-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
-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$local
```
Note: Clang's default -fpic is more aggressive than GCC -fpic: interprocedural
optimizations (including inlining) are available but local aliases are not used.
-fpic -fsemantic-interposition can disable interprocedural optimizations.
Depends on D101872
Reviewed By: peter.smith
Differential Revision: https://reviews.llvm.org/D101873
The idea is that the CC1 default for ELF should set dso_local on default
visibility external linkage definitions in the default -mrelocation-model pic
mode (-fpic/-fPIC) to match COFF/Mach-O and make output IR similar.
The refactoring is made available by 2820a2ca3a.
Currently only x86 supports local aliases. We move the decision to the driver.
There are three CC1 states:
* -fsemantic-interposition: make some linkages interposable and make default visibility external linkage definitions dso_preemptable.
* (default): selected if the target supports .Lfoo$local: make default visibility external linkage definitions dso_local
* -fhalf-no-semantic-interposition: if neither option is set or the target does not support .Lfoo$local: like -fno-semantic-interposition but local aliases are not used. So references can be interposed if not optimized out.
Add -fhalf-no-semantic-interposition to a few tests using the half-based semantic interposition behavior.
-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.)
Fangrui Song