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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - bextr not generated"
href="http://llvm.org/bugs/show_bug.cgi?id=17028">17028</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>bextr not generated
</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>
<tr>
<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>Backend: X86
</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>$ ./clang -v
clang version 3.4 (trunk 189409)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
$ cat bextr.c
int foo(int x)
{
return (x >> 4) & 0xfff; /* extract bits 15:4 */
}
$ ./clang -S -O3 -march=core-avx2 bextr.c -o /dev/stdout
.section __TEXT,__text,regular,pure_instructions
.globl _foo
.align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0: ## %entry
pushq %rbp
Ltmp2:
.cfi_def_cfa_offset 16
Ltmp3:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp4:
.cfi_def_cfa_register %rbp
shrl $4, %edi
andl $4095, %edi ## imm = 0xFFF
movl %edi, %eax
popq %rbp
ret
.cfi_endproc
...
I was expecting the codegen to use the new BMI instruction "bextr" here.
Something like this would be smaller and probably faster than the current
shift/and/mov codegen:
mov 0x0C04, %eax
bextr %eax, %edi, %eax</pre>
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