[llvm-dev] Bug or incorrect use of inline asm? (original) (raw)

Johan Engelen via llvm-dev llvm-dev at lists.llvm.org
Sat Aug 5 05:27:45 PDT 2017

On Fri, Aug 4, 2017 at 11:55 AM, Johan Engelen <jbc.engelen at gmail.com> wrote:

On Thu, Aug 3, 2017 at 6:19 PM, Tim Northover <t.p.northover at gmail.com> wrote:

2017-08-03 8:58 GMT-07:00 Johan Engelen via llvm-dev <_ _llvm-dev at lists.llvm.org>: > The error is gone after removing (or reducing) the alignment of %a. This > makes me believe that our inline asm syntax is correct to add an offset to a > pointer: " 4+$0 ".

The AT&T syntax for a displaced address doesn't have the '+'; it's just "4(%rsp)" so you should change the IR to "4$0". LLVM appears to be optimizing the addressing mode, and the optimizer is being more lenient on this detail so you're getting lucky when the +4 can be combined with an existing %rsp offset. Things break down when part of the offset is a linktime constant: ... So depending on the pointer argument, I need to write a + or without +, complicating automatic codegen.

The problem is worse. The following code is accepted depending on PIC or non-PIC compilation:

source_filename = "asanasm.d"
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-linux-gnu"

@_D7asanasm9constantsG2k = global [2 x i32] [i32 66051, i32 66051]

define void @_D7asanasm8offconstFZv() {
  call void asm sideeffect "movdqa 4$0, %xmm7", "*m,~{xmm7}"([2 x i32]*
@_D7asanasm9constantsG2k)
  ret void
}

I get no errors with PIC:

llc test.ll -o test.s -O0 -relocation-model=pic The asm output: movq _D7asanasm9constantsG2k at GOTPCREL(%rip), %rax #InlineASM movdqa 4(%rax), %xmm7

But without PIC:

llc test.ll -o test.s -O0 -relocation-model=static :1:10: error: unexpected token in argument list movdqa 4_D7asanasm9constantsG2k(%rip), %xmm7 ^

So it appears that applying an offset to a pointer variable will break using either "4+$0" or "4$0", and you have to be lucky to guess correctly what's going to happen with the pointer argument passed into the inline asm call. (my problem would be solved when "4+$0" would always be accepted)

I now feel this is a bug in LLVM, do you agree? Unfortunately, I do not know how to work around the current problem.

Thanks, Johan -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170805/fcadb323/attachment.html>

More information about the llvm-dev mailing list