[PATCH] D142469: [KnownBits] Add blsi and blsmsk

[Jay Foad via Phabricator via llvm-commits]

foad created this revision.
Herald added subscribers: StephenFan, hiraditya.
Herald added a project: All.
foad requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D142469

Files:
  llvm/include/llvm/Support/KnownBits.h
  llvm/lib/Support/KnownBits.cpp


Index: llvm/lib/Support/KnownBits.cpp
===================================================================
--- llvm/lib/Support/KnownBits.cpp
+++ llvm/lib/Support/KnownBits.cpp
@@ -624,20 +624,23 @@
 }
 
 KnownBits KnownBits::blsi() const {
-  KnownBits Known(getBitWidth());
-  unsigned Min = countMinTrailingZeros();
+  unsigned BitWidth = getBitWidth();
+  KnownBits Known(Zero, APInt(BitWidth, 0));
   unsigned Max = countMaxTrailingZeros();
-  Known.Zero.setLowBits(Min);
-  Known.Zero.setBitsFrom(Max + 0);
+  Known.Zero.setBitsFrom(std::min(Max + 1, BitWidth));
+  unsigned Min = countMinTrailingZeros();
+  if (Max == Min && Max < BitWidth)
+    Known.One.setBit(Max);
   return Known;
 }
 
 KnownBits KnownBits::blsmsk() const {
-  KnownBits Known(getBitWidth());
-  unsigned Min = countMinTrailingZeros();
+  unsigned BitWidth = getBitWidth();
+  KnownBits Known(BitWidth);
   unsigned Max = countMaxTrailingZeros();
-  Known.One.setLowBits(Min + 1);
-  Known.Zero.setBitsFrom(Max + 0);
+  Known.Zero.setBitsFrom(std::min(Max + 1, BitWidth));
+  unsigned Min = countMinTrailingZeros();
+  Known.One.setLowBits(std::min(Min + 1, BitWidth));
   return Known;
 }
 
Index: llvm/include/llvm/Support/KnownBits.h
===================================================================
--- llvm/include/llvm/Support/KnownBits.h
+++ llvm/include/llvm/Support/KnownBits.h
@@ -422,10 +422,12 @@
     return KnownBits(Zero.reverseBits(), One.reverseBits());
   }
 
-  /// Compute known bits for X & -X.
+  /// Compute known bits for X & -X. The name comes from the X86 BMI instruction
+  /// BLSI.
   KnownBits blsi() const;
 
-  /// Compute known bits for X ^ (X - 1).
+  /// Compute known bits for X ^ (X - 1). The name comes from the X86 BMI
+  /// instruction BLSMSK.
   KnownBits blsmsk() const;
 
   bool operator==(const KnownBits &Other) const {


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