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<base href="https://bugs.llvm.org/">
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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 - Failure to remove redundant check"
href="https://bugs.llvm.org/show_bug.cgi?id=34282">34282</a>
</td>
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<tr>
<th>Summary</th>
<td>Failure to remove redundant check
</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>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</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>aivchenk@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>For the following function:
int foo(char* C, int a, int b, int c, int d, int e, int f) {
int y1 = C[0] ? a : b;
int y2 = C[0] || C[1] ? c : d;
int y3 = C[0] || (C[1] && C[2]) ? e : f;
return C[0] ? y1 + y2 : C[1] ? y2 : y3;
}
<span class="quote">> </span >
.. we generate bad code:
# BB#0:
cmpb $0, (%rdi)
je .LBB0_1
# BB#5:
addl %esi, %ecx
movl %ecx, %eax
retq
.LBB0_1:
movl 8(%rsp), %eax
movb 1(%rdi), %dl
testb %dl, %dl
je .LBB0_3
# BB#2:
cmpb $0, 2(%rdi)
cmovnel %r9d, %eax
.LBB0_3:
testb %dl, %dl
cmovel %eax, %ecx
movl %ecx, %eax
retq
Just for comparison, here is GCC version:
cmpb $0, (%rdi)
leal (%rsi,%rcx), %eax
jne .L1
cmpb $0, 1(%rdi)
movl %ecx, %eax
cmove 8(%rsp), %eax
.L1:
rep ret
The main problem with LLVM code is that we do redundant "cmpb $0, 2(%rdi)"
check (while GCC removed that): C[2] does not influence the function return
value. As far as I can say, all other problems with our code, like doing testb
twice for cmov condition, are the result of not deleting the C[2] check</pre>
</div>
</p>
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