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[ValueTracking] Fix recurrence detection to check both PHI operands.

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Summary:
Currently we fail to compute known bits for recurrences where the
first incoming value is the start value of the recurrence.

Instead of exiting the loop when the first incoming value is not
the step of the recurrence, continue to check the second incoming
value.

The original code uses a loop to handle both cases, but incorrectly
exits instead of continuing.

Reviewers: lebedev.ri, spatel, nikic

Reviewed By: lebedev.ri

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66216

llvm-svn: 369088
This commit is contained in:
Florian Hahn 2019-08-16 09:15:02 +00:00
parent 41b64b38c5
commit 6ecdafe314
2 changed files with 3 additions and 3 deletions
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2
lib/Analysis/ValueTracking.cpp
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@ -1373,7 +1373,7 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
else if (LR == I)
L = LL;
else
break;
continue; // Check for recurrence with L and R flipped.
// Ok, we have a PHI of the form L op= R. Check for low
// zero bits.
computeKnownBits(R, Known2, Depth + 1, Q);

4
test/Transforms/InstCombine/phi-known-bits-operand-order.ll
View File

@ -19,8 +19,8 @@ define void @phi_recurrence_start_first() {
; CHECK-NEXT: br i1 [[COND_V]], label [[FOR_COND11:%.*]], label [[FOR_COND26]]
; CHECK: for.cond11:
; CHECK-NEXT: [[I_1:%.*]] = phi i32 [ [[START]], [[IF_THEN]] ], [ [[STEP:%.*]], [[FOR_COND11]] ]
; CHECK-NEXT: [[CMP13:%.*]] = icmp slt i32 [[I_1]], 100
; CHECK-NEXT: [[STEP]] = add nsw i32 [[I_1]], 1
; CHECK-NEXT: [[CMP13:%.*]] = icmp ult i32 [[I_1]], 100
; CHECK-NEXT: [[STEP]] = add nuw nsw i32 [[I_1]], 1
; CHECK-NEXT: br i1 [[CMP13]], label [[FOR_COND11]], label [[WHILE_END]]
; CHECK: for.cond26:
; CHECK-NEXT: br label [[WHILE_COND]]