[ValueTracking] Add basic computeKnownBits support for llvm.abs intrinsic

This includes basic support for computeKnownBits on abs. I've left FIXMEs for more complicated things we could do.

Differential Revision: https://reviews.llvm.org/D84963

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 3bf09b3..3b8c0fc 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1642,6 +1642,17 @@ static void computeKnownBitsFromOperator(const Operator *I,
     if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       default: break;
+      case Intrinsic::abs:
+        computeKnownBits(I->getOperand(0), Known2, Depth + 1, Q);
+        // Otherwise, if this call is undefined for INT_MIN, the result is
+        // positive.
+        if (match(II->getArgOperand(1), m_One()))
+          Known.Zero.setSignBit();
+        // Absolute value preserves trailing zero count.
+        Known.Zero.setLowBits(Known2.Zero.countTrailingOnes());
+        // FIXME: Handle known negative/non-negative input?
+        // FIXME: Calculate the negated Known bits and combine them?
+        break;
       case Intrinsic::bitreverse:
         computeKnownBits(I->getOperand(0), DemandedElts, Known2, Depth + 1, Q);
         Known.Zero |= Known2.Zero.reverseBits();