From 68a20c7f36d1d51cc46c0bd17384c16bc7818fa2 Mon Sep 17 00:00:00 2001 From: Fangrui Song Date: Mon, 10 May 2021 09:43:33 -0700 Subject: [PATCH] [clang] Support -fpic -fno-semantic-interposition for AArch64 -fno-semantic-interposition (only effective with -fpic) can optimize default visibility external linkage (non-ifunc-non-COMDAT) variable access and function calls to avoid GOT/PLT, by using local aliases, e.g. ``` int var; __attribute__((optnone)) int fun(int x) { return x * x; } int test() { return fun(var); } ``` -fpic (var and fun are dso_preemptable) ``` test: // @test adrp x8, :got:var ldr x8, [x8, :got_lo12:var] ldr w0, [x8] // fun is preemptible by default in ld -shared mode. ld will create a PLT. b fun ``` vs -fpic -fno-semantic-interposition (var and fun are dso_local) ``` test: // @test .Ltest$local: adrp x8, .Lvar$local ldr w0, [x8, :lo12:.Lvar$local] // The assembler either resolves .Lfun$local at assembly time, or produces a // relocation referencing a non-preemptible section symbol (which can avoid PLT). b .Lfun$local ``` Note: Clang's default -fpic is more aggressive than GCC -fpic: interprocedural optimizations (including inlining) are available but local aliases are not used. -fpic -fsemantic-interposition can disable interprocedural optimizations. Depends on D101872 Reviewed By: peter.smith Differential Revision: https://reviews.llvm.org/D101873 --- clang/lib/Driver/ToolChains/Clang.cpp | 2 +- clang/test/Driver/fsemantic-interposition.c | 3 ++- 2 files changed, 3 insertions(+), 2 deletions(-) diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp index 97a92e6..c4ea67e 100644 --- a/clang/lib/Driver/ToolChains/Clang.cpp +++ b/clang/lib/Driver/ToolChains/Clang.cpp @@ -4722,7 +4722,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA, options::OPT_fno_semantic_interposition); if (RelocationModel != llvm::Reloc::Static && !IsPIE) { // The supported targets need to call AsmPrinter::getSymbolPreferLocal. - bool SupportsLocalAlias = Triple.isX86(); + bool SupportsLocalAlias = Triple.isAArch64() || Triple.isX86(); if (!A) CmdArgs.push_back("-fhalf-no-semantic-interposition"); else if (A->getOption().matches(options::OPT_fsemantic_interposition)) diff --git a/clang/test/Driver/fsemantic-interposition.c b/clang/test/Driver/fsemantic-interposition.c index af76daf..3060e8a4 100644 --- a/clang/test/Driver/fsemantic-interposition.c +++ b/clang/test/Driver/fsemantic-interposition.c @@ -10,6 +10,7 @@ /// If -fno-semantic-interposition is specified and the target supports local /// aliases, neither CC1 option is set. +// RUN: %clang -target aarch64 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s // RUN: %clang -target i386 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s // RUN: %clang -target x86_64 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s // NO-NOT: "-fsemantic-interposition" @@ -20,8 +21,8 @@ /// local aliases, use the traditional half-baked behavor: interprocedural /// optimizations are allowed but local aliases are not used. If references are /// not optimized out, semantic interposition at runtime is possible. -// RUN: %clang -target aarch64 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=HALF %s // RUN: %clang -target ppc64le %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=HALF %s +// RUN: %clang -target riscv64 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=HALF %s // RUN: %clang -target x86_64 %s -Werror -fPIC -c -### 2>&1 | FileCheck --check-prefix=HALF %s // -- 2.7.4