[LoopBoundSplit] Check if latch is operand of PHI induction variable

[ParkHanbum]

A problem occurs when conditionally comparing the PHI derived variable from the outer loop with ICmp in the inner loop of a nested loop. This is because the latch in the inner loop is not an operand of the outer loop PHI, causing an assert to occur when getIncomingValueForBlock is called.

The issue is resolved by verifying whether the latch is an operand of the current PHI.

Fixed: #156793, #61655

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: hanbeom (ParkHanbum)

Changes

A problem occurs when conditionally comparing the PHI derived variable from the outer loop with ICmp in the inner loop of a nested loop. This is because the latch in the inner loop is not an operand of the outer loop PHI, causing an assert to occur when getIncomingValueForBlock is called.

The issue is resolved by verifying whether the latch is an operand of the current PHI.

Fixed: #156793, #61655

---

Full diff: https://github.com/llvm/llvm-project/pull/167227.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/LoopBoundSplit.cpp (+3-1)
• (modified) llvm/test/Transforms/LoopBoundSplit/loop-bound-split.ll (+83)

diff --git a/llvm/lib/Transforms/Scalar/LoopBoundSplit.cpp b/llvm/lib/Transforms/Scalar/LoopBoundSplit.cpp
index 73f1942849ac2..e1859509ccbbd 100644
--- a/llvm/lib/Transforms/Scalar/LoopBoundSplit.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopBoundSplit.cpp
@@ -72,7 +72,9 @@ static void analyzeICmp(ScalarEvolution &SE, ICmpInst *ICmp,
     // value from backedge.
     if (Cond.AddRecSCEV && isa<PHINode>(Cond.AddRecValue)) {
       PHINode *PN = cast<PHINode>(Cond.AddRecValue);
-      Cond.NonPHIAddRecValue = PN->getIncomingValueForBlock(L.getLoopLatch());
+      int Idx = PN->getBasicBlockIndex(L.getLoopLatch());
+      if (Idx >= 0)
+        Cond.NonPHIAddRecValue = PN->getIncomingValue(Idx);
     }
   }
 }
diff --git a/llvm/test/Transforms/LoopBoundSplit/loop-bound-split.ll b/llvm/test/Transforms/LoopBoundSplit/loop-bound-split.ll
index 433b0f051598e..cbfd03964b574 100644
--- a/llvm/test/Transforms/LoopBoundSplit/loop-bound-split.ll
+++ b/llvm/test/Transforms/LoopBoundSplit/loop-bound-split.ll
@@ -955,3 +955,86 @@ exit:
 }
 
 declare i64 @llvm.umax.i64(i64, i64)
+
+define void @outer_iv_used_within_inner_icmp(i32 %k) {
+; CHECK-LABEL: @outer_iv_used_within_inner_icmp(
+; CHECK-NEXT:  for.body.lr.ph:
+; CHECK-NEXT:    [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[K:%.*]], i32 1)
+; CHECK-NEXT:    [[TMP0:%.*]] = add nsw i32 [[SMAX]], -1
+; CHECK-NEXT:    [[NEW_BOUND:%.*]] = call i32 @llvm.smin.i32(i32 [[TMP0]], i32 2)
+; CHECK-NEXT:    br label [[OUTER_HEADER:%.*]]
+; CHECK:       outer.header:
+; CHECK-NEXT:    [[J:%.*]] = phi i32 [ 0, [[FOR_BODY_LR_PH:%.*]] ], [ [[J_INC:%.*]], [[OUTER_INC:%.*]] ]
+; CHECK-NEXT:    br label [[OUTER_HEADER_SPLIT:%.*]]
+; CHECK:       outer.header.split:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[I_05:%.*]] = phi i32 [ 0, [[OUTER_HEADER_SPLIT]] ], [ [[INC:%.*]], [[FOR_INC:%.*]] ]
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp ult i32 [[J]], 2
+; CHECK-NEXT:    br i1 true, label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       if.else:
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       for.inc:
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[I_05]], 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[INC]], [[NEW_BOUND]]
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY]], label [[OUTER_HEADER_SPLIT_SPLIT:%.*]]
+; CHECK:       outer.header.split.split:
+; CHECK-NEXT:    [[I_05_LCSSA:%.*]] = phi i32 [ [[INC]], [[FOR_INC]] ]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne i32 [[I_05_LCSSA]], [[K]]
+; CHECK-NEXT:    br i1 [[TMP1]], label [[FOR_BODY_SPLIT_PREHEADER:%.*]], label [[OUTER_INC]]
+; CHECK:       for.body.split.preheader:
+; CHECK-NEXT:    br label [[FOR_BODY_SPLIT:%.*]]
+; CHECK:       for.body.split:
+; CHECK-NEXT:    [[I_05_SPLIT:%.*]] = phi i32 [ [[INC_SPLIT:%.*]], [[FOR_INC_SPLIT:%.*]] ], [ [[I_05_LCSSA]], [[FOR_BODY_SPLIT_PREHEADER]] ]
+; CHECK-NEXT:    [[CMP1_SPLIT:%.*]] = icmp ult i32 [[J]], 2
+; CHECK-NEXT:    br i1 false, label [[IF_THEN_SPLIT:%.*]], label [[IF_ELSE_SPLIT:%.*]]
+; CHECK:       if.else.split:
+; CHECK-NEXT:    br label [[FOR_INC_SPLIT]]
+; CHECK:       if.then.split:
+; CHECK-NEXT:    br label [[FOR_INC_SPLIT]]
+; CHECK:       for.inc.split:
+; CHECK-NEXT:    [[INC_SPLIT]] = add nsw i32 [[I_05_SPLIT]], 1
+; CHECK-NEXT:    [[CMP_SPLIT:%.*]] = icmp slt i32 [[INC_SPLIT]], [[K]]
+; CHECK-NEXT:    br i1 [[CMP_SPLIT]], label [[FOR_BODY_SPLIT]], label [[OUTER_INC_LOOPEXIT:%.*]]
+; CHECK:       outer.inc.loopexit:
+; CHECK-NEXT:    br label [[OUTER_INC]]
+; CHECK:       outer.inc:
+; CHECK-NEXT:    [[J_INC]] = add nsw i32 [[J]], 1
+; CHECK-NEXT:    [[OUTER_CMP:%.*]] = icmp slt i32 [[J_INC]], [[K]]
+; CHECK-NEXT:    br i1 [[OUTER_CMP]], label [[OUTER_HEADER]], label [[FOR_END:%.*]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+for.body.lr.ph:
+  br label %outer.header
+
+outer.header:
+  %j = phi i32 [ 0, %for.body.lr.ph ], [ %j.inc, %outer.inc ]
+  br label %for.body
+
+for.body:
+  %i.05 = phi i32 [ 0, %outer.header ], [ %inc, %for.inc ]
+  %cmp1 = icmp ult i32 %j, 2
+  br i1 %cmp1, label %if.then, label %if.else
+
+if.then:
+  br label %for.inc
+
+if.else:
+  br label %for.inc
+
+for.inc:
+  %inc = add nsw i32 %i.05, 1
+  %cmp = icmp slt i32 %inc, %k
+  br i1 %cmp, label %for.body, label %outer.inc
+
+outer.inc:
+  %j.inc = add nsw i32 %j, 1
+  %outer.cmp = icmp slt i32 %j.inc, %k
+  br i1 %outer.cmp, label %outer.header, label %for.end
+
+for.end:
+  ret void
+}