[LV] Fix FindLastIV reduction for epilogue vectorization. (#120395)

Following 0e528ac, this patch adjusts
the resume value of VPReductionPHIRecipe for FindLastIV reductions.
Replacing the resume value with:

  ResumeValue = ResumeValue == StartValue ? SentinelValue : ResumeValue;

This addressed the correctness issue when the start value might not be
less than the minimum value of a monotonically increasing induction
variable.

Thanks Florian Hahn for the help.

---------

Co-authored-by: Florian Hahn <[email protected]>

Author: Mel-Chen

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7691,6 +7691,20 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
            "AnyOf expected to start by comparing main resume value to original "
            "start value");
     MainResumeValue = Cmp->getOperand(0);
+  } else if (RecurrenceDescriptor::isFindLastIVRecurrenceKind(
+                 RdxDesc.getRecurrenceKind())) {
+    using namespace llvm::PatternMatch;
+    Value *Cmp, *OrigResumeV;
+    bool IsExpectedPattern =
+        match(MainResumeValue, m_Select(m_OneUse(m_Value(Cmp)),
+                                        m_Specific(RdxDesc.getSentinelValue()),
+                                        m_Value(OrigResumeV))) &&
+        match(Cmp,
+              m_SpecificICmp(ICmpInst::ICMP_EQ, m_Specific(OrigResumeV),
+                             m_Specific(RdxDesc.getRecurrenceStartValue())));
+    assert(IsExpectedPattern && "Unexpected reduction resume pattern");
+    (void)IsExpectedPattern;
+    MainResumeValue = OrigResumeV;
   }
   PHINode *MainResumePhi = cast<PHINode>(MainResumeValue);
 
@@ -10413,6 +10427,19 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
             cast<Instruction>(ResumeV)->getParent()->getFirstNonPHI());
         ResumeV =
             Builder.CreateICmpNE(ResumeV, RdxDesc.getRecurrenceStartValue());
+      } else if (RecurrenceDescriptor::isFindLastIVRecurrenceKind(RK)) {
+        // VPReductionPHIRecipe for FindLastIV reductions requires an adjustment
+        // to the resume value. The resume value is adjusted to the sentinel
+        // value when the final value from the main vector loop equals the start
+        // value. This ensures correctness when the start value might not be
+        // less than the minimum value of a monotonically increasing induction
+        // variable.
+        IRBuilder<> Builder(
+            cast<Instruction>(ResumeV)->getParent()->getFirstNonPHI());
+        Value *Cmp =
+            Builder.CreateICmpEQ(ResumeV, RdxDesc.getRecurrenceStartValue());
+        ResumeV =
+            Builder.CreateSelect(Cmp, RdxDesc.getSentinelValue(), ResumeV);
       }
     } else {
       // Retrieve the induction resume values for wide inductions from

llvm/test/Transforms/LoopVectorize/epilog-iv-select-cmp.ll

@@ -40,7 +40,9 @@ define i64 @select_icmp_const(ptr %a, i64 %n) {
 ; CHECK-NEXT:    br i1 [[MIN_EPILOG_ITERS_CHECK]], label %[[VEC_EPILOG_SCALAR_PH]], label %[[VEC_EPILOG_PH]]
 ; CHECK:       [[VEC_EPILOG_PH]]:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], %[[VEC_EPILOG_ITER_CHECK]] ], [ 0, %[[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
-; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi i64 [ [[RDX_SELECT]], %[[VEC_EPILOG_ITER_CHECK]] ], [ 3, %[[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
+; CHECK-NEXT:    [[BC_MERGE_RDX1:%.*]] = phi i64 [ [[RDX_SELECT]], %[[VEC_EPILOG_ITER_CHECK]] ], [ 3, %[[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
+; CHECK-NEXT:    [[TMP14:%.*]] = icmp eq i64 [[BC_MERGE_RDX1]], 3
+; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = select i1 [[TMP14]], i64 -9223372036854775808, i64 [[BC_MERGE_RDX1]]
 ; CHECK-NEXT:    [[N_MOD_VF2:%.*]] = urem i64 [[N]], 4
 ; CHECK-NEXT:    [[N_VEC3:%.*]] = sub i64 [[N]], [[N_MOD_VF2]]
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <4 x i64> poison, i64 [[BC_RESUME_VAL]], i64 0
@@ -144,7 +146,9 @@ define i64 @select_fcmp_const_fast(ptr %a, i64 %n) {
 ; CHECK-NEXT:    br i1 [[MIN_EPILOG_ITERS_CHECK]], label %[[VEC_EPILOG_SCALAR_PH]], label %[[VEC_EPILOG_PH]]
 ; CHECK:       [[VEC_EPILOG_PH]]:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], %[[VEC_EPILOG_ITER_CHECK]] ], [ 0, %[[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
-; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi i64 [ [[RDX_SELECT]], %[[VEC_EPILOG_ITER_CHECK]] ], [ 2, %[[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
+; CHECK-NEXT:    [[BC_MERGE_RDX1:%.*]] = phi i64 [ [[RDX_SELECT]], %[[VEC_EPILOG_ITER_CHECK]] ], [ 2, %[[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
+; CHECK-NEXT:    [[TMP14:%.*]] = icmp eq i64 [[BC_MERGE_RDX1]], 2
+; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = select i1 [[TMP14]], i64 -9223372036854775808, i64 [[BC_MERGE_RDX1]]
 ; CHECK-NEXT:    [[N_MOD_VF2:%.*]] = urem i64 [[N]], 4
 ; CHECK-NEXT:    [[N_VEC3:%.*]] = sub i64 [[N]], [[N_MOD_VF2]]
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <4 x i64> poison, i64 [[BC_RESUME_VAL]], i64 0