[PATCH] D59473: [ValueTracking] Avoid redundant known bits calculation in computeOverflowForSignedAdd()

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: spatel, lebedev.ri.
Herald added subscribers: llvm-commits, jdoerfert, hiraditya.
Herald added a project: LLVM.

This is a tangent to D59386 <https://reviews.llvm.org/D59386>, which got me thinking about the performance of this code...

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 the ripple logic. 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).


Repository:
  rL LLVM

https://reviews.llvm.org/D59473

Files:
  llvm/lib/Analysis/ValueTracking.cpp


Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -4182,19 +4182,19 @@
     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. Check if there is an assumption about the sign of the
+  // result.
   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;


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