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

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

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 8027c2706ee..127c3dcc35c 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -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);