[ConstantRange] Add fromKnownBits() method

Following the suggestion in D59450, I'm moving the code for constructing
a ConstantRange from KnownBits out of ValueTracking, which also allows us
to test this code independently.

I'm adding this method to ConstantRange rather than KnownBits (which
would have been a bit nicer API wise) to avoid creating a dependency
from Support to IR, where ConstantRange lives.

Differential Revision: https://reviews.llvm.org/D59475

llvm-svn: 356339

llvm/include/llvm/IR/ConstantRange.h

@@ -41,6 +41,7 @@ namespace llvm {
 
 class MDNode;
 class raw_ostream;
+struct KnownBits;
 
 /// This class represents a range of values.
 class LLVM_NODISCARD ConstantRange {
@@ -58,6 +59,11 @@ public:
   /// assert out if the two APInt's are not the same bit width.
   ConstantRange(APInt Lower, APInt Upper);
 
+  /// Initialize a range based on a known bits constraint. The IsSigned flag
+  /// indicates whether the constant range should not wrap in the signed or
+  /// unsigned domain.
+  static ConstantRange fromKnownBits(const KnownBits &Known, bool IsSigned);
+
   /// Produce the smallest range such that all values that may satisfy the given
   /// predicate with any value contained within Other is contained in the
   /// returned range.  Formally, this returns a superset of

llvm/lib/Analysis/ValueTracking.cpp

@@ -4077,13 +4077,6 @@ static OverflowResult mapOverflowResult(ConstantRange::OverflowResult OR) {
   llvm_unreachable("Unknown OverflowResult");
 }
 
-static ConstantRange constantRangeFromKnownBits(const KnownBits &Known) {
-  if (Known.isUnknown())
-    return ConstantRange(Known.getBitWidth(), /* full */ true);
-
-  return ConstantRange(Known.One, ~Known.Zero + 1);
-}
-
 OverflowResult llvm::computeOverflowForUnsignedAdd(
     const Value *LHS, const Value *RHS, const DataLayout &DL,
     AssumptionCache *AC, const Instruction *CxtI, const DominatorTree *DT,
@@ -4092,8 +4085,10 @@ OverflowResult llvm::computeOverflowForUnsignedAdd(
                                         nullptr, UseInstrInfo);
   KnownBits RHSKnown = computeKnownBits(RHS, DL, /*Depth=*/0, AC, CxtI, DT,
                                         nullptr, UseInstrInfo);
-  ConstantRange LHSRange = constantRangeFromKnownBits(LHSKnown);
-  ConstantRange RHSRange = constantRangeFromKnownBits(RHSKnown);
+  ConstantRange LHSRange =
+      ConstantRange::fromKnownBits(LHSKnown, /*signed*/ false);
+  ConstantRange RHSRange =
+      ConstantRange::fromKnownBits(RHSKnown, /*signed*/ false);
   return mapOverflowResult(LHSRange.unsignedAddMayOverflow(RHSRange));
 }
 
@@ -4208,8 +4203,10 @@ OverflowResult llvm::computeOverflowForUnsignedSub(const Value *LHS,
                                                    const DominatorTree *DT) {
   KnownBits LHSKnown = computeKnownBits(LHS, DL, /*Depth=*/0, AC, CxtI, DT);
   KnownBits RHSKnown = computeKnownBits(RHS, DL, /*Depth=*/0, AC, CxtI, DT);
-  ConstantRange LHSRange = constantRangeFromKnownBits(LHSKnown);
-  ConstantRange RHSRange = constantRangeFromKnownBits(RHSKnown);
+  ConstantRange LHSRange =
+      ConstantRange::fromKnownBits(LHSKnown, /*signed*/ false);
+  ConstantRange RHSRange =
+      ConstantRange::fromKnownBits(RHSKnown, /*signed*/ false);
   return mapOverflowResult(LHSRange.unsignedSubMayOverflow(RHSRange));
 }
 

llvm/lib/IR/ConstantRange.cpp

@@ -31,6 +31,7 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
@@ -53,6 +54,24 @@ ConstantRange::ConstantRange(APInt L, APInt U)
          "Lower == Upper, but they aren't min or max value!");
 }
 
