[KnownBits] Add computeForAddCarry()

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358297 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/Support/KnownBits.h b/include/llvm/Support/KnownBits.h
index ed32cd2b576f2d1ad9cb8c601eac245721124330..07fd94e29a1fb6f4670359621bbee9c3557ee6ee 100644 (file)
--- a/include/llvm/Support/KnownBits.h
+++ b/include/llvm/Support/KnownBits.h
@@ -202,6 +202,10 @@ public:
     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/lib/Support/KnownBits.cpp b/lib/Support/KnownBits.cpp
index 9988314fabb9b0cc6af8dcc9956a43359c5213c9..a6c591fca31219fc85d776ff758fdba61e93a264 100644 (file)
--- a/lib/Support/KnownBits.cpp
+++ b/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/unittests/Support/CMakeLists.txt b/unittests/Support/CMakeLists.txt
index 12c983df50ccd1ffe72c025c3c601190f4418f40..b1cefddf9dee74fb9679b46f887af726c82705ce 100644 (file)
--- a/unittests/Support/CMakeLists.txt
+++ b/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/unittests/Support/KnownBitsTest.cpp b/unittests/Support/KnownBitsTest.cpp
new file mode 100644 (file)
index 0000000..c2b3b12
--- /dev/null
+++ b/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