2075f2b296
-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
31 lines
2.0 KiB
C
31 lines
2.0 KiB
C
// RUN: %clang -target x86_64 %s -Werror -fpic -fsemantic-interposition -c -### 2>&1 | FileCheck %s
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// RUN: %clang -target x86_64 %s -Werror -fPIC -fsemantic-interposition -c -### 2>&1 | FileCheck %s
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// CHECK: "-fsemantic-interposition"
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/// No-op for -fno-pic/-fpie.
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// RUN: %clang -target x86_64 %s -Werror -fsemantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NOOP %s
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// RUN: %clang -target x86_64 %s -Werror -fPIE -fsemantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NOOP %s
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// NOOP-NOT: "-fsemantic-interposition"
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// NOOP-NOT: "-fno-semantic-interposition"
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/// If -fno-semantic-interposition is specified and the target supports local
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/// aliases, neither CC1 option is set.
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// RUN: %clang -target aarch64 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s
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// RUN: %clang -target riscv32 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s
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// RUN: %clang -target riscv64 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s
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// RUN: %clang -target i386 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s
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// RUN: %clang -target x86_64 %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=NO %s
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// NO-NOT: "-fsemantic-interposition"
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// NO-NOT: "-fhalf-no-semantic-interposition"
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/// If neither -fsemantic-interposition or -fno-semantic-interposition is specified,
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/// or -fno-semantic-interposition is specified but the target does not support
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/// local aliases, use the traditional half-baked behavor: interprocedural
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/// optimizations are allowed but local aliases are not used. If references are
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/// not optimized out, semantic interposition at runtime is possible.
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// RUN: %clang -target ppc64le %s -Werror -fPIC -fno-semantic-interposition -c -### 2>&1 | FileCheck --check-prefix=HALF %s
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// RUN: %clang -target x86_64 %s -Werror -fPIC -c -### 2>&1 | FileCheck --check-prefix=HALF %s
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//
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// HALF: "-fhalf-no-semantic-interposition"
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