[PATCH] D34162: [InstCombine] Set correct insertion point for selects generated while folding phis

[Anna Thomas via Phabricator via llvm-commits]

anna updated this revision to Diff 102858.
anna added a comment.

Updated with a better test case from spatel.
Added comments to clarify that there maybe an increase in IR.


https://reviews.llvm.org/D34162

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/phi-select-constant.ll


Index: test/Transforms/InstCombine/phi-select-constant.ll
===================================================================
--- test/Transforms/InstCombine/phi-select-constant.ll
+++ test/Transforms/InstCombine/phi-select-constant.ll
@@ -55,3 +55,32 @@
  %sel = select <4 x i1> %phinode, <4 x i64> zeroinitializer, <4 x i64> <i64 124, i64 125, i64 126, i64 127>
  ret <4 x i64> %sel
 }
+
+; Test PR33364
+; Insert the generated select into the same block as the incoming phi value.
+; phi has constant vectors along with a single non-constant vector as operands.
+define <2 x i8> @vec3(i1 %cond1, i1 %cond2, <2 x i1> %x, <2 x i8> %y, <2 x i8> %z) {
+; CHECK-LABEL: @vec3
+; CHECK-LABEL: entry:
+; CHECK-NEXT: [[PHITMP1:%.*]] = shufflevector <2 x i8> %y, <2 x i8> %z, <2 x i32> <i32 2, i32 1>
+entry:
+  br i1 %cond1, label %if1, label %else
+
+; CHECK-LABEL: if1:
+; CHECK-NEXT: [[PHITMP2:%.*]] = shufflevector <2 x i8> %y, <2 x i8> %z, <2 x i32> <i32 0, i32 3>
+if1:
+  br i1 %cond2, label %if2, label %else
+
+; CHECK-LABEL: if2:
+; CHECK-NEXT: [[PHITMP3:%.*]] = select <2 x i1> %x, <2 x i8> %y, <2 x i8> %z
+if2:
+  br label %else
+
+; CHECK-LABEL: else:
+; CHECK-NEXT: [[PHITMP4:%.*]] = phi <2 x i8> [ [[PHITMP3]], %if2 ], [ [[PHITMP1]], %entry ], [ [[PHITMP2]], %if1 ]
+; CHECK-NEXT: ret <2 x i8> [[PHITMP4]]
+else:
+  %phi = phi <2 x i1> [ %x, %if2 ], [ <i1 0, i1 1>, %entry ], [ <i1 1, i1 0>, %if1 ]
+  %sel = select <2 x i1> %phi, <2 x i8> %y, <2 x i8> %z
+  ret <2 x i8> %sel
+}
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -939,9 +939,19 @@
       // `TrueVInPred`.
       if (InC && !isa<ConstantExpr>(InC) && isa<ConstantInt>(InC))
         InV = InC->isNullValue() ? FalseVInPred : TrueVInPred;
-      else
+      else {
+        // Generate the select in the same block as PN's current incoming block.
+        // Note: ThisBB need not be the NonConstBB because vector constants
+        // which are constants by definition are handled here.
+        // FIXME: This can lead to an increase in IR generation because we might
+        // generate selects for vector constant phi operand, that could not be
+        // folded to TrueVInPred or FalseVInPred as done for ConstantInt. For
+        // non-vector phis, this transformation was always profitable because
+        // the select would be generated exactly once in the NonConstBB.
+        Builder->SetInsertPoint(ThisBB->getTerminator());
         InV = Builder->CreateSelect(PN->getIncomingValue(i),
                                     TrueVInPred, FalseVInPred, "phitmp");
+      }
       NewPN->addIncoming(InV, ThisBB);
     }
   } else if (CmpInst *CI = dyn_cast<CmpInst>(&I)) {


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