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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 --- - bit-scan-reverse / count-leading-zeros loop not recognized"
href="http://llvm.org/bugs/show_bug.cgi?id=17130">17130</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>bit-scan-reverse / count-leading-zeros loop not recognized
</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>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kkhoo@perfwizard.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>This is very similar to <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - bit-scan-forward / count-trailing-zeros loop not recognized"
href="show_bug.cgi?id=17128">bug 17128</a> (count trailing zeros), but I'm filing a
separate bug just in case the solution involves some differences in
implementation in the compiler.
On the surface, this one maps to existing hardware instructions (Power =
ctlzw/d, ARM = clz) more easily than count-trailing-zeros. x86 has bsr/lzcnt as
well as bsf/tzcnt.
Here's a test file with a couple of implementations of count-leading-zeros.
Currently, llvm recognizes that both functions have bit-testing loops ('bt'
instructions is generated), but doesn't know that they can be simplified to
count-leading-zeros:
$ ./clang -v
clang version 3.4 (trunk 189776)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
$ cat lzcnt.c
int lzcnt(int x) {
int count = 0;
int i = 31;
while (i>=0 && (((x >> i) & 0x1) == 0)) {
i--;
count++;
}
return count;
}
int lzcnt2(int x) {
int count = 0;
int i;
for (i=0; i<32; i++) {
if ((x & (1 << (31-i))) != 0) break;
count++;
}
return count;
}
$ ./clang -S -O3 -fomit-frame-pointer -march=core-avx2 -o /dev/stdout lzcnt.c
.section __TEXT,__text,regular,pure_instructions
.globl _lzcnt
.align 4, 0x90
_lzcnt: ## @lzcnt
.cfi_startproc
## BB#0: ## %entry
xorl %eax, %eax
movl $32, %ecx
.align 4, 0x90
LBB0_1: ## %land.rhs
## =>This Inner Loop Header: Depth=1
decl %ecx
btl %ecx, %edi
jb LBB0_3
## BB#2: ## %while.body
## in Loop: Header=BB0_1 Depth=1
incl %eax
testl %ecx, %ecx
jg LBB0_1
LBB0_3: ## %while.end
ret
.cfi_endproc
.globl _lzcnt2
.align 4, 0x90
_lzcnt2: ## @lzcnt2
.cfi_startproc
## BB#0: ## %entry
xorl %eax, %eax
movl $31, %ecx
.align 4, 0x90
LBB1_1: ## %for.body
## =>This Inner Loop Header: Depth=1
btl %ecx, %edi
jb LBB1_3
## BB#2: ## %if.end
## in Loop: Header=BB1_1 Depth=1
decl %ecx
incl %eax
cmpl $32, %eax
jl LBB1_1
LBB1_3: ## %for.end
ret
.cfi_endproc</pre>
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