[LLVMbugs] [Bug 15462] New: Inefficient zeroing sequence on Sandybridge
bugzilla-daemon at llvm.org
bugzilla-daemon at llvm.org
Wed Mar 6 15:50:44 PST 2013
http://llvm.org/bugs/show_bug.cgi?id=15462
Bug ID: 15462
Summary: Inefficient zeroing sequence on Sandybridge
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedbugs at nondot.org
Reporter: nrotem at apple.com
CC: llvmbugs at cs.uiuc.edu
Classification: Unclassified
The following IR sequence is combined during DAGCombine into a wide 256-bit
store.
define void @foo(i64* %p) {
store i64 0, i64* %p
%q = getelementptr i64* %p, i64 1
store i64 0, i64* %q
%r = getelementptr i64* %p, i64 2
store i64 0, i64* %r
%s = getelementptr i64* %p, i64 3
store i64 0, i64* %s
ret void
}
But the lowering of Store operations does not know that this is a zero and it
creates a split store which is inefficient.
bin/Debug/llc -O2 -mtriple=x86_64-apple-darwin12
.section __TEXT,__text,regular,pure_instructions
.globl _foo
.align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0:
vxorps %xmm0, %xmm0, %xmm0
vextractf128 $1, %ymm0, %xmm1 <-------- bah
vmovups %xmm1, 16(%rdi) <-------- bah
vmovups %xmm0, (%rdi)
vzeroupper
ret
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