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

Stephen Checkoway via llvm-dev
2020-01-08T02:04:13Z

> 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
>
>
>
>
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--
Stephen Checkoway