http://llvm.org/bugs/show_bug.cgi?id=17172
Bug ID: 17172
Summary: missed opportunity to use absolute value vector
instruction (vpabsd)
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
The following code is performing an absolute value operation on a 4 integer
array. LLVM is generating the straightforward unrolled loop code, but not
recognizing that these operations could be mapped to a single 'VPABSD'
instruction.
$ ./clang -v
clang version 3.4 (trunk 189776)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
$ cat vpabsd.c
void foo(int *x) {
int i;
for (i=0; i<4; i++) {
x[i] = x[i] < 0 ? -x[i] : x[i];
}
}
$ ./clang -S -O3 -fomit-frame-pointer -march=corei7-avx -o /dev/stdout vpabsd.c
.section __TEXT,__text,regular,pure_instructions
.globl _foo
.align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0: ## %entry
movl (%rdi), %eax
movl %eax, %ecx
negl %ecx
cmovll %eax, %ecx
movl %ecx, (%rdi)
movl 4(%rdi), %eax
movl %eax, %ecx
negl %ecx
cmovll %eax, %ecx
movl %ecx, 4(%rdi)
movl 8(%rdi), %eax
movl %eax, %ecx
negl %ecx
cmovll %eax, %ecx
movl %ecx, 8(%rdi)
movl 12(%rdi), %eax
movl %eax, %ecx
negl %ecx
cmovll %eax, %ecx
movl %ecx, 12(%rdi)
ret
--
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