[ConstantRange] Introduce getMinSignedBits() method

Similar to the ConstantRange::getActiveBits(), and to similarly-named
methods in APInt, returns the bitwidth needed to represent
the given signed constant range

Author: LebedevRI

llvm/include/llvm/IR/ConstantRange.h

@@ -265,6 +265,10 @@ class LLVM_NODISCARD ConstantRange {
   /// in this range.
   unsigned getActiveBits() const;
 
+  /// Compute the maximal number of bits needed to represent every value
+  /// in this signed range.
+  unsigned getMinSignedBits() const;
+
   /// Subtract the specified constant from the endpoints of this constant range.
   ConstantRange subtract(const APInt &CI) const;
 

llvm/lib/IR/ConstantRange.cpp

@@ -420,6 +420,14 @@ unsigned ConstantRange::getActiveBits() const {
   return getUnsignedMax().getActiveBits();
 }
 
+unsigned ConstantRange::getMinSignedBits() const {
+  if (isEmptySet())
+    return 0;
+
+  return std::max(getSignedMin().getMinSignedBits(),
+                  getSignedMax().getMinSignedBits());
+}
+
 ConstantRange ConstantRange::subtract(const APInt &Val) const {
   assert(Val.getBitWidth() == getBitWidth() && "Wrong bit width");
   // If the set is empty or full, don't modify the endpoints.

llvm/unittests/IR/ConstantRangeTest.cpp

@@ -668,6 +668,32 @@ TEST_F(ConstantRangeTest, losslessUnsignedTruncationZeroext) {
   });
 }
 
+TEST_F(ConstantRangeTest, getMinSignedBits) {
+  unsigned Bits = 4;
+  EnumerateConstantRanges(Bits, [&](const ConstantRange &CR) {
+    unsigned Exact = 0;
+    ForeachNumInConstantRange(CR, [&](const APInt &N) {
+      Exact = std::max(Exact, N.getMinSignedBits());
+    });
+
+    unsigned ResultCR = CR.getMinSignedBits();
+    EXPECT_EQ(Exact, ResultCR);
+  });
+}
+TEST_F(ConstantRangeTest, losslessSignedTruncationSignext) {
+  unsigned Bits = 4;
+  EnumerateConstantRanges(Bits, [&](const ConstantRange &CR) {
+    unsigned MinBitWidth = CR.getMinSignedBits();
+    if (MinBitWidth == 0) {
+      EXPECT_TRUE(CR.isEmptySet());
+      return;
+    }
+    if (MinBitWidth == Bits)
+      return;
+    EXPECT_EQ(CR, CR.truncate(MinBitWidth).signExtend(Bits));
+  });
+}
+
 TEST_F(ConstantRangeTest, SubtractAPInt) {
   EXPECT_EQ(Full.subtract(APInt(16, 4)), Full);
   EXPECT_EQ(Empty.subtract(APInt(16, 4)), Empty);