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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 - Better code for (x & 7) == 6)"
href="https://bugs.llvm.org/show_bug.cgi?id=42695">42695</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Better code for (x & 7) == 6)
</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>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>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>int f(unsigned i) {
if ((i & 7) == 6) return 1;
return i;
}
int g(unsigned i) {
if (((i - 6) & 7) == 0) return 1;
return i;
}
Clang -O3 trunk
f(unsigned int): # @f(unsigned int)
mov eax, edi
and eax, 7
cmp eax, 6
mov eax, 1
cmovne eax, edi
ret
g(unsigned int): # @g(unsigned int)
lea eax, [rdi + 2]
test al, 7
mov eax, 1
cmovne eax, edi
ret
f should produce same code as g. But more typical version - 'f' - produces
worse code..
define dso_local i32 @_Z1fj(i32) local_unnamed_addr #0 {
%2 = and i32 %0, 7
%3 = icmp eq i32 %2, 6
%4 = select i1 %3, i32 1, i32 %0
ret i32 %4
}
define dso_local i32 @_Z1gj(i32) local_unnamed_addr #0 {
%2 = add i32 %0, 2
%3 = and i32 %2, 7
%4 = icmp eq i32 %3, 0
%5 = select i1 %4, i32 1, i32 %0
ret i32 %5
}
g has more IR intructions, but for X86, it looks better.. Probably missed
canonicalization? But maybe g is worse on other backends, I didn't check it.
(X86 fold only?).
<a href="https://godbolt.org/z/XMyGJR">https://godbolt.org/z/XMyGJR</a></pre>
</div>
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