Recommit r333226 "[ValueTracking] Teach computeKnownBits that the result of an absolute value pattern that uses nsw flag is always positive."

Libfuzzer tests have been fixed to prevent being optimized.

Original commit message:

If the nsw flag is used in the absolute value then it is undefined for INT_MIN. For all other value it will produce a positive number. So we can assume the result is positive.

This breaks some InstCombine abs/nabs combining tests because we simplify the second compare from known bits rather than as the whole pattern. Looks like we can probably fix it by adding a neg+abs/nabs combine to just swap the select operands. N

Differential Revision: https://reviews.llvm.org/D47041

llvm-svn: 333300

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 7f33c67..5f5d1be 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1078,6 +1078,12 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
       // leading zero bits.
       MaxHighZeros =
           std::max(Known.countMinLeadingZeros(), Known2.countMinLeadingZeros());
+    } else if (SPF == SPF_ABS) {
+      // RHS from matchSelectPattern returns the negation part of abs pattern.
+      // If the negate has an NSW flag we can assume the sign bit of the result
+      // will be 0 because that makes abs(INT_MIN) undefined.
+      if (cast<Instruction>(RHS)->hasNoSignedWrap())
+        MaxHighZeros = 1;
     }
 
     // Only known if known in both the LHS and RHS.