aware the rest of the file is already using LLVM code style, but I think that's was just an oversight. I'll probably change the code style in this file to LLDB's and it would keep the git history a bit simpler.

[asm]

lldb/unittests/Core/MangledTest.cpp

72

Sure no problem. This is my first contribution to LLVM. I just mimicked the code around.

Just to check I understand. You're looking for something like this?

ConstString manged_lname("_RRR");
Mangled the_mangled(mangled_name);
ConstString the_demangled = the_mangled.GetDemangledName();

EXPECT_STREQ("", the_demangled.GetCString());

[teemperor]

lldb/unittests/Core/MangledTest.cpp

72

Jepp, that's the LLDB code style! Thank you!

[clayborg]

Once teemperor's issues are resolved this looks good to me!

[asm]

code style fixes, use modern log

[teemperor]

LGTM, thanks! (And congrats on your first patch)

I'll land this for you next week just to give the others another chance to take a look.

[asm]

And thank you for the quick review!

[asm]

@teemperor @clayborg Is this good to go? :)

[clayborg]

Yes, good to go!

lldb/include/lldb/Core/Mangled.h

enum NamePreference {
     ePreferMangled,
     ePreferDemangled,
     ePreferDemangledWithoutArguments
   };
 
   enum ManglingScheme {
     eManglingSchemeNone = 0,
     eManglingSchemeMSVC,
-    eManglingSchemeItanium
+    eManglingSchemeItanium,
+    eManglingSchemeRustV0
   };
 
   /// Default constructor.
   ///
   /// Initialize with both mangled and demangled names empty.
   Mangled() = default;
 
   /// Construct with name.

lldb/source/Core/Mangled.cpp

 
 Mangled::ManglingScheme Mangled::GetManglingScheme(llvm::StringRef const name) {
   if (name.empty())
     return Mangled::eManglingSchemeNone;
 
   if (name.startswith("?"))
     return Mangled::eManglingSchemeMSVC;
 
+  if (name.startswith("_R"))
+    return Mangled::eManglingSchemeRustV0;
+
   if (name.startswith("_Z"))
     return Mangled::eManglingSchemeItanium;
 
   // ___Z is a clang extension of block invocations
   if (name.startswith("___Z"))
     return Mangled::eManglingSchemeItanium;
 
   return Mangled::eManglingSchemeNone;
if (demangled_cstr)
       LLDB_LOGF(log, "demangled itanium: %s -> \"%s\"", M, demangled_cstr);
     else
       LLDB_LOGF(log, "demangled itanium: %s -> error: failed to demangle", M);
   }
 
   return demangled_cstr;
 }
 
+static char *GetRustV0DemangledStr(const char *M) {
+  char *demangled_cstr = llvm::rustDemangle(M, nullptr, nullptr, nullptr);
+
+  if (Log *log = lldb_private::GetLogIfAllCategoriesSet(LIBLLDB_LOG_DEMANGLE)) {
+    if (demangled_cstr && demangled_cstr[0])
+      LLDB_LOG(log, "demangled rustv0: {0} -> \"{1}\"", M, demangled_cstr);
+    else
+      LLDB_LOG(log, "demangled rustv0: {0} -> error: failed to demangle", M);
+  }
+
+  return demangled_cstr;
+}
+
 // Explicit demangling for scheduled requests during batch processing. This
 // makes use of ItaniumPartialDemangler's rich demangle info
 bool Mangled::DemangleWithRichManglingInfo(
     RichManglingContext &context, SkipMangledNameFn *skip_mangled_name) {
   // Others are not meant to arrive here. ObjC names or C's main() for example
   // have their names stored in m_demangled, while m_mangled is empty.
   assert(m_mangled);
 
if (m_demangled.IsEmpty()) {
       // Cannot demangle it, so don't try parsing.
       return false;
     } else {
       // Demangled successfully, we can try and parse it with
       // CPlusPlusLanguage::MethodName.
       return context.FromCxxMethodName(m_demangled);
     }
   }
+
+  case eManglingSchemeRustV0:
+    // Rich demangling scheme is not supported for Rust
+    return false;
   }
   llvm_unreachable("Fully covered switch above!");
 }
 
 // Generate the demangled name on demand using this accessor. Code in this
 // class will need to use this accessor if it wishes to decode the demangled
 // name. The result is cached and will be kept until a new string value is
 // supplied to this object, or until the end of the object's lifetime.
if (mangling_scheme != eManglingSchemeNone &&
       switch (mangling_scheme) {
       case eManglingSchemeMSVC:
         demangled_name = GetMSVCDemangledStr(mangled_name);
         break;
       case eManglingSchemeItanium: {
         demangled_name = GetItaniumDemangledStr(mangled_name);
         break;
       }
+      case eManglingSchemeRustV0:
+        demangled_name = GetRustV0DemangledStr(mangled_name);
+        break;
       case eManglingSchemeNone:
         llvm_unreachable("eManglingSchemeNone was handled already");
       }
       if (demangled_name) {
         m_demangled.SetStringWithMangledCounterpart(
             llvm::StringRef(demangled_name), m_mangled);
         free(demangled_name);
       }

lldb/source/Symbol/Symtab.cpp

case Mangled::eManglingSchemeItanium: {
     // Include this name in the index.
     return false;
   }
 
   // No filters for this scheme yet. Include all names in indexing.
   case Mangled::eManglingSchemeMSVC:
     return false;
 
+  // No filters for this scheme yet. Include all names in indexing.
+  case Mangled::eManglingSchemeRustV0:
+    return false;
+
   // Don't try and demangle things we can't categorize.
   case Mangled::eManglingSchemeNone:
     return true;
   }
   llvm_unreachable("unknown scheme!");
 }
 
 void Symtab::InitNameIndexes() {

lldb/unittests/Core/MangledTest.cpp

 TEST(MangledTest, EmptyForInvalidName) {
   ConstString MangledName("_ZN1a1b1cmxktpEEvm");
   Mangled TheMangled(MangledName);
   ConstString TheDemangled = TheMangled.GetDemangledName();
 
   EXPECT_STREQ("", TheDemangled.GetCString());
 }
 
+TEST(MangledTest, ResultForValidRustV0Name) {
+  ConstString mangled_name("_RNvC1a4main");
+  Mangled the_mangled(mangled_name);
+  ConstString the_demangled = the_mangled.GetDemangledName();
+
+  ConstString expected_result("a::main");
+  EXPECT_STREQ(expected_result.GetCString(), the_demangled.GetCString());
+}
+
+TEST(MangledTest, EmptyForInvalidRustV0Name) {
+  ConstString mangled_name("_RRR");
+  Mangled the_mangled(mangled_name);
+  ConstString the_demangled = the_mangled.GetDemangledName();
+
+  EXPECT_STREQ("", the_demangled.GetCString());
+}
+
 TEST(MangledTest, NameIndexes_FindFunctionSymbols) {
   SubsystemRAII<FileSystem, HostInfo, ObjectFileELF, SymbolFileSymtab>
       subsystems;
 
   auto ExpectedFile = TestFile::fromYaml(R"(
 --- !ELF
 FileHeader:      
   Class:           ELFCLASS64