[llvm] db81324 - [Support] Add KnownBits::countMaxSignedBits(). Make KnownBits::countMinSignBits() always return at least 1.

[Craig Topper via llvm-commits]

Author: Craig Topper
Date: 2022-01-02T23:27:16-08:00
New Revision: db81324c8da56cd498b58d0081b49b08095ec534

URL: https://github.com/llvm/llvm-project/commit/db81324c8da56cd498b58d0081b49b08095ec534
DIFF: https://github.com/llvm/llvm-project/commit/db81324c8da56cd498b58d0081b49b08095ec534.diff

LOG: [Support] Add KnownBits::countMaxSignedBits(). Make KnownBits::countMinSignBits() always return at least 1.

Even if we don't have any known bits, we can assume that there is
at least 1 sign bit. This is consistent with ComputeNumSignBits
which always returns at least 1.

Add KnownBits::countMaxSignedBits() which computes the number of
bits needed to represent all signed values with those known bits.
This is the signed equivalent of countMaxActiveBits().

Split from D116469.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D116500

Added: 
    

Modified: 
    llvm/include/llvm/Support/KnownBits.h
    llvm/unittests/Support/KnownBitsTest.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index 1f32760a6fd1b..1674bad21e951 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -249,7 +249,14 @@ struct KnownBits {
       return countMinLeadingZeros();
     if (isNegative())
       return countMinLeadingOnes();
-    return 0;
+    // Every value has at least 1 sign bit.
+    return 1;
+  }
+
+  /// Returns the maximum number of bits needed to represent all possible
+  /// signed values with these known bits.
+  unsigned countMaxSignedBits() const {
+    return getBitWidth() - countMinSignBits() + 1;
   }
 
   /// Returns the maximum number of trailing zero bits possible.

diff  --git a/llvm/unittests/Support/KnownBitsTest.cpp b/llvm/unittests/Support/KnownBitsTest.cpp
index f9631f29902f5..d41402b69fc0c 100644
--- a/llvm/unittests/Support/KnownBitsTest.cpp
+++ b/llvm/unittests/Support/KnownBitsTest.cpp
@@ -442,6 +442,17 @@ TEST(KnownBitsTest, CountMaxActiveBits) {
   });
 }
 
+TEST(KnownBitsTest, CountMaxSignedBits) {
+  unsigned Bits = 4;
+  ForeachKnownBits(Bits, [&](const KnownBits &Known) {
+    unsigned Expected = 0;
+    ForeachNumInKnownBits(Known, [&](const APInt &N) {
+      Expected = std::max(Expected, N.getMinSignedBits());
+    });
+    EXPECT_EQ(Expected, Known.countMaxSignedBits());
+  });
+}
+
 TEST(KnownBitsTest, SExtOrTrunc) {
   const unsigned NarrowerSize = 4;
   const unsigned BaseSize = 6;