bwendling wrote:
> This generally looks reasonable to me, with two main concerns:
>
> * Don't we need to handle callbr as well?
Probably, but I want to do that in a follow-up commit because it's likely to be
more complex.
> * It seems like the semantics of `"rm"` input constraints get changed by the
> prepare pass without any indication in the IR.
Could you give an example?
> Another issue I found is that this code:
>
> ```
> unsigned test(unsigned v) {
> unsigned ret;
> __asm__("movl %1, %0" : "=rm"(ret) : "rm"(v));
> return ret;
> }
> ```
>
> Omits the inline asm completely at `-O0` now. I haven't looked deeply, but I
> suspect the reason is that the inline asm was originally marked
> `memory(read)`, which is no longer true after the transform.
This is something that Clang isn't able to handle currently even at `-O2`:
```
$ clang -o - -O0 -S ~/llvm/a.c -m64
.text
.file "a.c"
.globl test # -- Begin function test
.p2align 4, 0x90
.type test,@function
test: # @test
.cfi_startproc
# %bb.0:
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
movl %edi, -4(%rbp)
movl -4(%rbp), %eax
movl %eax, -12(%rbp)
#APP
/home/morbo/llvm/a.c:3:11: error: invalid operand for instruction
3 | __asm__("movl %1, %0" : "=rm"(ret) : "rm"(v));
| ^
<inline asm>:1:18: note: instantiated into assembly here
1 | movl -12(%rbp), -8(%rbp)
| ^~~~~~~~
#NO_APP
movl -8(%rbp), %eax
popq %rbp
.cfi_def_cfa %rsp, 8
retq
.Lfunc_end0:
.size test, .Lfunc_end0-test
.cfi_endproc
# -- End function
.ident "Debian clang version 19.1.7 (10.1+build1)"
.section ".note.GNU-stack","",@progbits
.addrsig
1 error generated.
$ clang -o - -O2 -S ~/llvm/a.c
.text
.file "a.c"
.globl test # -- Begin function test
.p2align 4, 0x90
.type test,@function
test: # @test
.cfi_startproc
# %bb.0:
movl %edi, -8(%rsp)
#APP
/home/morbo/llvm/a.c:3:11: error: invalid operand for instruction
3 | __asm__("movl %1, %0" : "=rm"(ret) : "rm"(v));
| ^
<inline asm>:1:17: note: instantiated into assembly here
1 | movl -8(%rsp), -4(%rsp)
| ^~~~~~~~
#NO_APP
movl -4(%rsp), %eax
retq
.Lfunc_end0:
.size test, .Lfunc_end0-test
.cfi_endproc
# -- End function
.ident "Debian clang version 19.1.7 (10.1+build1)"
.section ".note.GNU-stack","",@progbits
.addrsig
1 error generated.
```
This is because the `mov` can't move between memory addresses. I'm not sure why
my change omits the assembly entirely though. I'll look into it.
https://github.com/llvm/llvm-project/pull/181973
_______________________________________________
cfe-commits mailing list
[email protected]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
