Make explicit -fno-semantic-interposition (in -fpic mode) infer dso_local

-fno-semantic-interposition is currently the CC1 default. (The opposite
disables some interprocedural optimizations.) However, it does not infer
dso_local: on most targets accesses to ExternalLinkage functions/variables
defined in the current module still need PLT/GOT.

This patch makes explicit -fno-semantic-interposition infer dso_local,
so that PLT/GOT can be eliminated if targets implement local aliases
for AsmPrinter::getSymbolPreferLocal (currently only x86).

Currently we check whether the module flag "SemanticInterposition" is 0.
If yes, infer dso_local. In the future, we can infer dso_local unless
"SemanticInterposition" is 1: frontends other than clang will also
benefit from the optimization if they don't bother setting the flag.
(There will be risks if they do want ELF interposition: they need to set
"SemanticInterposition" to 1.)

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index b4bc027..f98490c 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -3049,6 +3049,9 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   }
 
   Opts.SemanticInterposition = Args.hasArg(OPT_fsemantic_interposition);
+  // An explicit -fno-semantic-interposition infers dso_local.
+  Opts.ExplicitNoSemanticInterposition =
+      Args.hasArg(OPT_fno_semantic_interposition);
 
   // -mrtd option
   if (Arg *A = Args.getLastArg(OPT_mrtd)) {