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List:       binutils
Subject:    Re: movl x@GOTPCREL+4(%rip), %eax
From:       "H.J. Lu via Binutils" <binutils () sourceware ! org>
Date:       2020-11-26 3:35:18
Message-ID: CAMe9rOpsA6w0_pqwBmBAbXsPjkEcmJpc7bOC=b1EaXmNs7ZZkQ () mail ! gmail ! com
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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.


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