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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [x86] optimizing for size results in bigger code because of repeated constant"
href="https://llvm.org/bugs/show_bug.cgi?id=27202">27202</a>
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<th>Summary</th>
<td>[x86] optimizing for size results in bigger code because of repeated constant
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>Noticed via <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [InstCombine] missing canonicalization for binop + compare against constant"
href="show_bug.cgi?id=27105">bug 27105</a>:
define i1 @foo(i32 %i) #0 {
%and = and i32 %i, 305419896
%cmp = icmp eq i32 %and, 305419896
ret i1 %cmp
}
attributes #0 = { "no-frame-pointer-elim"="false" }
$ ./llc maskcmp.ll -filetype=obj ; otool -tv maskcmp.o
...
0000000000000000 andl $0x12345678, %edi ## imm = 0x12345678
0000000000000006 cmpl $0x12345678, %edi ## imm = 0x12345678
000000000000000c sete %al
000000000000000f retq
Now add an 'optsize' attribute:
define i1 @foo(i32 %i) #0 {
%and = and i32 %i, 305419896
%cmp = icmp eq i32 %and, 305419896
ret i1 %cmp
}
attributes #0 = { optsize "no-frame-pointer-elim"="false" }
$ ./llc maskcmp.ll -filetype=obj ; otool -tv maskcmp.o
...
0000000000000000 andl $0x12345678, %edi ## imm = 0x12345678
0000000000000006 movl $0x12345678, %eax ## imm = 0x12345678
000000000000000b cmpl %eax, %edi
000000000000000d sete %al
0000000000000010 retq
We loaded the constant into a register, but we didn't actually use it for the
'and', so the code got bigger rather than smaller.
This is probably related to <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [x86] avoid big bad immediates in the instruction stream, part 2: hoist constants"
href="show_bug.cgi?id=24448">bug 24448</a>, and may be a bug introduced with:
<a href="http://reviews.llvm.org/rL244601">http://reviews.llvm.org/rL244601</a></pre>
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</p>
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