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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - preserve_mostcc on X86 incorrectly restores RAX after storing return value"
href="https://bugs.llvm.org/show_bug.cgi?id=49445">49445</a>
</td>
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<th>Summary</th>
<td>preserve_mostcc on X86 incorrectly restores RAX after storing return value
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jannh@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, juergen@apple.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, pengfei.wang@intel.com, spatel+llvm@rotateright.com
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<p>
<div>
<pre>When a function with preserve_mostcc calling convention returns a value through
RAX, the epilogue incorrectly restores RAX after the return value has been
written into it.
Minimal testcase:
```
target triple = "x86_64-unknown-linux-gnu"
define preserve_mostcc i32 @foobar() {
ret i32 1
}
```
Resulting assembly from `llc -o=- test2.ll`:
```
.text
.file "test2.ll"
.globl foobar # -- Begin function foobar
.p2align 4, 0x90
.type foobar,@function
foobar: # @foobar
.cfi_startproc
# %bb.0:
pushq %rax
.cfi_def_cfa_offset 16
.cfi_offset %rax, -16
movl $1, %eax
popq %rax
.cfi_def_cfa_offset 8
retq
.Lfunc_end0:
.size foobar, .Lfunc_end0-foobar
.cfi_endproc
# -- End function
.section ".note.GNU-stack","",@progbits
```
I think this essentially means that `__attribute__((preserve_most))` is broken
for any function that returns a value on X86?
Simple test with C code:
```
$ cat demo.c
#include <stdio.h>
__attribute__((preserve_most))
__attribute__((noinline))
unsigned int get_number(void) {
return 0x41414141;
}
int main(void) {
printf("get_number() = 0x%x\n", get_number());
return 0;
}
$ /h/git/foreign/llvmp-build-debug/bin/clang-12 -o demo demo.c -Wall
$ ./demo
get_number() = 0x401140
$
```</pre>
</div>
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