[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

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/unittests/Support/KnownBitsTest.cpp b/llvm/unittests/Support/KnownBitsTest.cpp
index f9631f2..d41402b 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;