[VPlan] Simplify select c, x, x -> x

[lukel97]

As noted in 1a9358c, some simplifications can produce a redundant select where the true and false operands are the same, which this patch removes.

The is_fpclass test was changed so the condition wasn't made dead.

I didn't notice any legacy-vplan cost model mismatches on llvm-test-suite or SPEC with this patch.

[llvmbot]

@llvm/pr-subscribers-vectorizers

@llvm/pr-subscribers-llvm-transforms

Author: Luke Lau (lukel97)

Changes

As noted in 1a9358c, some simplifications can produce a redundant select where the true and false operands are the same.

This simplifies these selects and adds m_Deferred to VPlanPatternMatch (just copied over from IR/PatternMatch) to simplify the pattern matching.

Some upcoming patches I plan on posting can also be made cleaner with m_Deferred.

The is_fpclass was changed so the condition wasn't made dead.

I didn't notice any legacy-vplan cost model mismatches on llvm-test-suite or SPEC with this patch.

---

Full diff: https://github.com/llvm/llvm-project/pull/133731.diff

5 Files Affected:

• (modified) llvm/lib/Transforms/Vectorize/VPlanPatternMatch.h (+21)
• (modified) llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp (+6)
• (modified) llvm/test/Transforms/LoopVectorize/X86/cost-model.ll (+1-6)
• (modified) llvm/test/Transforms/LoopVectorize/is_fpclass.ll (+5-5)
• (modified) llvm/test/Transforms/LoopVectorize/select-reduction.ll (+1-6)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanPatternMatch.h b/llvm/lib/Transforms/Vectorize/VPlanPatternMatch.h
index 3a727866a2875..24d1a16d617cf 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanPatternMatch.h
+++ b/llvm/lib/Transforms/Vectorize/VPlanPatternMatch.h
@@ -66,6 +66,27 @@ struct specificval_ty {
 
 inline specificval_ty m_Specific(const VPValue *VPV) { return VPV; }
 
+/// Stores a reference to the VPValue *, not the VPValue * itself,
+/// thus can be used in commutative matchers.
+template <typename Class> struct deferredval_ty {
+  Class *const &Val;
+
+  deferredval_ty(Class *const &V) : Val(V) {}
+
+  template <typename ITy> bool match(ITy *const V) { return V == Val; }
+};
+
+/// Like m_Specific(), but works if the specific value to match is determined
+/// as part of the same match() expression. For example:
+/// m_Mul(m_VPValue(X), m_Specific(X)) is incorrect, because m_Specific() will
+/// bind X before the pattern match starts.
+/// m_Mul(m_VPValue(X), m_Deferred(X)) is correct, and will check against
+/// whichever value m_VPValue(X) populated.
+inline deferredval_ty<VPValue> m_Deferred(VPValue *const &V) { return V; }
+inline deferredval_ty<const VPValue> m_Deferred(const VPValue *const &V) {
+  return V;
+}
+
 /// Match a specified integer value or vector of all elements of that
 /// value. \p BitWidth optionally specifies the bitwidth the matched constant
 /// must have. If it is 0, the matched constant can have any bitwidth.
diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
index a2e6182cf16e2..879cc247aec81 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
@@ -1059,6 +1059,12 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
     return;
   }
 
+  if (match(&R, m_Select(m_VPValue(), m_VPValue(X), m_Deferred(X)))) {
+    R.getVPSingleValue()->replaceAllUsesWith(X);
+    R.eraseFromParent();
+    return;
+  }
+
   if (match(&R, m_c_Mul(m_VPValue(A), m_SpecificInt(1))))
     return R.getVPSingleValue()->replaceAllUsesWith(A);
 
