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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 bug - spurious shift in partial word test"
href="http://llvm.org/bugs/show_bug.cgi?id=17783">17783</a>
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<th>Summary</th>
<td>Optimization bug - spurious shift in partial word test
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>maurice.marks@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>In the situation where a partial word is tested, lets say >0, by shifting left
to get the sign bit into the msb and testing llvm is inserting a spurious right
shift instruction.
For example this IR:
...
%0 = load i64* %a.addr, align 8
%shl = shl i64 %0, 28
%cmp = icmp sgt i64 %shl, 0
...
results in
...
shlq $28, %rdi
sarq $28, %rdi ; <<< spurious shift
testq %rdi, %rdi
gcc doesnt have this problem. It just emits the shift and test.
The reason appears to be that the instruction combining pass decides that the
shift and test is equivalent to a test on the partial word, in this case an
I36.
>From the -debug log:
....
INSTCOMBINE ITERATION #0 on testit
IC: ADDING: 10 instrs to worklist
IC: Visiting: %shl = shl i64 %a, 28
IC: Visiting: %cmp = icmp sgt i64 %shl, 0
IC: ADD: %0 = trunc i64 %a to i36
IC: Old = %cmp = icmp sgt i64 %shl, 0
New = <badref> = icmp sgt i36 %0, 0
IC: ADD: %cmp = icmp sgt i36 %0, 0
IC: ERASE %1 = icmp sgt i64 %shl, 0
IC: ADD: %shl = shl i64 %a, 28
IC: DCE: %shl = shl i64 %a, 28
--- etc
So apparently the extra shift is inserted to restore the I36, although it is
never referenced again.
Here is a little C test program, try it at -O3 in both gcc and clang and you
will see the problem:
#include <stdint.h>
uint8_t testit(uint64_t a) {
return ((int64_t) (a << 28) > 0) ;
}</pre>
</div>
</p>
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