[all-commits] [llvm/llvm-project] 68a20c: [clang] Support -fpic -fno-semantic-interposition ...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 68a20c7f36d1d51cc46c0bd17384c16bc7818fa2
      https://github.com/llvm/llvm-project/commit/68a20c7f36d1d51cc46c0bd17384c16bc7818fa2
  Author: Fangrui Song <i at maskray.me>
  Date:   2021-05-10 (Mon, 10 May 2021)

  Changed paths:
    M clang/lib/Driver/ToolChains/Clang.cpp
    M clang/test/Driver/fsemantic-interposition.c

  Log Message:
  -----------
  [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