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[clang] Support -fpic -fno-semantic-interposition for RISCV 

-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:
.LBB1_1:
        auipc   a0, %got_pcrel_hi(var)
        ld      a0, %pcrel_lo(.LBB1_1)(a0)
        lw      a0, 0(a0)
// fun is preemptible by default in ld -shared mode. ld will create a PLT.
        tail    fun@plt
```

vs -fpic -fno-semantic-interposition (var and fun are dso_local)
```
test:
.Ltest$local:
.LBB1_1:
        auipc   a0, %pcrel_hi(.Lvar$local)
        addi    a0, a0, %pcrel_lo(.LBB1_1)
        lw      a0, 0(a0)
// The assembler either resolves .Lfun$local at assembly time (-mno-relax
// -fno-function-sections), or produces a relocation referencing a non-preemptible
// local symbol (which can avoid PLT).
        tail    .Lfun$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 D101875

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D101876
This commit is contained in:
Fangrui Song 2021-05-11 11:38:32 -07:00
parent b2f227c6c8
commit 2075f2b296
2 changed files with 4 additions and 2 deletions
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3
clang/lib/Driver/ToolChains/Clang.cpp
View File

@ -4716,7 +4716,8 @@ 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.isAArch64() || Triple.isX86();
bool SupportsLocalAlias =
Triple.isAArch64() || Triple.isRISCV() || Triple.isX86();
if (!A)
CmdArgs.push_back("-fhalf-no-semantic-interposition");
else if (A->getOption().matches(options::OPT_fsemantic_interposition))

3
clang/test/Driver/fsemantic-interposition.c
View File

@ -11,6 +11,8 @@
/// 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 riscv32 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s
// RUN: %clang -target riscv64 %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"
@ -22,7 +24,6 @@
/// optimizations are allowed but local aliases are not used. If references are
/// not optimized out, semantic interposition at runtime is possible.
// 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
//