[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

1 files changed, 65 insertions, 0 deletions

diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
new file mode 100644
index 00000000000..ac790ebed35
--- /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;
+}