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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 - Failure to combine rotate through a select"
href="https://bugs.llvm.org/show_bug.cgi?id=45898">45898</a>
</td>
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<tr>
<th>Summary</th>
<td>Failure to combine rotate through a select
</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>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<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>llvm-dev@redking.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>lebedev.ri@gmail.com, llvm-bugs@lists.llvm.org, spatel+llvm@rotateright.com
</td>
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<div>
<pre>Not sure whether this can be done in instcombine or must wait until DAG.
<a href="https://c.godbolt.org/z/Pam6xk">https://c.godbolt.org/z/Pam6xk</a>
#include <stdint.h>
uint8_t rotl_2(uint8_t x) {
uint8_t Xlo = (x<<2);
uint8_t Xhi = (x>>6);
uint8_t Xrot = Xlo|Xhi;
return (x < 128 ? Xrot : Xlo);
}
we have a conditional use of a rotate pattern, which unfortunately gets broken
up by the select
define zeroext i8 @rotl_2(i8 zeroext %0) {
%2 = shl i8 %0, 2
%3 = lshr i8 %0, 6
%4 = icmp slt i8 %0, 0
%5 = select i1 %4, i8 0, i8 %3
%6 = or i8 %5, %2
ret i8 %6
}
resulting in:
rotl_2:
leal (,%rdi,4), %ecx
movl %edi, %eax
shrb $6, %al
xorl %edx, %edx
testb %dil, %dil
movzbl %al, %eax
cmovsl %edx, %eax
orb %cl, %al
retq
while gcc manages:
rotl_2:
movl %edi, %edx
leal 0(,%rdi,4), %eax
rolb $2, %dl
testb %dil, %dil
cmovns %edx, %eax
ret</pre>
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