[KnownBits] Add computeForAddCarry()

nikic · nikic · commit 7671fc71f62a · 2019-04-12T18:18:08.000Z
This is for D60460. computeForAddSub() essentially already supports carries because it has to deal with subtractions. This revision extracts a lower-level computeForAddCarry() function, which allows computing the known bits for add (carry known zero), sub (carry known one) and addcarry (carry unknown). As we don't seem to have any yet, I've added a unit test file for KnownBits and exhaustive tests for the new computeForAddCarry() functionality, as well the existing computeForAddSub() function. Differential Revision: https://reviews.llvm.org/D60522 llvm-svn: 358297
diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
@@ -202,6 +202,10 @@ struct KnownBits {
     return getBitWidth() - Zero.countPopulation();
   }
 
+  /// Compute known bits resulting from adding LHS, RHS and a 1-bit Carry.
+  static KnownBits computeForAddCarry(
+      const KnownBits &LHS, const KnownBits &RHS, const KnownBits &Carry);
+
   /// Compute known bits resulting from adding LHS and RHS.
   static KnownBits computeForAddSub(bool Add, bool NSW, const KnownBits &LHS,
                                     KnownBits RHS);
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
@@ -15,18 +15,14 @@
 
 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;
-  }
+static KnownBits computeForAddCarry(
+    const KnownBits &LHS, const KnownBits &RHS,
+    bool CarryZero, bool CarryOne) {
+  assert(!(CarryZero && CarryOne) &&
+         "Carry can't be zero and one at the same time");
 
-  APInt PossibleSumZero = ~LHS.Zero + ~RHS.Zero + CarryIn;
-  APInt PossibleSumOne = LHS.One + RHS.One + CarryIn;
+  APInt PossibleSumZero = ~LHS.Zero + ~RHS.Zero + !CarryZero;
+  APInt PossibleSumOne = LHS.One + RHS.One + CarryOne;
 
   // Compute known bits of the carry.
   APInt CarryKnownZero = ~(PossibleSumZero ^ LHS.Zero ^ RHS.Zero);
@@ -45,9 +41,32 @@ KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
   KnownBits KnownOut;
   KnownOut.Zero = ~std::move(PossibleSumZero) & Known;
   KnownOut.One = std::move(PossibleSumOne) & Known;
+  return KnownOut;
+}
+
+KnownBits KnownBits::computeForAddCarry(
+    const KnownBits &LHS, const KnownBits &RHS, const KnownBits &Carry) {
+  assert(Carry.getBitWidth() == 1 && "Carry must be 1-bit");
+  return ::computeForAddCarry(
+      LHS, RHS, Carry.Zero.getBoolValue(), Carry.One.getBoolValue());
+}
+
+KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
+                                      const KnownBits &LHS, KnownBits RHS) {
+  KnownBits KnownOut;
+  if (Add) {
+    // Sum = LHS + RHS + 0
+    KnownOut = ::computeForAddCarry(
+        LHS, RHS, /*CarryZero*/true, /*CarryOne*/false);
+  } else {
+    // Sum = LHS + ~RHS + 1
+    std::swap(RHS.Zero, RHS.One);
+    KnownOut = ::computeForAddCarry(
+        LHS, RHS, /*CarryZero*/false, /*CarryOne*/true);
+  }
 
