[LV] Fix predication for branches with matching true and false succs.

Currently due to the edge caching, we create wrong predicates for
branches with matching true and false successors. We will cache the
condition for the edge from the true successor, and then lookup the same
edge (src and dst are the same) for the edge to the false successor.

If both successors match, the condition should always be true. At the
moment, we cannot really create constant VPValues, but we can just
create a true condition as X | !X. Later passes will clean that up.

Fixes PR44488.

Reviewers: rengolin, hsaito, fhahn, Ayal, dorit, gilr

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D73079

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -6729,7 +6729,7 @@ VPValue *VPRecipeBuilder::createEdgeMask(BasicBlock *Src, BasicBlock *Dst,
   BranchInst *BI = dyn_cast<BranchInst>(Src->getTerminator());
   assert(BI && "Unexpected terminator found");
 
-  if (!BI->isConditional())
+  if (!BI->isConditional() || BI->getSuccessor(0) == BI->getSuccessor(1))
     return EdgeMaskCache[Edge] = SrcMask;
 
   VPValue *EdgeMask = Plan->getVPValue(BI->getCondition());

llvm/test/Transforms/LoopVectorize/pr44488-predication.ll

@@ -0,0 +1,79 @@
+; RUN: opt < %s -force-vector-width=2 -force-vector-interleave=1 -loop-vectorize -S | FileCheck %s
+
+; Test case for PR44488. Checks that the correct predicates are created for
+; branches where true and false successors are equal. See the checks involving
+; CMP1 and CMP2.
+
+@v_38 = global i16 12061, align 1
+@v_39 = global i16 11333, align 1
+
+define i16 @test_true_and_false_branch_equal() {
+; CHECK-LABEL: @test_true_and_false_branch_equal(
+; CHECK-LABEL: vector.body:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, %vector.ph ], [ [[INDEX_NEXT:%.*]], [[PRED_SREM_CONTINUE2:%.*]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i32 [[INDEX]] to i16
+; CHECK-NEXT:    [[OFFSET_IDX:%.*]] = add i16 99, [[TMP0]]
+; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <2 x i16> undef, i16 [[OFFSET_IDX]], i32 0
+; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <2 x i16> [[BROADCAST_SPLATINSERT]], <2 x i16> undef, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[INDUCTION:%.*]] = add <2 x i16> [[BROADCAST_SPLAT]], <i16 0, i16 1>
+; CHECK-NEXT:    [[TMP1:%.*]] = add i16 [[OFFSET_IDX]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = load i16, i16* @v_38, align 1
+; CHECK-NEXT:    [[TMP3:%.*]] = load i16, i16* @v_38, align 1
+; CHECK-NEXT:    [[TMP4:%.*]] = insertelement <2 x i16> undef, i16 [[TMP2]], i32 0
+; CHECK-NEXT:    [[TMP5:%.*]] = insertelement <2 x i16> [[TMP4]], i16 [[TMP3]], i32 1
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq <2 x i16> [[TMP5]], <i16 32767, i16 32767>
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp eq <2 x i16> [[TMP5]], zeroinitializer
+; CHECK-NEXT:    [[NOT_CMP2:%.*]] = xor <2 x i1> [[CMP2]], <i1 true, i1 true>
+; CHECK-NEXT:    [[PRED1:%.*]] = extractelement <2 x i1> [[NOT_CMP2]], i32 0
+; CHECK-NEXT:    br i1 [[PRED1]], label [[PRED_SREM_IF:%.*]], label [[PRED_SREM_CONTINUE:%.*]]
+; CHECK:       pred.srem.if:
+; CHECK-NEXT:    [[TMP14:%.*]] = srem i16 5786, [[TMP2]]
+; CHECK-NEXT:    [[TMP15:%.*]] = insertelement <2 x i16> undef, i16 [[TMP14]], i32 0
+; CHECK-NEXT:    br label [[PRED_SREM_CONTINUE]]
+; CHECK:       pred.srem.continue:
+; CHECK-NEXT:    [[TMP16:%.*]] = phi <2 x i16> [ undef, %vector.body ], [ [[TMP15]], [[PRED_SREM_IF]] ]
+; CHECK-NEXT:    [[PRED2:%.*]] = extractelement <2 x i1> [[NOT_CMP2]], i32 1
+; CHECK-NEXT:    br i1 [[PRED2]], label [[PRED_SREM_IF1:%.*]], label [[PRED_SREM_CONTINUE2]]
+; CHECK:       pred.srem.if1:
+; CHECK-NEXT:    [[TMP18:%.*]] = srem i16 5786, [[TMP3]]
+; CHECK-NEXT:    [[TMP19:%.*]] = insertelement <2 x i16> [[TMP16]], i16 [[TMP18]], i32 1
+; CHECK-NEXT:    br label [[PRED_SREM_CONTINUE2]]
+; CHECK:       pred.srem.continue2:
+; CHECK-NEXT:    [[TMP20:%.*]] = phi <2 x i16> [ [[TMP16]], [[PRED_SREM_CONTINUE]] ], [ [[TMP19]], [[PRED_SREM_IF1]] ]
+; CHECK-NEXT:    [[PREDPHI:%.*]] = select <2 x i1> [[CMP2]], <2 x i16> <i16 5786, i16 5786>, <2 x i16> [[TMP20]]
+; CHECK-NEXT:    [[TMP22:%.*]] = extractelement <2 x i16> [[PREDPHI]], i32 0
+; CHECK-NEXT:    store i16 [[TMP22]], i16* @v_39, align 1
+; CHECK-NEXT:    [[TMP23:%.*]] = extractelement <2 x i16> [[PREDPHI]], i32 1
+; CHECK-NEXT:    store i16 [[TMP23]], i16* @v_39, align 1
+; CHECK-NEXT:    [[INDEX_NEXT]] = add i32 [[INDEX]], 2
+; CHECK-NEXT:    [[TMP24:%.*]] = icmp eq i32 [[INDEX_NEXT]], 12
+; CHECK-NEXT:    br i1 [[TMP24]], label %middle.block, label %vector.body, !llvm.loop !0
+;
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %entry, %for.latch
+  %i.07 = phi i16 [ 99, %entry ], [ %inc7, %for.latch ]
+  %lv = load i16, i16* @v_38, align 1
+  %cmp1 = icmp eq i16 %lv, 32767
+  br i1 %cmp1, label %cond.end, label %cond.end
+
+cond.end:                                         ; preds = %for.body, %for.body
+  %cmp2 = icmp eq i16 %lv, 0
+  br i1 %cmp2, label %for.latch, label %cond.false4
+
+cond.false4:                                      ; preds = %cond.end
+  %rem = srem i16 5786, %lv
+  br label %for.latch
+
+for.latch:                                        ; preds = %cond.end, %cond.false4
+  %cond6 = phi i16 [ %rem, %cond.false4 ], [ 5786, %cond.end ]
+  store i16 %cond6, i16* @v_39, align 1
+  %inc7 = add nsw i16 %i.07, 1
+  %cmp = icmp slt i16 %inc7, 111
+  br i1 %cmp, label %for.body, label %exit
+
+exit:                                 ; preds = %for.latch
+  %rv = load i16, i16* @v_39, align 1
+  ret i16 %rv
+}