[PATCH] D126895: [SLP] Phi inputs that come from an unreachable block should be undef.

Thu Jun 2 10:02:11 PDT 2022

vporpo created this revision.
vporpo added reviewers: ABataev, RKSimon, hvdijk.
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Up until now when we generate a vector phi we create a poison value if the input
comes from a BB that is not reachable from the input.

For example:

  bb1
    %foo = ...
    br bb2
  
  unreachable_bb:
    br bb2
  
  bb2:
    %phi0 = phi [%foo0, %bb1], [0, %unreachable_bb]
    %phi1 = phi [%foo1, %bb1], [1, %unreachable_bb]

When we vectorize the phi, we are generating:

  %vec_phi = phi [%vec_foo, %bb1], [ poison, %unreachable_bb ]

I think that this is incorrect, the input should be undef and not poison.
The value coming from an unreachable block from entry is uninitialized, which
IIUC is modeled by undef.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D126895

Files:
  llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
  llvm/test/Transforms/SLPVectorizer/X86/landing_pad.ll
  llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll
  llvm/test/Transforms/SLPVectorizer/X86/phi-unreachable-input-block.ll
  llvm/test/Transforms/SLPVectorizer/X86/reduction-same-vals.ll
  llvm/test/Transforms/SLPVectorizer/X86/unreachable.ll


Index: llvm/test/Transforms/SLPVectorizer/X86/unreachable.ll
===================================================================

