[libcxxabi][demangle] create helper for std::string_view::starts_with

[nickdesaulniers]

Summary

Add C++ 20 style starts_with to replace StringView::startsWith in
LLVMDemangle. Due to library layering (LLVMSupport depends on
LLVMDemangle), we add the utility header under llvm/Demangle, instead of
llvm/ADT or llvm/Support.

Modify this in libcxxabi, then copy this over to llvm.

[nickdesaulniers]

(note to reviewers: it's on my todo list to backport my recent changes to llvm's demangle to libcxxabi's demangle; didn't realize there were two in tree until after making changes to the downstream one. Luckily, they are simply 1:1 copies)

[nickdesaulniers]

libcxxabi/src/demangle/StringViewExtras.h

21

"DemangleConfig.h" is needed for this defition

[nickdesaulniers]

• test upload without D148555

[nickdesaulniers]

• add DemangleConfig.h

[nickdesaulniers]

I wonder if this new file should get an entry in clang/docs/tools/clang-formatted-files.txt?

[nickdesaulniers]

• add new file to clang/docs/tools/clang-formatted-files.txt

[MaskRay]

This was added to ADT in https://reviews.llvm.org/D148367 then reverted in https://reviews.llvm.org/D148547 because it was a layering violation pointed out in https://reviews.llvm.org/D148384#4270356.

This description requires readers to read the links. How about make the description self-contained with a brief message?

Add C++ 20 style starts_with to replace StringView::startsWith in LLVMDemangle. Due to library layering (LLVMSupport depends on LLVMDemangle), we add the utility header under llvm/Demangle, instead of llvm/ADT or llvm/Support.

[MaskRay]

A member from #libc_abi needs to review this as well to make this differential green :)

[nickdesaulniers]

• use @MaskRay's recommended commit description, thanks @MaskRay!

[phosek]

LGTM

clang/docs/tools/clang-formatted-files.txt

 llvm/include/llvm/DebugInfo/Symbolize/DIFetcher.h
 llvm/include/llvm/DebugInfo/Symbolize/DIPrinter.h
 llvm/include/llvm/DebugInfo/Symbolize/Symbolize.h
 llvm/include/llvm/Debuginfod/Debuginfod.h
 llvm/include/llvm/Debuginfod/DIFetcher.h
 llvm/include/llvm/Debuginfod/HTTPClient.h
 llvm/include/llvm/Demangle/Demangle.h
 llvm/include/llvm/Demangle/StringView.h
+llvm/include/llvm/Demangle/StringViewExtras.h
 llvm/include/llvm/Demangle/Utility.h
 llvm/include/llvm/DWARFLinker/DWARFLinker.h
 llvm/include/llvm/DWARFLinker/DWARFLinkerCompileUnit.h
 llvm/include/llvm/DWARFLinker/DWARFLinkerDeclContext.h
 llvm/include/llvm/DWARFLinker/DWARFStreamer.h
 llvm/include/llvm/DWP/DWP.h
 llvm/include/llvm/DWP/DWPError.h
 llvm/include/llvm/DWP/DWPStringPool.h

libcxxabi/src/demangle/StringViewExtras.h

+//===--- StringViewExtras.h -------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// There are two copies of this file in the source tree.  The one under
+// libcxxabi is the original and the one under llvm is the copy.  Use
+// cp-to-llvm.sh to update the copy.  See README.txt for more details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef DEMANGLE_STRINGVIEW_H
+#define DEMANGLE_STRINGVIEW_H
+
+#include "DemangleConfig.h"
+
+#include <string_view>
+
+DEMANGLE_NAMESPACE_BEGIN
+
+inline bool starts_with(std::string_view self, char C) {
+  return !self.empty() && self.front() == C;
+}
+
+inline bool starts_with(std::string_view haystack, std::string_view needle) {
+  return haystack.substr(0, needle.size()) == needle;
+}
+
+DEMANGLE_NAMESPACE_END
+
+#endif

libcxxabi/src/demangle/cp-to-llvm.sh

 #!/bin/bash
 
 # Copies the 'demangle' library, excluding 'DemangleConfig.h', to llvm. If no
 # llvm directory is specified, then assume a monorepo layout.
 
 set -e
 
 cd $(dirname $0)
-HDRS="ItaniumDemangle.h ItaniumNodes.def StringView.h Utility.h"
+HDRS="ItaniumDemangle.h ItaniumNodes.def StringView.h StringViewExtras.h Utility.h"
 LLVM_DEMANGLE_DIR=$1
 
 if [[ -z "$LLVM_DEMANGLE_DIR" ]]; then
     LLVM_DEMANGLE_DIR="../../../llvm/include/llvm/Demangle"
 fi
 
 if [[ ! -d "$LLVM_DEMANGLE_DIR" ]]; then
     echo "No such directory: $LLVM_DEMANGLE_DIR" >&2

llvm/include/llvm/Demangle/StringViewExtras.h

+//===--- StringViewExtras.h ----------*- mode:c++;eval:(read-only-mode) -*-===//
+//       Do not edit! See README.txt.
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// There are two copies of this file in the source tree.  The one under
+// libcxxabi is the original and the one under llvm is the copy.  Use
+// cp-to-llvm.sh to update the copy.  See README.txt for more details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef DEMANGLE_STRINGVIEW_H
+#define DEMANGLE_STRINGVIEW_H
+
+#include "DemangleConfig.h"
+
+#include <string_view>
+
+DEMANGLE_NAMESPACE_BEGIN
+
+inline bool starts_with(std::string_view self, char C) {
+  return !self.empty() && self.front() == C;
+}
+
+inline bool starts_with(std::string_view haystack, std::string_view needle) {
+  return haystack.substr(0, needle.size()) == needle;
+}
+
+DEMANGLE_NAMESPACE_END
+
+#endif