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[InstCombine] canonicalize a scalar-select-of-vectors to vector select

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This pattern may arise more frequently with an enhancement to SLP vectorization suggested in PR42755:
https://bugs.llvm.org/show_bug.cgi?id=42755
...but we should handle this pattern to make things easier for the backend either way.

For all in-tree targets that I looked at, codegen for typical vector sizes looks better when we change
to a vector select, so this is safe to do without a cost model (in other words, as a target-independent
canonicalization).

For example, if the condition of the select is a scalar, we end up with something like this on x86:

	vpcmpgtd	%xmm0, %xmm1, %xmm0
	vpextrb	$12, %xmm0, %eax
	testb	$1, %al
	jne	LBB0_2
  ## %bb.1:
	vmovaps	%xmm3, %xmm2
  LBB0_2:
	vmovaps	%xmm2, %xmm0

Rather than the splat-condition variant:

	vpcmpgtd	%xmm0, %xmm1, %xmm0
	vpshufd	$255, %xmm0, %xmm0      ## xmm0 = xmm0[3,3,3,3]
	vblendvps	%xmm0, %xmm2, %xmm3, %xmm0

Differential Revision: https://reviews.llvm.org/D66095

llvm-svn: 369140
This commit is contained in:
Sanjay Patel 2019-08-16 18:51:30 +00:00
parent 2728670419
commit a657e27486
2 changed files with 37 additions and 4 deletions
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27
lib/Transforms/InstCombine/InstCombineSelect.cpp
View File

@ -1696,6 +1696,30 @@ static Instruction *canonicalizeSelectToShuffle(SelectInst &SI) {
ConstantVector::get(Mask));
}
/// If we have a select of vectors with a scalar condition, try to convert that
/// to a vector select by splatting the condition. A splat may get folded with
/// other operations in IR and having all operands of a select be vector types
/// is likely better for vector codegen.
static Instruction *canonicalizeScalarSelectOfVecs(
SelectInst &Sel, InstCombiner::BuilderTy &Builder) {
Type *Ty = Sel.getType();
if (!Ty->isVectorTy())
return nullptr;
// We can replace a single-use extract with constant index.
Value *Cond = Sel.getCondition();
if (!match(Cond, m_OneUse(m_ExtractElement(m_Value(), m_ConstantInt()))))
return nullptr;
// select (extelt V, Index), T, F --> select (splat V, Index), T, F
// Splatting the extracted condition reduces code (we could directly create a
// splat shuffle of the source vector to eliminate the intermediate step).
unsigned NumElts = Ty->getVectorNumElements();
Value *SplatCond = Builder.CreateVectorSplat(NumElts, Cond);
Sel.setCondition(SplatCond);
return &Sel;
}
/// Reuse bitcasted operands between a compare and select:
/// select (cmp (bitcast C), (bitcast D)), (bitcast' C), (bitcast' D) -->
/// bitcast (select (cmp (bitcast C), (bitcast D)), (bitcast C), (bitcast D))
@ -1992,6 +2016,9 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
if (Instruction *I = canonicalizeSelectToShuffle(SI))
return I;
if (Instruction *I = canonicalizeScalarSelectOfVecs(SI, Builder))
return I;
// Canonicalize a one-use integer compare with a non-canonical predicate by
// inverting the predicate and swapping the select operands. This matches a
// compare canonicalization for conditional branches.

14
test/Transforms/InstCombine/select-extractelement.ll
View File

@ -146,8 +146,8 @@ entry:
define <4 x i32> @extract_cond(<4 x i32> %x, <4 x i32> %y, <4 x i1> %condv) {
; CHECK-LABEL: @extract_cond(
; CHECK-NEXT: [[COND:%.*]] = extractelement <4 x i1> [[CONDV:%.*]], i32 3
; CHECK-NEXT: [[R:%.*]] = select i1 [[COND]], <4 x i32> [[X:%.*]], <4 x i32> [[Y:%.*]]
; CHECK-NEXT: [[DOTSPLAT:%.*]] = shufflevector <4 x i1> [[CONDV:%.*]], <4 x i1> undef, <4 x i32> <i32 3, i32 3, i32 3, i32 3>
; CHECK-NEXT: [[R:%.*]] = select <4 x i1> [[DOTSPLAT]], <4 x i32> [[X:%.*]], <4 x i32> [[Y:%.*]]
; CHECK-NEXT: ret <4 x i32> [[R]]
;
%cond = extractelement <4 x i1> %condv, i32 3
@ -168,6 +168,8 @@ define <4 x i32> @splat_cond(<4 x i32> %x, <4 x i32> %y, <4 x i1> %condv) {
declare void @extra_use(i1)
; Negative test
define <4 x i32> @extract_cond_extra_use(<4 x i32> %x, <4 x i32> %y, <4 x i1> %condv) {
; CHECK-LABEL: @extract_cond_extra_use(
; CHECK-NEXT: [[COND:%.*]] = extractelement <4 x i1> [[CONDV:%.*]], i32 3
@ -181,6 +183,8 @@ define <4 x i32> @extract_cond_extra_use(<4 x i32> %x, <4 x i32> %y, <4 x i1> %c
ret <4 x i32> %r
}
; Negative test
define <4 x i32> @extract_cond_variable_index(<4 x i32> %x, <4 x i32> %y, <4 x i1> %condv, i32 %index) {
; CHECK-LABEL: @extract_cond_variable_index(
; CHECK-NEXT: [[COND:%.*]] = extractelement <4 x i1> [[CONDV:%.*]], i32 [[INDEX:%.*]]
@ -192,10 +196,12 @@ define <4 x i32> @extract_cond_variable_index(<4 x i32> %x, <4 x i32> %y, <4 x i
ret <4 x i32> %r
}
; IR shuffle can alter the number of elements in the vector, so this is ok.
define <4 x i32> @extract_cond_type_mismatch(<4 x i32> %x, <4 x i32> %y, <5 x i1> %condv) {
; CHECK-LABEL: @extract_cond_type_mismatch(
; CHECK-NEXT: [[COND:%.*]] = extractelement <5 x i1> [[CONDV:%.*]], i32 1
; CHECK-NEXT: [[R:%.*]] = select i1 [[COND]], <4 x i32> [[X:%.*]], <4 x i32> [[Y:%.*]]
; CHECK-NEXT: [[DOTSPLAT:%.*]] = shufflevector <5 x i1> [[CONDV:%.*]], <5 x i1> undef, <4 x i32> <i32 1, i32 1, i32 1, i32 1>
; CHECK-NEXT: [[R:%.*]] = select <4 x i1> [[DOTSPLAT]], <4 x i32> [[X:%.*]], <4 x i32> [[Y:%.*]]
; CHECK-NEXT: ret <4 x i32> [[R]]
;
%cond = extractelement <5 x i1> %condv, i32 1