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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - instcombine miscompile after r237539"
href="https://llvm.org/bugs/show_bug.cgi?id=23583">23583</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>instcombine miscompile after r237539
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hans@chromium.org
</td>
</tr>
<tr>
<th>CC</th>
<td>james.molloy@arm.com, llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider this reduced test case from a 64-bit Chromium build on Linux:
int f() __attribute__((noinline));
int f() { volatile int x = -1; return x; }
void ParseUniformName(unsigned long *outSubscript) {
int index = f();
if (index >= 0) {
*outSubscript = index;
} else {
*outSubscript = 0xffffffffu;
}
}
#include <stdio.h>
int main() {
unsigned long index = 0xdeadbeef;
ParseUniformName(&index);
printf("index = 0x%zx\n", index);
return 0;
}
The expected output is 0xffffffff, but compiling with ToT Clang at -O3 yields
0xffffffffffffffff instead.
This seems to have regressed at r237539.
Asm before:
_Z16ParseUniformNamePm: # @_Z16ParseUniformNamePm
.cfi_startproc
# BB#0: # %entry
pushq %rbx
.Ltmp0:
.cfi_def_cfa_offset 16
.Ltmp1:
.cfi_offset %rbx, -16
movq %rdi, %rbx
callq _Z1fv
testl %eax, %eax
cltq
movl $4294967295, %ecx # imm = 0xFFFFFFFF
cmovnsq %rax, %rcx
movq %rcx, (%rbx)
popq %rbx
retq
Asm after:
_Z16ParseUniformNamePm: # @_Z16ParseUniformNamePm
.cfi_startproc
# BB#0: # %entry
pushq %rbx
.Ltmp0:
.cfi_def_cfa_offset 16
.Ltmp1:
.cfi_offset %rbx, -16
movq %rdi, %rbx
callq _Z1fv
cmpl $-2, %eax
movl $-1, %ecx
cmovgl %eax, %ecx
movslq %ecx, %rax
movq %rax, (%rbx)
popq %rbx
retq</pre>
</div>
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