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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [x86] Redundant XORs generated withwith BSR from "__builtin_clz"."
href="https://bugs.llvm.org/show_bug.cgi?id=47603">47603</a>
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<th>Summary</th>
<td>[x86] Redundant XORs generated withwith BSR from "__builtin_clz".
</td>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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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>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>ToHe_EMA@gmx.de
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</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Consider the following simple C function to be compiled with Clang:
char Func(unsigned DataOffset)
{
return 31 ^ __builtin_clz(DataOffset);
}
Unfortunately it compiles to this assembly:
bsr eax, edi
xor eax, 31
xor al, 31
ret
The relevant LLVM IR is as follows:
declare i32 @llvm.ctlz.i32(i32, i1 immarg) #1
define signext i8 @Func(i32 %0) {
%2 = tail call i32 @llvm.ctlz.i32(i32 %0, i1 true)
%3 = trunc i32 %2 to i8
%4 = xor i8 %3, 31
ret i8 %4
}
Obviously the two XORs should be entirely eliminated in the generated machine
code. This problem only occurs if the return type is a 8 bit integer (char).
For 16 bit, 32 bit and 64 bit integers the xors are elided correctly. Another
variant that leads to the same issue (without an 8 bit integer) is this:
extern char Lookup[32];
int LookupFunc(unsigned DataOffset)
{
return Lookup[31 ^ __builtin_clz(DataOffset)];
}
With results in the following relevant LLVM IR code:
@Lookup = external dso_local local_unnamed_addr global [32 x i8], align 16
declare i32 @llvm.ctlz.i32(i32, i1 immarg) #1
define i32 @LookupFuncj(i32 %0) {
%2 = tail call i32 @llvm.ctlz.i32(i32 %0, i1 true)
%3 = xor i32 %2, 31
%4 = zext i32 %3 to i64
%5 = getelementptr inbounds [32 x i8], [32 x i8]* @Lookup, i64 0, i64 %4
%6 = load i8, i8* %5, align 1
%7 = sext i8 %6 to i32
ret i32 %7
}</pre>
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