[ValueTracking] Avoid redundant known bits calculation in computeOverflowForSignedAdd()

We're already computing the known bits of the operands here. If the
known bits of the operands can determine the sign bit of the result,
we'll already catch this in signedAddMayOverflow(). The only other
way (and as the comment already indicates) we'll get new information
from computing known bits on the whole add, is if there's an assumption
on it.

As such, we change the code to only compute known bits from assumptions,
instead of computing full known bits on the add (which would unnecessarily
recompute the known bits of the operands as well).

Differential Revision: https://reviews.llvm.org/D59473

llvm-svn: 356785

Author: nikic

llvm/lib/Analysis/ValueTracking.cpp

@@ -4146,19 +4146,21 @@ static OverflowResult computeOverflowForSignedAdd(const Value *LHS,
     return OverflowResult::MayOverflow;
 
   // If the sign of Add is the same as at least one of the operands, this add
-  // CANNOT overflow. This is particularly useful when the sum is
-  // @llvm.assume'ed non-negative rather than proved so from analyzing its
-  // operands.
+  // CANNOT overflow. If this can be determined from the known bits of the
+  // operands the above signedAddMayOverflow() check will have already done so.
+  // The only other way to improve on the known bits is from an assumption, so
+  // call computeKnownBitsFromAssume() directly.
   bool LHSOrRHSKnownNonNegative =
       (LHSKnown.isNonNegative() || RHSKnown.isNonNegative());
   bool LHSOrRHSKnownNegative =
       (LHSKnown.isNegative() || RHSKnown.isNegative());
   if (LHSOrRHSKnownNonNegative || LHSOrRHSKnownNegative) {
-    KnownBits AddKnown = computeKnownBits(Add, DL, /*Depth=*/0, AC, CxtI, DT);
+    KnownBits AddKnown(LHSKnown.getBitWidth());
+    computeKnownBitsFromAssume(
+        Add, AddKnown, /*Depth=*/0, Query(DL, AC, CxtI, DT, true));
     if ((AddKnown.isNonNegative() && LHSOrRHSKnownNonNegative) ||
-        (AddKnown.isNegative() && LHSOrRHSKnownNegative)) {
+        (AddKnown.isNegative() && LHSOrRHSKnownNegative))
       return OverflowResult::NeverOverflows;
-    }
   }
 
   return OverflowResult::MayOverflow;