[ValueTracking][InstCombine] Introduce and use ComputeMinSignedBits

This introduces a new ComputeMinSignedBits method for ValueTracking that
returns the BitWidth - SignBits + 1 from ComputeSignBits, and represents
the minimum bit size for the value as a signed integer.  Similar to the
existing APInt::getMinSignedBits method, this can make some of the
reasoning around ComputeSignBits more natural.

See https://reviews.llvm.org/D112298

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 0585a63..0e07017 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -396,6 +396,14 @@ unsigned llvm::ComputeNumSignBits(const Value *V, const DataLayout &DL,
       V, Depth, Query(DL, AC, safeCxtI(V, CxtI), DT, UseInstrInfo));
 }
 
+unsigned llvm::ComputeMinSignedBits(const Value *V, const DataLayout &DL,
+                                    unsigned Depth, AssumptionCache *AC,
+                                    const Instruction *CxtI,
+                                    const DominatorTree *DT) {
+  unsigned SignBits = ComputeNumSignBits(V, DL, Depth, AC, CxtI, DT);
+  return V->getType()->getScalarSizeInBits() - SignBits + 1;
+}
+
 static void computeKnownBitsAddSub(bool Add, const Value *Op0, const Value *Op1,
                                    bool NSW, const APInt &DemandedElts,
                                    KnownBits &KnownOut, KnownBits &Known2,