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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - optimization of (x>>8)==0 breaks other comparisons with x"
href="http://llvm.org/bugs/show_bug.cgi?id=19964">19964</a>
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<th>Summary</th>
<td>optimization of (x>>8)==0 breaks other comparisons with x
</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>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>nlewycky@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Testcase:
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
define zeroext i1 @between(i32 %bf.load, i32 %n) {
entry:
%bf.lshr = lshr i32 %bf.load, 16
%cmp2 = icmp eq i32 %bf.lshr, 0
%cmp5 = icmp uge i32 %bf.lshr, %n
%.cmp5 = or i1 %cmp2, %cmp5
ret i1 %.cmp5
}
We codegen this wrong in x86 by lowering the "lshr" into an "andl" for the
comparison against zero, but also use that masked value to compare against %n.
nlewycky@ducttape:~$ llvm/Debug+Asserts/bin/llc x2.ll -o -
.text
.file "x2.ll"
.globl between
.align 16, 0x90
.type between,@function
between: # @between
.cfi_startproc
# BB#0: # %entry
andl $-65536, %edi # imm = 0xFFFFFFFFFFFF0000
sete %cl
cmpl %esi, %edi ;; nope! either %edi needs to be shifted right or
%esi needs to be shifted left
setae %al
orb %cl, %al
retq
.Ltmp0:
.size between, .Ltmp0-between
.cfi_endproc
.section ".note.GNU-stack","",@progbits
We produce correct code with llc -O0.</pre>
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