diff options
| -rw-r--r-- | include/llvm/Instruction.h | 21 |
1 files changed, 17 insertions, 4 deletions
diff --git a/include/llvm/Instruction.h b/include/llvm/Instruction.h index 1d1f0f4..924b195 100644 --- a/include/llvm/Instruction.h +++ b/include/llvm/Instruction.h @@ -182,19 +182,32 @@ public: /// mayHaveSideEffects - Return true if the instruction may have side effects. /// + /// Note that this does not consider malloc and alloca to have side + /// effects because the newly allocated memory is completely invisible to + /// instructions which don't used the returned value. For cases where this + /// matters, isSafeToSpeculativelyExecute may be more appropriate. bool mayHaveSideEffects() const { return mayWriteToMemory() || mayThrow(); } /// isSafeToSpeculativelyExecute - Return true if the instruction does not /// have any effects besides calculating the result and does not have - /// undefined behavior. Unlike in mayHaveSideEffects(), allocating memory - /// is considered an effect. + /// undefined behavior. + /// + /// This method never returns true for an instruction that returns true for + /// mayHaveSideEffects; however, this method also does some other checks in + /// addition. It checks for undefined behavior, like dividing by zero or + /// loading from an invalid pointer (but not for undefined results, like a + /// shift with a shift amount larger than the width of the result). It checks + /// for malloc and alloca because speculatively executing them might cause a + /// memory leak. It also returns false for instructions related to control + /// flow, specifically terminators and PHI nodes. /// /// This method only looks at the instruction itself and its operands, so if /// this method returns true, it is safe to move the instruction as long as - /// the operands still dominate it. However, care must be taken with - /// instructions which read memory. + /// the correct dominance relationships for the operands and users hold. + /// However, this method can return true for instructions that read memory; + /// for such instructions, moving them may change the resulting value. bool isSafeToSpeculativelyExecute() const; /// Methods for support type inquiry through isa, cast, and dyn_cast: |