[KnownBits] Add a sextOrTrunc method

[qcolombet]

Summary

We already offer zextOrTrunc and it seems natural to offer the
same capability for sign extension.

This patch is a preparatory addition useful for future computeKnownBits
developments.

[lebedev.ri]

Could use a test, but it generally makes sense to have such a method.

[RKSimon]

LGTM

[qcolombet]

• Add a test for the new method

[qcolombet]

Hi @lebedev.ri ,

Could use a test, but it generally makes sense to have such a method.

I didn't see a test for zextOrTrunc that I could mimic so I come up with something.

Let me know what do you think.

Cheers,
-Quentin

[lebedev.ri]

Seems fine

llvm/unittests/Support/KnownBitsTest.cpp

205 ↗

(On Diff #296807)

I guess i was thinking of something like this:

TEST(KnownBitsTest, GetMinMaxVal) {
  const unsigned NarrowerSize = 4;
  const unsigned BaseSize = 6;
  const unsigned WiderSize = 8;
  ForeachKnownBits(BaseSize, [&](const KnownBits &KnownBase) {
    for (unsigned Size : {NarrowerSize, BaseSize, WiderSize}) {
      KnownBits Test;
      if(Size == BaseSize)
        Test = KnownBase
      else
        Test = Size < BaseSize ? KnownBase.trunc(Size) :  KnownBase.sext(Size);
      EXPECT_EQ(Test, KnownBase.sextOrTrunc(Size));
    }
  });
}

which seems pretty tautological i guess, so maybe we should go with your test.

llvm/include/llvm/Support/KnownBits.h

public:
   KnownBits zextOrTrunc(unsigned BitWidth) const {
     if (BitWidth > getBitWidth())
       return zext(BitWidth);
     if (BitWidth < getBitWidth())
       return trunc(BitWidth);
     return *this;
   }
 
+  /// Return known bits for a sign extension or truncation of the value we're
+  /// tracking.
+  KnownBits sextOrTrunc(unsigned BitWidth) const {
+    if (BitWidth > getBitWidth())
+      return sext(BitWidth);
+    if (BitWidth < getBitWidth())
+      return trunc(BitWidth);
+    return *this;
+  }
+
   /// Return a KnownBits with the extracted bits
   /// [bitPosition,bitPosition+numBits).
   KnownBits extractBits(unsigned NumBits, unsigned BitPosition) const {
     return KnownBits(Zero.extractBits(NumBits, BitPosition),
                      One.extractBits(NumBits, BitPosition));
   }
 
   /// Return KnownBits based on this, but updated given that the underlying