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.)
diff --git a/clang/test/CodeGen/semantic-interposition.c b/clang/test/CodeGen/semantic-interposition.c
index 43656e3..3d6c5f2 100644
--- a/clang/test/CodeGen/semantic-interposition.c
+++ b/clang/test/CodeGen/semantic-interposition.c
@@ -1,5 +1,9 @@
// RUN: %clang_cc1 -emit-llvm -fsemantic-interposition %s -o - | FileCheck --check-prefix=INTERPOSITION %s
// RUN: %clang_cc1 -emit-llvm %s -o - | FileCheck --check-prefix=NO %s
+/// With explicit -fno-semantic-interposition, add a module flag to inform the
+/// backend that dso_local can be inferred.
+// RUN: %clang_cc1 -emit-llvm -fno-semantic-interposition %s -o - | FileCheck --check-prefix=EXPLICIT_NO %s
// INTERPOSITION: !{{[0-9]+}} = !{i32 1, !"SemanticInterposition", i32 1}
// NO-NOT: "SemanticInterposition"
+// EXPLICIT_NO: !{{[0-9]+}} = !{i32 1, !"SemanticInterposition", i32 0}