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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 - optimize smax(x, -1) with bit-hack"
href="https://bugs.llvm.org/show_bug.cgi?id=49895">49895</a>
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<tr>
<th>Summary</th>
<td>optimize smax(x, -1) with bit-hack
</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>All
</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>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Noticed while investigating idioms for signum functions...
If we have an smax with -1:
declare i32 @llvm.smax.i32(i32, i32)
define i32 @smax(i32 %x) {
%m = call i32 @llvm.smax.i32(i32 %x, i32 -1)
ret i32 %m
}
or:
define i32 @cmpsel(i32 %x) {
%a = icmp sgt i32 %x, -1
%m = select i1 %a, i32 %x, i32 -1
ret i32 %m
}
It seems most targets would do better to convert to arithmetic shift + logic:
define i32 @tgt(i32 %x) {
%a = ashr i32 %x, 31
%m = or i32 %x, %a
ret i32 %m
}
<a href="https://alive2.llvm.org/ce/z/AJYAmp">https://alive2.llvm.org/ce/z/AJYAmp</a>
AArch64:
cmp w0, #0
csinv w0, w0, wzr, ge
vs.
orr w0, w0, w0, asr #31
PowerPC64:
li 4, -1
cmpwi 3, -1
rldic 4, 4, 0, 32
iselgt 3, 3, 4
vs.
srawi 4, 3, 31
or 3, 3, 4
x86:
testl %edi, %edi
movl $-1, %eax
cmovnsl %edi, %eax
vs.
movl %edi, %eax
sarl $31, %eax
orl %edi, %eax</pre>
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