--- llvm/test/Transforms/SLPVectorizer/X86/unreachable.ll
+++ llvm/test/Transforms/SLPVectorizer/X86/unreachable.ll
@@ -15,7 +15,7 @@
 ; CHECK-NEXT:    [[BAD:%.*]] = fadd float [[BAD]], 0.000000e+00
 ; CHECK-NEXT:    br label [[BB2]]
 ; CHECK:       bb2:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi <4 x i32> [ poison, [[BB1:%.*]] ], [ <i32 2, i32 2, i32 2, i32 2>, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi <4 x i32> [ undef, [[BB1:%.*]] ], [ <i32 2, i32 2, i32 2, i32 2>, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i32* [[X:%.*]] to <4 x i32>*
 ; CHECK-NEXT:    store <4 x i32> [[TMP0]], <4 x i32>* [[TMP1]], align 4
 ; CHECK-NEXT:    ret void
Index: llvm/test/Transforms/SLPVectorizer/X86/reduction-same-vals.ll
===================================================================
--- llvm/test/Transforms/SLPVectorizer/X86/reduction-same-vals.ll
+++ llvm/test/Transforms/SLPVectorizer/X86/reduction-same-vals.ll
@@ -8,7 +8,7 @@
 ; CHECK:       bb2:
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi <2 x i32> [ poison, [[BB2:%.*]] ], [ zeroinitializer, [[BB1:%.*]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi <2 x i32> [ undef, [[BB2:%.*]] ], [ zeroinitializer, [[BB1:%.*]] ]
 ; CHECK-NEXT:    [[SHUFFLE:%.*]] = shufflevector <2 x i32> [[TMP0]], <2 x i32> poison, <4 x i32> <i32 0, i32 0, i32 0, i32 1>
 ; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <2 x i32> [[TMP0]], <2 x i32> poison, <8 x i32> zeroinitializer
 ; CHECK-NEXT:    [[TMP2:%.*]] = call i32 @llvm.vector.reduce.mul.v8i32(<8 x i32> [[TMP1]])
Index: llvm/test/Transforms/SLPVectorizer/X86/phi-unreachable-input-block.ll
===================================================================
--- llvm/test/Transforms/SLPVectorizer/X86/phi-unreachable-input-block.ll
+++ llvm/test/Transforms/SLPVectorizer/X86/phi-unreachable-input-block.ll
@@ -15,7 +15,7 @@
 ; CHECK:       bb2:
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[TMP2:%.*]] = phi <2 x i8> [ [[TMP1]], [[BB1:%.*]] ], [ poison, [[BB2:%.*]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = phi <2 x i8> [ [[TMP1]], [[BB1:%.*]] ], [ undef, [[BB2:%.*]] ]
 ; CHECK-NEXT:    [[TMP3:%.*]] = zext <2 x i8> [[TMP2]] to <2 x i64>
 ; CHECK-NEXT:    [[GEP0:%.*]] = getelementptr i64, i64* [[PTR:%.*]], i64 0
 ; CHECK-NEXT:    [[TMP4:%.*]] = bitcast i64* [[GEP0]] to <2 x i64>*
@@ -50,7 +50,7 @@
 ; CHECK:       bb2:
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[TMP2:%.*]] = phi <2 x i8> [ [[TMP1]], [[BB1:%.*]] ], [ poison, [[BB2:%.*]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = phi <2 x i8> [ [[TMP1]], [[BB1:%.*]] ], [ undef, [[BB2:%.*]] ]
 ; CHECK-NEXT:    [[TMP3:%.*]] = zext <2 x i8> [[TMP2]] to <2 x i64>
 ; CHECK-NEXT:    [[GEP0:%.*]] = getelementptr i64, i64* [[PTR:%.*]], i64 0
 ; CHECK-NEXT:    [[TMP4:%.*]] = bitcast i64* [[GEP0]] to <2 x i64>*
@@ -85,7 +85,7 @@
 ; CHECK:       bb2:
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[TMP2:%.*]] = phi <2 x i8> [ [[TMP1]], [[BB1:%.*]] ], [ poison, [[BB2:%.*]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = phi <2 x i8> [ [[TMP1]], [[BB1:%.*]] ], [ undef, [[BB2:%.*]] ]
 ; CHECK-NEXT:    [[TMP3:%.*]] = zext <2 x i8> [[TMP2]] to <2 x i64>
 ; CHECK-NEXT:    [[GEP0:%.*]] = getelementptr i64, i64* [[PTR:%.*]], i64 0
 ; CHECK-NEXT:    [[TMP4:%.*]] = bitcast i64* [[GEP0]] to <2 x i64>*
Index: llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll
===================================================================
--- llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll
+++ llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll
@@ -116,7 +116,7 @@
 ; CHECK:       bb2:
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi <2 x i32> [ undef, [[BB1]] ], [ poison, [[BB2:%.*]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi <2 x i32> [ undef, [[BB1]] ], [ undef, [[BB2:%.*]] ]
 ; CHECK-NEXT:    [[SHUFFLE:%.*]] = shufflevector <2 x i32> [[TMP0]], <2 x i32> poison, <8 x i32> <i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 1>
 ; CHECK-NEXT:    [[TMP1:%.*]] = extractelement <8 x i32> [[SHUFFLE]], i32 0
 ; CHECK-NEXT:    [[TMP2:%.*]] = insertelement <16 x i32> poison, i32 [[TMP1]], i32 0
Index: llvm/test/Transforms/SLPVectorizer/X86/landing_pad.ll
===================================================================
--- llvm/test/Transforms/SLPVectorizer/X86/landing_pad.ll
+++ llvm/test/Transforms/SLPVectorizer/X86/landing_pad.ll
@@ -10,7 +10,7 @@
 ; CHECK:       bb2.loopexit:
 ; CHECK-NEXT:    br label [[BB2:%.*]]
 ; CHECK:       bb2:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi <4 x i32> [ [[SHUFFLE:%.*]], [[BB9:%.*]] ], [ poison, [[BB2_LOOPEXIT:%.*]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi <4 x i32> [ [[SHUFFLE:%.*]], [[BB9:%.*]] ], [ undef, [[BB2_LOOPEXIT:%.*]] ]
 ; CHECK-NEXT:    ret void
 ; CHECK:       bb3:
 ; CHECK-NEXT:    [[TMP1:%.*]] = phi <2 x i32> [ [[TMP5:%.*]], [[BB6:%.*]] ], [ poison, [[BB1:%.*]] ]
Index: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -4751,7 +4751,7 @@
       SmallVector<ValueList, 2> OperandsVec;
       for (unsigned I = 0, E = PH->getNumIncomingValues(); I < E; ++I) {
         if (!DT->isReachableFromEntry(PH->getIncomingBlock(I))) {
-          ValueList Operands(VL.size(), PoisonValue::get(PH->getType()));
+          ValueList Operands(VL.size(), UndefValue::get(PH->getType()));
           TE->setOperand(I, Operands);
           OperandsVec.push_back(Operands);
           continue;


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