[llvm-dev] Inline assembly in intel syntax mishandling i constraint

[Eric Astor via llvm-dev]

This is in fact fixed on trunk, but after LLVM 9.0 (see
https://godbolt.org/z/pKHlqr); I believe the fix was
https://reviews.llvm.org/D71436.

On Tue, Jan 7, 2020 at 9:04 PM Stephen Checkoway via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> > On Jan 7, 2020, at 18:41, Craig Topper <craig.topper at gmail.com> wrote:
> >
> > What version of llvm are you using? This looks like it may be fixed on
> trunk.
>
> After poking at my installation of rust, I'm not entirely sure what
> version of LLVM it uses. Looking at the GitHub page, it looks like Rust
> maintains their own copy of llvm and cherry picks commits. The C example
> was compiled with 6.0.
>
> If it's fixed in LLVM, then I'll file a bug with Rust.
>
> Thanks,
>
> Steve
>
> >
> > ~Craig
> >
> >
> > On Tue, Jan 7, 2020 at 2:44 PM Stephen Checkoway via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > Hi all,
> >
> > I'm getting rather odd behavior from a call asm inteldialect(). TL;DR is
> "mov reg, $0" with a "i" constraint on $0 is behaving identical to "mov
> reg, dword ptr [$0]" and differently from "movl $0, reg" in AT&T syntax.
> >
> >
> > I'm not sure how to get clang to emit an inteldialect, so for this
> example, I'm emitting llvm and then modifying the resultant .ll file. (I
> get similar behavior with rust's asm!(… : "intel") so I'm assuming that's
> what rust is using, although I didn't verify this).
> >
> > Here's the example
> >
> > static int foo;
> > static int bar;
> >
> > void _start(void) {
> >   asm volatile("movl %0, %%eax" : : "i"(&foo));
> >   asm volatile("movl %0, %%ebx" : : "i"(&bar));
> > }
> >
> > This produces
> > define void @_start() #0 {
> >   call void asm sideeffect "movl $0, %eax",
> "i,~{dirflag},~{fpsr},~{flags}"(i32* @foo) #1, !srcloc !3
> >   call void asm sideeffect "movl $0, %ebx",
> "i,~{dirflag},~{fpsr},~{flags}"(i32* @bar) #1, !srcloc !4
> >   ret void
> > }
> >
> > When assembled, I get the expected output
> >  80480a3:       b8 b0 90 04 08          mov    eax,0x80490b0
> >  80480a8:       bb b4 90 04 08          mov    ebx,0x80490b4
> >
> >
> > After modifying the second one to be
> >   call void asm sideeffect inteldialect "mov ebx, $0",
> "i,~{dirflag},~{fpsr},~{flags}"(i32* @bar) #1, !srcloc !4
> > and assembling, I get the unexpected output
> >  80480a3:       b8 b0 90 04 08          mov    eax,0x80490b0
> >  80480a8:       8b 1d b4 90 04 08       mov    ebx,DWORD PTR ds:0x80490b4
> >
> > This is identical to the output I get if I change the assembly template
> to "mov ebx, dword ptr [$0]"
> >
> > I think the underlying issue here is that whichever variant of Intel
> syntax this supports (MASM?) treats
> > mov reg, symbol
> > as a load and it wants
> > mov reg, offset symbol
> >
> > E.g., if I ask Clang to output assembly in Intel syntax via -mllvm
> --x86-asm-syntax=intel, I get
> >         #APP
> >         mov     eax, offset foo
> >         #NO_APP
> >         #APP
> >
> >         mov     ebx, dword ptr [bar]
> >
> >         #NO_APP
> >
> > (I have no idea where those extra newlines are coming from.)
> >
> > If I try to change the assembly template to "mov ebx, offset $0" it
> complains about multiple symbols being present:
> > <inline asm>:2:18: error: cannot use more than one symbol in memory
> operand
> >         mov ebx, offset bar
> >
> > I attached my source file and my modified .ll file. I compiled the
> source file with
> >
> >     clang -m32 a.c -ffreestanding -nostdlib -S -emit-llvm
> >
> > $ clang --version
> > clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
> > Target: x86_64-pc-linux-gnu
> > Thread model: posix
> > InstalledDir: /usr/bin
> >
> > Is this an LLVM bug or am I misusing inteldialect?
> >
> > Thank you,
> >
> > Steve
> >
> > --
> > Stephen Checkoway
> >
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> --
> Stephen Checkoway
>
>
>
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