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

H.J. Lu hjl.tools@gmail.com
Tue Nov 24 22:38:36 GMT 2020 
On Tue, Nov 24, 2020 at 10:46 AM Fangrui Song <i@maskray.me> wrote:
>
>
> On 2020-11-24, H.J. Lu wrote:
> >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.
>
> Thanks. Would be nice clarifying this in the x86-64 psABI.

https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/16

> >
> >> * 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.
>
> Filed https://sourceware.org/bugzilla/show_bug.cgi?id=26939
>

https://sourceware.org/pipermail/binutils/2020-November/114272.html
-- 
H.J.
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