[KnownBits][ValueTracking] Move the math for calculating known bits for add/sub into a static method in KnownBits object
I want to reuse this code in SimplifyDemandedBits handling of Add/Sub. This will make that easier.
Wonder if we should use it in SelectionDAG's computeKnownBits too.
Differential Revision: https://reviews.llvm.org/D36433
llvm-svn: 310378
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
new file mode 100644
index 0000000..ac790eb
--- /dev/null
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -0,0 +1,65 @@
+//===-- KnownBits.cpp - Stores known zeros/ones ---------------------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains a class for representing known zeros and ones used by
+// computeKnownBits.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/KnownBits.h"
+
+using namespace llvm;
+
+KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
+ const KnownBits &LHS, KnownBits RHS) {
+ // Carry in a 1 for a subtract, rather than 0.
+ bool CarryIn = false;
+ if (!Add) {
+ // Sum = LHS + ~RHS + 1
+ std::swap(RHS.Zero, RHS.One);
+ CarryIn = true;
+ }
+
+ APInt PossibleSumZero = ~LHS.Zero + ~RHS.Zero + CarryIn;
+ APInt PossibleSumOne = LHS.One + RHS.One + CarryIn;
+
+ // Compute known bits of the carry.
+ APInt CarryKnownZero = ~(PossibleSumZero ^ LHS.Zero ^ RHS.Zero);
+ APInt CarryKnownOne = PossibleSumOne ^ LHS.One ^ RHS.One;
+
+ // Compute set of known bits (where all three relevant bits are known).
+ APInt LHSKnownUnion = LHS.Zero | LHS.One;
+ APInt RHSKnownUnion = RHS.Zero | RHS.One;
+ APInt CarryKnownUnion = std::move(CarryKnownZero) | CarryKnownOne;
+ APInt Known = std::move(LHSKnownUnion) & RHSKnownUnion & CarryKnownUnion;
+
+ assert((PossibleSumZero & Known) == (PossibleSumOne & Known) &&
+ "known bits of sum differ");
+
+ // Compute known bits of the result.
+ KnownBits KnownOut;
+ KnownOut.Zero = ~std::move(PossibleSumZero) & Known;
+ KnownOut.One = std::move(PossibleSumOne) & Known;
+
+ // Are we still trying to solve for the sign bit?
+ if (!Known.isSignBitSet()) {
+ if (NSW) {
+ // Adding two non-negative numbers, or subtracting a negative number from
+ // a non-negative one, can't wrap into negative.
+ if (LHS.isNonNegative() && RHS.isNonNegative())
+ KnownOut.makeNonNegative();
+ // Adding two negative numbers, or subtracting a non-negative number from
+ // a negative one, can't wrap into non-negative.
+ else if (LHS.isNegative() && RHS.isNegative())
+ KnownOut.makeNegative();
+ }
+ }
+
+ return KnownOut;
+}