[PATCH] D66095: [InstCombine] canonicalize a scalar-select-of-vectors to vector select

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: vporpo, ABataev, dtemirbulatov.
Herald added subscribers: hiraditya, mcrosier.
Herald added a project: LLVM.

This pattern may arise with an enhancement to SLP vectorization suggested in PR42755:
https://bugs.llvm.org/show_bug.cgi?id=42755

For all in-tree targets that I looked at, codegen looks better when we change to a vector select, so I think 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


https://reviews.llvm.org/D66095

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/select-extractelement.ll


Index: llvm/test/Transforms/InstCombine/select-extractelement.ll
===================================================================
--- llvm/test/Transforms/InstCombine/select-extractelement.ll
+++ llvm/test/Transforms/InstCombine/select-extractelement.ll
@@ -146,8 +146,8 @@
 
 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 @@
 
 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 @@
   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 @@
   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
Index: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1550,6 +1550,30 @@
                                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))
@@ -1846,6 +1870,9 @@
   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.


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