diff options
| -rw-r--r-- | include/llvm/Analysis/ConstantFolding.h | 13 | ||||
| -rw-r--r-- | lib/Analysis/ConstantFolding.cpp | 21 |
2 files changed, 18 insertions, 16 deletions
diff --git a/include/llvm/Analysis/ConstantFolding.h b/include/llvm/Analysis/ConstantFolding.h index e2675eb2d4..f6b1f5ab99 100644 --- a/include/llvm/Analysis/ConstantFolding.h +++ b/include/llvm/Analysis/ConstantFolding.h @@ -7,7 +7,8 @@ // //===----------------------------------------------------------------------===// // -// This file declares routines for folding instructions into constants. +// This file declares routines for folding instructions into constants when all +// operands are constants, for example "sub i32 1, 0" -> "1". // // Also, to supplement the basic VMCore ConstantExpr simplifications, // this file declares some additional folding routines that can make use of @@ -27,11 +28,11 @@ namespace llvm { class Function; class Type; -/// ConstantFoldInstruction - Attempt to constant fold the specified -/// instruction. If successful, the constant result is returned, if not, null -/// is returned. Note that this function can only fail when attempting to fold -/// instructions like loads and stores, which have no constant expression form. -/// +/// ConstantFoldInstruction - Try to constant fold the specified instruction. +/// If successful, the constant result is returned, if not, null is returned. +/// Note that this fails if not all of the operands are constant. Otherwise, +/// this function can only fail when attempting to fold instructions like loads +/// and stores, which have no constant expression form. Constant *ConstantFoldInstruction(Instruction *I, const TargetData *TD = 0); /// ConstantFoldConstantExpression - Attempt to fold the constant expression diff --git a/lib/Analysis/ConstantFolding.cpp b/lib/Analysis/ConstantFolding.cpp index 82b406f204..9d92853c43 100644 --- a/lib/Analysis/ConstantFolding.cpp +++ b/lib/Analysis/ConstantFolding.cpp @@ -695,22 +695,23 @@ static Constant *SymbolicallyEvaluateGEP(Constant *const *Ops, unsigned NumOps, // Constant Folding public APIs //===----------------------------------------------------------------------===// - -/// ConstantFoldInstruction - Attempt to constant fold the specified -/// instruction. If successful, the constant result is returned, if not, null -/// is returned. Note that this function can only fail when attempting to fold -/// instructions like loads and stores, which have no constant expression form. -/// +/// ConstantFoldInstruction - Try to constant fold the specified instruction. +/// If successful, the constant result is returned, if not, null is returned. +/// Note that this fails if not all of the operands are constant. Otherwise, +/// this function can only fail when attempting to fold instructions like loads +/// and stores, which have no constant expression form. Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) { - // Handle PHI nodes specially here... + // Handle PHI nodes quickly here... if (PHINode *PN = dyn_cast<PHINode>(I)) { Constant *CommonValue = 0; for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { Value *Incoming = PN->getIncomingValue(i); - // If the incoming value is equal to the phi node itself or is undef then - // skip it. - if (Incoming == PN || isa<UndefValue>(Incoming)) + // If the incoming value is undef then skip it. Note that while we could + // skip the value if it is equal to the phi node itself we choose not to + // because that would break the rule that constant folding only applies if + // all operands are constants. + if (isa<UndefValue>(Incoming)) continue; // If the incoming value is not a constant, or is a different constant to // the one we saw previously, then give up. |