[cfe-dev] Clang x86-64 can't move segment registers

[Craig Topper via cfe-dev]

I think if you press the binary button on the godbolt output it will fail
on gcc. I don’t think it feeds it to the assembler without that.

On Tue, Jun 5, 2018 at 10:28 AM Tobias Hahn <tobias.hahn at ableton.com> wrote:

> Thanks, that fixes it. I'll file it against PLCrashReporter. FWIW, gcc
> trunk still seems to accept it: https://godbolt.org/g/un7WJM
>
> Thanks again!
> Tobias
>
> > Am 05.06.2018 um 17:59 schrieb Craig Topper <craig.topper at gmail.com>:
> >
> > It should be movw. The segment registers are only 16-bits wide. We used
> to accept it because the Intel documentation sort of makes it look valid
> because you can use a 64-bit GPR as the destination, but it doesn't make
> any sense to use 64-bits when writing to memory. GNU assembler 2.30 feels
> the same way so it probably doesn't work in gcc either.
> >
> > ~Craig
> >
> >
> > On Tue, Jun 5, 2018 at 8:39 AM Tobias Hahn via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
> > Since clang 6, the following code produces `error: invalid operand for
> instruction`:
> >
> > ```c++
> > void f()
> > {
> >   asm("movq %cs, 160(%rsp);");
> >   asm("movq %fs, 168(%rsp);");
> >   asm("movq %gs, 176(%rsp);");
> > }
> > ```
> > (see https://godbolt.org/g/CgPBb9)
> >
> > Two questions: 1. Is this a bug or a feature? 2. Is this a clang
> bug/feature or an llvm one (a.k.a what is the right place to report this)?
> >
> > (Background: I ran into this while trying to roll clang to a new version
> in a project containing PLCrashReporter, which uses these instructions
> while capturing a thread context, see [1]. I'm now trying to figure out if
> this is a compiler bug or if PLCrashReporter needs to be patched.)
> >
> > Thanks!
> > Tobias
> >
> > [1]
> https://github.com/plausiblelabs/plcrashreporter/blob/master/Source/PLCrashAsyncThread_current.S#L96
> >
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> >
> >
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>
> Ableton AG, Schoenhauser Allee 6-7, 10119 Berlin, Germany
> Sitz (Registered Office) Berlin, Amtsgericht Berlin-Charlottenburg, HRB
> 72838
> Vorstand (Management Board): Gerhard Behles, Jan Bohl
> Vorsitzender des Aufsichtsrats (Chair of the Supervisory Board): Uwe Struck
>
>
> --
~Craig
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