Inline assembly in intel syntax mishandling i constraint

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1

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

a.c (151 Bytes)

bug.ll (1.27 KB)

2

What version of llvm are you using? This looks like it may be fixed on trunk.

3

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

4

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.

5

Great, thanks! I had no idea I could use Compiler Explorer with LLVM IR directly.

Steve