[InstCombine] Minor cleanup of cast simplification code [NFC]

Summary:
This patch cleans up parts of InstCombine to raise its compliance with the LLVM coding standards and to increase its readability. The changes and according rationale are summarized in the following:

- Rename `ShouldOptimizeCast()` to `shouldOptimizeCast()` since functions should start with a lower case letter.

- Move `shouldOptimizeCast()` from InstCombineCasts.cpp to InstCombineAndOrXor.cpp since it's only used there.

- Simplify interface of `shouldOptimizeCast()`.

- Minor code style adaptions in `shouldOptimizeCast()`.

- Remove the documentation on the function definition of `shouldOptimizeCast()` since it just repeats the documentation on its declaration. Also enhance the documentation on its declaration with more information describing its intended use and make it doxygen-compliant.

- Change a comment in `foldCastedBitwiseLogic()` from `fold (logic (cast A), (cast B)) -> (cast (logic A, B))` to `fold logic(cast(A), cast(B)) -> cast(logic(A, B))` since the surrounding comments use this format.

- Remove comment `Only do this if the casts both really cause code to be generated.` in `foldCastedBitwiseLogic()` since it just repeats parts of the documentation of `shouldOptimizeCast()` and does not help to improve readability.

- Simplify the interface of `isEliminableCastPair()`.

- Removed the documentation on the function definition of `isEliminableCastPair()` which only contained obvious statements about its implementation. Instead added more general doxygen-compliant documentation to its declaration.

- Renamed parameter `DoXform` of `transformZExtIcmp()` to `DoTransform` to make its intention clearer.

- Moved documentation of `transformZExtIcmp()` from its definition to its declaration and made it doxygen-compliant.

Reviewers: vtjnash, grosser

Subscribers: majnemer, llvm-commits

Differential Revision: https://reviews.llvm.org/D22449

Contributed-by: Matthias Reisinger
llvm-svn: 275964

1 files changed, 38 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index aa421ff594f..f199e1dc547 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -355,14 +355,16 @@ private:
                             SmallVectorImpl<Value *> &NewIndices);
   Instruction *FoldOpIntoSelect(Instruction &Op, SelectInst *SI);
 
-  /// \brief Classify whether a cast is worth optimizing.
+  /// Classify whether a cast is worth optimizing.
   ///
-  /// Returns true if the cast from "V to Ty" actually results in any code
-  /// being generated and is interesting to optimize out. If the cast can be
-  /// eliminated by some other simple transformation, we prefer to do the
-  /// simplification first.
-  bool ShouldOptimizeCast(Instruction::CastOps opcode, const Value *V,
-                          Type *Ty);
+  /// This is a helper to decide whether the simplification of
+  /// logic(cast(A), cast(B)) to cast(logic(A, B)) should be performed.
+  ///
+  /// \param CI The cast we are interested in.
+  ///
+  /// \return true if this cast actually results in any code being generated and
+  /// if it cannot already be eliminated by some other transformation.
+  bool shouldOptimizeCast(CastInst *CI);
 
   /// \brief Try to optimize a sequence of instructions checking if an operation
   /// on LHS and RHS overflows.
@@ -385,8 +387,22 @@ private:
   bool transformConstExprCastCall(CallSite CS);
   Instruction *transformCallThroughTrampoline(CallSite CS,
                                               IntrinsicInst *Tramp);
-  Instruction *transformZExtICmp(ICmpInst *ICI, Instruction &CI,
-                                 bool DoXform = true);
+
+  /// Transform (zext icmp) to bitwise / integer operations in order to
+  /// eliminate it.
+  ///
+  /// \param ICI The icmp of the (zext icmp) pair we are interested in.
+  /// \parem CI The zext of the (zext icmp) pair we are interested in.
+  /// \param DoTransform Pass false to just test whether the given (zext icmp)
+  /// would be transformed. Pass true to actually perform the transformation.
+  ///
+  /// \return null if the transformation cannot be performed. If the
+  /// transformation can be performed the new instruction that replaces the
+  /// (zext icmp) pair will be returned (if \p DoTransform is false the
+  /// unmodified \p ICI will be returned in this case).
+  Instruction *transformZExtICmp(ICmpInst *ICI, ZExtInst &CI,
+                                 bool DoTransform = true);
+
   Instruction *transformSExtICmp(ICmpInst *ICI, Instruction &CI);
   bool WillNotOverflowSignedAdd(Value *LHS, Value *RHS, Instruction &CxtI);
   bool WillNotOverflowSignedSub(Value *LHS, Value *RHS, Instruction &CxtI);
@@ -397,6 +413,19 @@ private:
   Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);
   Instruction *foldCastedBitwiseLogic(BinaryOperator &I);
 
+  /// Determine if a pair of casts can be replaced by a single cast.
+  ///
+  /// \param CI1 The first of a pair of casts.
+  /// \param CI2 The second of a pair of casts.
+  ///
+  /// \return 0 if the cast pair cannot be eliminated, otherwise returns an
+  /// Instruction::CastOps value for a cast that can replace the pair, casting
+  /// CI1->getSrcTy() to CI2->getDstTy().
+  ///
+  /// \see CastInst::isEliminableCastPair
+  Instruction::CastOps isEliminableCastPair(const CastInst *CI1,
+                                            const CastInst *CI2);
+
 public:
   /// \brief Inserts an instruction \p New before instruction \p Old
   ///