<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incorrect order of evaluation for || somehow involving ! of _Bool"
href="https://bugs.llvm.org/show_bug.cgi?id=35363">35363</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Incorrect order of evaluation for || somehow involving ! of _Bool
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>5.0
</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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jseward@acm.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Incorrect order of evaluation for || somehow involving ! of _Bool
For the following input (in C)
void ff(void);
void gg(void);
_Bool hh(int*);
void test()
{
int a;
_Bool q = hh(&a);
if (!q || (a == 42))
ff();
else
gg();
}
"clang version 5.0.0 (tags/RELEASE_500/final) (llvm/tags/RELEASE_500/final
312553)" on x86_64-Linux, at -O and above, produces code that firsts tests
|a == 42| and only then |!q|:
test: # @test
.cfi_startproc
# BB#0:
pushq %rax
.Lcfi0:
.cfi_def_cfa_offset 16
leaq 4(%rsp), %rdi
callq hh
cmpl $42, 4(%rsp)
je .LBB0_2
# BB#1:
testb %al, %al
je .LBB0_2
# BB#3:
callq gg
popq %rax
retq
.LBB0_2:
callq ff
popq %rax
retq
I don't see how it can justify comparing |a==42| before looking at the
return value from hh.
I notice also that it produces the "expected" order of tests (testb, then
cmpl) for either of the following changes:
* using |unsigned char| or |int| or |long| instead of |_Bool|
* changing the first term from |!q| to simply |q|
It's as if the compiler believes that ! of _Bool is undefined behavior in
C, so it can do what it likes. But I can't imagine why it would believe
that.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>