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

[Fangrui Song]

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.

>
>> * 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

>Thanks.
>
>-- 
>H.J.