movl x@GOTPCREL+4(%rip), %eax

[H.J. Lu]

On Tue, Nov 24, 2020 at 9:44 AM Fangrui Song <i@maskray.me> wrote:
>
> Clang emits movl x@GOTPCREL+4(%rip), %eax for the following code:
>
>    void x();
>    int foo() { return (long)x >> 32; }
>
> 3 questions:
>
> * Is movl x@GOTPCREL+4(%rip) valid?

Yes.

> * If no, should GNU as reject the expression?
> * If yes, should GNU as emit an R_X86_64_GOTPCRELX instead of R_X86_64_GOTPCREL? (If yes, ideally x86-64 psABI should document that this case cannot be optimized)

R_X86_64_GOTPCRELX can be generated.

> * If yes, gold should be fixed to not relax R_X86_64_GOTPCRELX if the
>    addend is not -4
>
>
> Let's compare the -no-pie linked output of `movl foo@GOTPCREL(%rip), %eax; movl foo@GOTPCREL+4(%rip), %eax`:
>
> GNU ld (the second instruction loads the high 32-bit address):
>
> 0000000000401000 <foo-0xd>:
>    401000:       48 c7 c0 0d 10 40 00    mov    rax,0x40100d
>    401007:       8b 05 ef 1f 00 00       mov    eax,DWORD PTR [rip+0x1fef]        # 402ffc <.got+0x4>
>
> gold (the second instruction loads the address of foo plus 4):
>
> 00000000004000e8 <foo-0xd>:
>    4000e8:       48 8d 05 06 00 00 00    lea    rax,[rip+0x6]        # 4000f5 <foo>
>    4000ef:       8d 05 04 00 00 00       lea    eax,[rip+0x4]        # 4000f9 <foo+0x4>

Please file a gold bug.

Thanks.

-- 
H.J.