diff --git a/llvm/test/Transforms/LoopVectorize/X86/cost-model.ll b/llvm/test/Transforms/LoopVectorize/X86/cost-model.ll
index 1f171414242bc..c39dd6ffe5f01 100644
--- a/llvm/test/Transforms/LoopVectorize/X86/cost-model.ll
+++ b/llvm/test/Transforms/LoopVectorize/X86/cost-model.ll
@@ -1046,16 +1046,11 @@ define i64 @live_in_known_1_via_scev() {
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x i64> [ <i64 3, i64 1, i64 1, i64 1>, [[VECTOR_PH]] ], [ [[VEC_PHI]], [[VECTOR_BODY]] ]
-; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i32> poison, i32 [[INDEX]], i64 0
-; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT]], <4 x i32> poison, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[VEC_IV:%.*]] = add <4 x i32> [[BROADCAST_SPLAT]], <i32 0, i32 1, i32 2, i32 3>
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp ule <4 x i32> [[VEC_IV]], splat (i32 5)
-; CHECK-NEXT:    [[TMP1:%.*]] = select <4 x i1> [[TMP0]], <4 x i64> [[VEC_PHI]], <4 x i64> [[VEC_PHI]]
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 4
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[INDEX_NEXT]], 8
 ; CHECK-NEXT:    br i1 [[TMP2]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP24:![0-9]+]]
 ; CHECK:       middle.block:
-; CHECK-NEXT:    [[TMP3:%.*]] = call i64 @llvm.vector.reduce.mul.v4i64(<4 x i64> [[TMP1]])
+; CHECK-NEXT:    [[TMP3:%.*]] = call i64 @llvm.vector.reduce.mul.v4i64(<4 x i64> [[VEC_PHI]])
 ; CHECK-NEXT:    br i1 true, label [[EXIT:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i32 [ 8, [[MIDDLE_BLOCK]] ], [ 0, [[PH]] ]
diff --git a/llvm/test/Transforms/LoopVectorize/is_fpclass.ll b/llvm/test/Transforms/LoopVectorize/is_fpclass.ll
index c429092a50e6a..9f82c93d3579d 100644
--- a/llvm/test/Transforms/LoopVectorize/is_fpclass.ll
+++ b/llvm/test/Transforms/LoopVectorize/is_fpclass.ll
@@ -9,12 +9,12 @@ define void @d() {
 ; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
-; CHECK-NEXT:    [[TMP6:%.*]] = load float, ptr null, align 4
-; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <2 x float> poison, float [[TMP6]], i64 0
+; CHECK-NEXT:    [[TMP0:%.*]] = load float, ptr null, align 4
+; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <2 x float> poison, float [[TMP0]], i64 0
 ; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <2 x float> [[BROADCAST_SPLATINSERT]], <2 x float> poison, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr float, ptr @d, i64 [[INDEX]]
 ; CHECK-NEXT:    [[TMP2:%.*]] = call <2 x i1> @llvm.is.fpclass.v2f32(<2 x float> [[BROADCAST_SPLAT]], i32 0)
-; CHECK-NEXT:    [[TMP3:%.*]] = select <2 x i1> [[TMP2]], <2 x float> zeroinitializer, <2 x float> zeroinitializer
+; CHECK-NEXT:    [[TMP3:%.*]] = select <2 x i1> [[TMP2]], <2 x float> zeroinitializer, <2 x float> splat (float 1.000000e+00)
 ; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr float, ptr [[TMP1]], i32 0
 ; CHECK-NEXT:    store <2 x float> [[TMP3]], ptr [[TMP4]], align 4
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
@@ -30,7 +30,7 @@ define void @d() {
 ; CHECK-NEXT:    [[I3:%.*]] = load float, ptr null, align 4
 ; CHECK-NEXT:    [[I4:%.*]] = getelementptr float, ptr @d, i64 [[I]]
 ; CHECK-NEXT:    [[I5:%.*]] = tail call i1 @llvm.is.fpclass.f32(float [[I3]], i32 0)
-; CHECK-NEXT:    [[I6:%.*]] = select i1 [[I5]], float 0.000000e+00, float 0.000000e+00
+; CHECK-NEXT:    [[I6:%.*]] = select i1 [[I5]], float 0.000000e+00, float 1.000000e+00
 ; CHECK-NEXT:    store float [[I6]], ptr [[I4]], align 4
 ; CHECK-NEXT:    [[I7]] = add i64 [[I]], 1
 ; CHECK-NEXT:    [[I8:%.*]] = icmp eq i64 [[I7]], 128
@@ -46,7 +46,7 @@ loop:
   %i3 = load float, ptr null, align 4
   %i4 = getelementptr float, ptr @d, i64 %i
   %i5 = tail call i1 @llvm.is.fpclass.f32(float %i3, i32 0)
-  %i6 = select i1 %i5, float 0.0, float 0.0
+  %i6 = select i1 %i5, float 0.0, float 1.0
   store float %i6, ptr %i4, align 4
   %i7 = add i64 %i, 1
   %i8 = icmp eq i64 %i7, 128
diff --git a/llvm/test/Transforms/LoopVectorize/select-reduction.ll b/llvm/test/Transforms/LoopVectorize/select-reduction.ll
index 836115f381382..16cc05da17a63 100644
--- a/llvm/test/Transforms/LoopVectorize/select-reduction.ll
+++ b/llvm/test/Transforms/LoopVectorize/select-reduction.ll
@@ -88,16 +88,11 @@ define i32 @pr66895_tail_fold_reduction_exit_inst_gets_simplified(i32 %n) {
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x i32> [ <i32 0, i32 1, i32 1, i32 1>, [[VECTOR_PH]] ], [ [[VEC_PHI]], [[VECTOR_BODY]] ]
-; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i32> poison, i32 [[INDEX]], i64 0
-; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT]], <4 x i32> poison, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[VEC_IV:%.*]] = add <4 x i32> [[BROADCAST_SPLAT]], <i32 0, i32 1, i32 2, i32 3>
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp ule <4 x i32> [[VEC_IV]], splat (i32 12)
-; CHECK-NEXT:    [[TMP1:%.*]] = select <4 x i1> [[TMP0]], <4 x i32> [[VEC_PHI]], <4 x i32> [[VEC_PHI]]
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 4
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[INDEX_NEXT]], 16
 ; CHECK-NEXT:    br i1 [[TMP2]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
 ; CHECK:       middle.block:
-; CHECK-NEXT:    [[TMP3:%.*]] = call i32 @llvm.vector.reduce.mul.v4i32(<4 x i32> [[TMP1]])
+; CHECK-NEXT:    [[TMP3:%.*]] = call i32 @llvm.vector.reduce.mul.v4i32(<4 x i32> [[VEC_PHI]])
 ; CHECK-NEXT:    br i1 true, label [[EXIT:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i32 [ -4, [[MIDDLE_BLOCK]] ], [ 12, [[ENTRY:%.*]] ]

[lukel97]

Actually I've just noticed the combine above can use m_Deferred. I've split off the m_Deferred bit into #133736 to keep it NFC

[fhahn]

LGTM, thanks. Looks like the redundant selects are the result of other simplifications I