> (The -fno-semantic-interposition thing is probably the biggest performance gap > between gcc -fpic and clang -fpic.) Yep, it is often confusing to users (who do not understand what ELF interposition is) that clang and gcc disagree on default flags here. Recently -Ofast was extended to imply -fno-semantic-interposition that will hopefully make more people notice this.
While doing that I have added per-symbol flag about interposition to the symbol table, so we can also support __atttribute__ ((semantic_interposition)) and __attribute__((no_semantic_interpoition)) if that would be useful for something.