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

[H.J. Lu]

On Wed, Nov 25, 2020 at 5:55 PM Fangrui Song <i@maskray.me> wrote:
>
> On 2020-11-24, H.J. Lu wrote:
> >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
>
> While I think rejecting addend!=-4 on the linker side is fine, I've been
> thinking whether the assembler should produce R_X86_64_GOTPCREL for
> x@GOTPCREL+4.
>
> If the whole purpose of GOTPCRELX is for optimization and
> x@GOTPCREL+4 definitely cannot be optimized, should the assembler not
> produce GOTPCRELX?

Since linker must check addend!=-4, GOTPCRELX is OK.

-- 
H.J.