   // Are we still trying to solve for the sign bit?
-  if (!Known.isSignBitSet()) {
+  if (!KnownOut.isNegative() && !KnownOut.isNonNegative()) {
     if (NSW) {
       // Adding two non-negative numbers, or subtracting a negative number from
       // a non-negative one, can't wrap into negative.
diff --git a/llvm/unittests/Support/CMakeLists.txt b/llvm/unittests/Support/CMakeLists.txt
@@ -34,6 +34,7 @@ add_llvm_unittest(SupportTests
   Host.cpp
   ItaniumManglingCanonicalizerTest.cpp
   JSONTest.cpp
+  KnownBitsTest.cpp
   LEB128Test.cpp
   LineIteratorTest.cpp
   LockFileManagerTest.cpp
diff --git a/llvm/unittests/Support/KnownBitsTest.cpp b/llvm/unittests/Support/KnownBitsTest.cpp
@@ -0,0 +1,130 @@
+//===- llvm/unittest/Support/KnownBitsTest.cpp - KnownBits tests ----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements unit tests for KnownBits functions.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/KnownBits.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+template<typename FnTy>
+void ForeachKnownBits(unsigned Bits, FnTy Fn) {
+  unsigned Max = 1 << Bits;
+  KnownBits Known(Bits);
+  for (unsigned Zero = 0; Zero < Max; ++Zero) {
+    for (unsigned One = 0; One < Max; ++One) {
+      Known.Zero = Zero;
+      Known.One = One;
+      if (Known.hasConflict())
+        continue;
+
+      Fn(Known);
+    }
+  }
+}
+
+template<typename FnTy>
+void ForeachNumInKnownBits(const KnownBits &Known, FnTy Fn) {
+  unsigned Bits = Known.getBitWidth();
+  unsigned Max = 1 << Bits;
+  for (unsigned N = 0; N < Max; ++N) {
+    APInt Num(Bits, N);
+    if ((Num & Known.Zero) != 0 || (~Num & Known.One) != 0)
+      continue;
+
+    Fn(Num);
+  }
+}
+
+TEST(KnownBitsTest, AddCarryExhaustive) {
+  unsigned Bits = 4;
+  ForeachKnownBits(Bits, [&](const KnownBits &Known1) {
+    ForeachKnownBits(Bits, [&](const KnownBits &Known2) {
+      ForeachKnownBits(1, [&](const KnownBits &KnownCarry) {
+        // Explicitly compute known bits of the addition by trying all
+        // possibilities.
+        KnownBits Known(Bits);
+        Known.Zero.setAllBits();
+        Known.One.setAllBits();
+        ForeachNumInKnownBits(Known1, [&](const APInt &N1) {
+          ForeachNumInKnownBits(Known2, [&](const APInt &N2) {
+            ForeachNumInKnownBits(KnownCarry, [&](const APInt &Carry) {
+              APInt Add = N1 + N2;
+              if (Carry.getBoolValue())
+                ++Add;
+
+              Known.One &= Add;
+              Known.Zero &= ~Add;
+            });
+          });
+        });
+
+        KnownBits KnownComputed = KnownBits::computeForAddCarry(
+            Known1, Known2, KnownCarry);
+        EXPECT_EQ(Known.Zero, KnownComputed.Zero);
+        EXPECT_EQ(Known.One, KnownComputed.One);
+      });
+    });
+  });
+}
+
+static void TestAddSubExhaustive(bool IsAdd) {
+  unsigned Bits = 4;
+  ForeachKnownBits(Bits, [&](const KnownBits &Known1) {
+    ForeachKnownBits(Bits, [&](const KnownBits &Known2) {
+      KnownBits Known(Bits), KnownNSW(Bits);
+      Known.Zero.setAllBits();
+      Known.One.setAllBits();
+      KnownNSW.Zero.setAllBits();
+      KnownNSW.One.setAllBits();
+
+      ForeachNumInKnownBits(Known1, [&](const APInt &N1) {
+        ForeachNumInKnownBits(Known2, [&](const APInt &N2) {
+          bool Overflow;
+          APInt Res;
+          if (IsAdd)
+            Res = N1.sadd_ov(N2, Overflow);
+          else
+            Res = N1.ssub_ov(N2, Overflow);
+
+          Known.One &= Res;
+          Known.Zero &= ~Res;
+
+          if (!Overflow) {
+            KnownNSW.One &= Res;
+            KnownNSW.Zero &= ~Res;
+          }
+        });
+      });
+
+      KnownBits KnownComputed = KnownBits::computeForAddSub(
+          IsAdd, /*NSW*/false, Known1, Known2);
+      EXPECT_EQ(Known.Zero, KnownComputed.Zero);
+      EXPECT_EQ(Known.One, KnownComputed.One);
+
+      // The NSW calculation is not precise, only check that it's
+      // conservatively correct.
+      KnownBits KnownNSWComputed = KnownBits::computeForAddSub(
+          IsAdd, /*NSW*/true, Known1, Known2);
+      EXPECT_TRUE(KnownNSWComputed.Zero.isSubsetOf(KnownNSW.Zero));
+      EXPECT_TRUE(KnownNSWComputed.One.isSubsetOf(KnownNSW.One));
+    });
+  });
+}
+
+TEST(KnownBitsTest, AddSubExhaustive) {
+  TestAddSubExhaustive(true);
+  TestAddSubExhaustive(false);
+}
+
+} // end anonymous namespace
