[libcxx-commits] [PATCH] D151260: [Demangle] avoid more std::string_view::substr

[Nick Desaulniers via Phabricator via libcxx-commits]

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In D148959 <https://reviews.llvm.org/D148959>, I removed usage of std::string_view::substr because it may
throw, and libcxxabi cannot use such code.  I missed one instance in
llvm::starts_with.  That is blocking copying the code back upstream in
D148566 <https://reviews.llvm.org/D148566>.

Mark these helpers noexcept (as they are in C++20) as well, to remind
future travelers.

Make these changes upstream, and copy them back downstream using
libcxxabi/src/demangle/cp-to-llvm.sh.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D151260

Files:
  libcxxabi/src/demangle/StringViewExtras.h
  llvm/include/llvm/Demangle/StringViewExtras.h


Index: llvm/include/llvm/Demangle/StringViewExtras.h
===================================================================
--- llvm/include/llvm/Demangle/StringViewExtras.h
+++ llvm/include/llvm/Demangle/StringViewExtras.h
@@ -21,12 +21,18 @@
 
 DEMANGLE_NAMESPACE_BEGIN
 
-inline bool starts_with(std::string_view self, char C) {
+inline bool starts_with(std::string_view self, char C) noexcept {
   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;
+inline bool starts_with(std::string_view haystack,
+                        std::string_view needle) noexcept {
+  if (needle.empty())
+    return true;
+  if (needle.length() > haystack.length())
+    return false;
+  haystack.remove_suffix(haystack.length() - needle.length());
+  return haystack == needle;
 }
 
 DEMANGLE_NAMESPACE_END
Index: libcxxabi/src/demangle/StringViewExtras.h
===================================================================
--- libcxxabi/src/demangle/StringViewExtras.h
+++ libcxxabi/src/demangle/StringViewExtras.h
@@ -21,12 +21,18 @@
 
 DEMANGLE_NAMESPACE_BEGIN
 
-inline bool starts_with(std::string_view self, char C) {
+inline bool starts_with(std::string_view self, char C) noexcept {
   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;
+inline bool starts_with(std::string_view haystack,
+                        std::string_view needle) noexcept {
+  if (needle.empty())
+    return true;
+  if (needle.length() > haystack.length())
+    return false;
+  haystack.remove_suffix(haystack.length() - needle.length());
+  return haystack == needle;
 }
 
 DEMANGLE_NAMESPACE_END


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