+ConstantRange ConstantRange::fromKnownBits(const KnownBits &Known,
+                                           bool IsSigned) {
+  if (Known.isUnknown())
+    return ConstantRange(Known.getBitWidth(), /* full */ true);
+
+  // For unsigned ranges, or signed ranges with known sign bit, create a simple
+  // range between the smallest and largest possible value.
+  if (!IsSigned || Known.isNegative() || Known.isNonNegative())
+    return ConstantRange(Known.One, ~Known.Zero + 1);
+
+  // If we don't know the sign bit, pick the lower bound as a negative number
+  // and the upper bound as a non-negative one.
+  APInt Lower = Known.One, Upper = ~Known.Zero;
+  Lower.setSignBit();
+  Upper.clearSignBit();
+  return ConstantRange(Lower, Upper + 1);
+}
+
 ConstantRange ConstantRange::makeAllowedICmpRegion(CmpInst::Predicate Pred,
                                                    const ConstantRange &CR) {
   if (CR.isEmptySet())

llvm/unittests/IR/ConstantRangeTest.cpp

@@ -9,6 +9,7 @@
 #include "llvm/IR/ConstantRange.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Operator.h"
+#include "llvm/Support/KnownBits.h"
 #include "gtest/gtest.h"
 
 using namespace llvm;
@@ -1406,4 +1407,68 @@ TEST_F(ConstantRangeTest, SignedSubOverflowExhautive) {
       });
 }
 
+TEST_F(ConstantRangeTest, FromKnownBits) {
+  KnownBits Unknown(16);
+  EXPECT_EQ(Full, ConstantRange::fromKnownBits(Unknown, /*signed*/false));
+  EXPECT_EQ(Full, ConstantRange::fromKnownBits(Unknown, /*signed*/true));
+
+  // .10..01. -> unsigned 01000010 (66)  to 11011011 (219)
+  //          -> signed   11000010 (194) to 01011011 (91)
+  KnownBits Known(8);
+  Known.Zero = 36;
+  Known.One = 66;
+  ConstantRange Unsigned(APInt(8, 66), APInt(8, 219 + 1));
+  ConstantRange Signed(APInt(8, 194), APInt(8, 91 + 1));
+  EXPECT_EQ(Unsigned, ConstantRange::fromKnownBits(Known, /*signed*/false));
+  EXPECT_EQ(Signed, ConstantRange::fromKnownBits(Known, /*signed*/true));
+
+  // 1.10.10. -> 10100100 (164) to 11101101 (237)
+  Known.Zero = 18;
+  Known.One = 164;
+  ConstantRange CR1(APInt(8, 164), APInt(8, 237 + 1));
+  EXPECT_EQ(CR1, ConstantRange::fromKnownBits(Known, /*signed*/false));
+  EXPECT_EQ(CR1, ConstantRange::fromKnownBits(Known, /*signed*/true));
+
+  // 01.0.1.0 -> 01000100 (68) to 01101110 (110)
+  Known.Zero = 145;
+  Known.One = 68;
+  ConstantRange CR2(APInt(8, 68), APInt(8, 110 + 1));
+  EXPECT_EQ(CR2, ConstantRange::fromKnownBits(Known, /*signed*/false));
+  EXPECT_EQ(CR2, ConstantRange::fromKnownBits(Known, /*signed*/true));
+}
+
+TEST_F(ConstantRangeTest, FromKnownBitsExhaustive) {
+  unsigned Bits = 4;
+  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() || Known.isUnknown())
+        continue;
+
+      APInt MinUnsigned = APInt::getMaxValue(Bits);
+      APInt MaxUnsigned = APInt::getMinValue(Bits);
+      APInt MinSigned = APInt::getSignedMaxValue(Bits);
+      APInt MaxSigned = APInt::getSignedMinValue(Bits);
+      for (unsigned N = 0; N < Max; ++N) {
+        APInt Num(Bits, N);
+        if ((Num & Known.Zero) != 0 || (~Num & Known.One) != 0)
+          continue;
+
+        if (Num.ult(MinUnsigned)) MinUnsigned = Num;
+        if (Num.ugt(MaxUnsigned)) MaxUnsigned = Num;
+        if (Num.slt(MinSigned)) MinSigned = Num;
+        if (Num.sgt(MaxSigned)) MaxSigned = Num;
+      }
+
+      ConstantRange UnsignedCR(MinUnsigned, MaxUnsigned + 1);
+      ConstantRange SignedCR(MinSigned, MaxSigned + 1);
+      EXPECT_EQ(UnsignedCR, ConstantRange::fromKnownBits(Known, false));
+      EXPECT_EQ(SignedCR, ConstantRange::fromKnownBits(Known, true));
+    }
+  }
+}
+
 }  // anonymous namespace