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[MemCpyOpt] Fix wrong merging adjacent nontemporal stores into memset calls.

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Pass MemCpyOpt doesn't check if a store instruction is nontemporal.
As a consequence, adjacent nontemporal stores are always merged into a
memset call.

Example:

;;;
define void @foo(<4 x float>* nocapture %p) {
entry:
  store <4 x float> zeroinitializer, <4 x float>* %p, align 16, !nontemporal !0
  %p1 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 1
  store <4 x float> zeroinitializer, <4 x float>* %p1, align 16, !nontemporal !0
  ret void
}

!0 = !{i32 1}
;;;

In this example, the two nontemporal stores are combined to a memset of zero
which does not preserve the nontemporal hint. Later on the backend (tested on a
x86-64 corei7) expands that memset call into a sequence of two normal 16-byte
aligned vector stores.

opt -memcpyopt example.ll -S -o - | llc -mcpu=corei7 -o -

Before:
  xorps  %xmm0, %xmm0
  movaps  %xmm0, 16(%rdi)
  movaps  %xmm0, (%rdi)

With this patch, we no longer merge nontemporal stores into calls to memset.
In this example, llc correctly expands the two stores into two movntps:
  xorps  %xmm0, %xmm0
  movntps %xmm0, 16(%rdi)
  movntps  %xmm0, (%rdi)

In theory, we could extend the usage of !nontemporal metadata to memcpy/memset
calls. However a change like that would only have the effect of forcing the
backend to expand !nontemporal memsets back to sequences of store instructions.
A memset library call would not have exactly the same semantic of a builtin
!nontemporal memset call. So, SelectionDAG will have to conservatively expand
it back to a sequence of !nontemporal stores (effectively undoing the merging).

Differential Revision: http://reviews.llvm.org/D13519


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249820 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrea Di Biagio 2015-10-09 10:53:41 +00:00
parent 4d9ce589f8
commit 12b807e83a
2 changed files with 59 additions and 0 deletions
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10
lib/Transforms/Scalar/MemCpyOptimizer.cpp
View File

@ -488,6 +488,16 @@ Instruction *MemCpyOpt::tryMergingIntoMemset(Instruction *StartInst,
bool MemCpyOpt::processStore(StoreInst *SI, BasicBlock::iterator &BBI) {
if (!SI->isSimple()) return false;
// Avoid merging nontemporal stores since the resulting
// memcpy/memset would not be able to preserve the nontemporal hint.
// In theory we could teach how to propagate the !nontemporal metadata to
// memset calls. However, that change would force the backend to
// conservatively expand !nontemporal memset calls back to sequences of
// store instructions (effectively undoing the merging).
if (SI->getMetadata(LLVMContext::MD_nontemporal))
return false;
const DataLayout &DL = SI->getModule()->getDataLayout();
// Detect cases where we're performing call slot forwarding, but

49
test/Transforms/MemCpyOpt/nontemporal.ll Normal file
View File

@ -0,0 +1,49 @@
; RUN: opt < %s -memcpyopt -S | FileCheck %s
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
; Verify that we don't combine nontemporal stores into memset calls.
define void @nontemporal_stores_1(<4 x float>* nocapture %dst) {
; CHECK-LABEL: @nontemporal_stores_1
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %dst, align 16, !nontemporal !0
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %ptr1, align 16, !nontemporal !0
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %ptr2, align 16, !nontemporal !0
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %ptr3, align 16, !nontemporal !0
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %ptr4, align 16, !nontemporal !0
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %ptr5, align 16, !nontemporal !0
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %ptr6, align 16, !nontemporal !0
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %ptr7, align 16, !nontemporal !0
; CHECK-NEXT: ret void
entry:
store <4 x float> zeroinitializer, <4 x float>* %dst, align 16, !nontemporal !0
%ptr1 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 1
store <4 x float> zeroinitializer, <4 x float>* %ptr1, align 16, !nontemporal !0
%ptr2 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 2
store <4 x float> zeroinitializer, <4 x float>* %ptr2, align 16, !nontemporal !0
%ptr3 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 3
store <4 x float> zeroinitializer, <4 x float>* %ptr3, align 16, !nontemporal !0
%ptr4 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 4
store <4 x float> zeroinitializer, <4 x float>* %ptr4, align 16, !nontemporal !0
%ptr5 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 5
store <4 x float> zeroinitializer, <4 x float>* %ptr5, align 16, !nontemporal !0
%ptr6 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 6
store <4 x float> zeroinitializer, <4 x float>* %ptr6, align 16, !nontemporal !0
%ptr7 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 7
store <4 x float> zeroinitializer, <4 x float>* %ptr7, align 16, !nontemporal !0
ret void
}
define void @nontemporal_stores_2(<4 x float>* nocapture %dst) {
; CHECK-LABEL: @nontemporal_stores_2
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %dst, align 16, !nontemporal !0
; CHECK: store <4 x float> zeroinitializer, <4 x float>* %ptr1, align 16, !nontemporal !0
; CHECK-NEXT: ret void
entry:
store <4 x float> zeroinitializer, <4 x float>* %dst, align 16, !nontemporal !0
%ptr1 = getelementptr inbounds <4 x float>, <4 x float>* %dst, i64 1
store <4 x float> zeroinitializer, <4 x float>* %ptr1, align 16, !nontemporal !0
ret void
}
!0 = !{i32 1}