[LLVMdev] isa and friends as an alternative to dynamic cast?

[Chris Lattner]

On Thu, 21 Apr 2005, Reid Spencer wrote:
> one or more of the libraries. Since we don't have that situation in
> LLVM, using the various Casting.h facilities allows all that "dynamic"
> casting to actually be done statically, hence no runtime performance
> penalty.

Reid is right.  isa on an instruction literally loads a field out of the 
instruction and compares it against a constant.  As another example, a 
good compiler (ahem, like LLVM :) ), turns this:

if (... = dyn_cast<LoadInst>(Y)) {
...
} else if (... = dyn_cast<StoreInst>(Y)) {
...
} else if (... = dyn_cast<AllocaInst>(Y)) {
...

Into:

switch (Y->getOpcode()) {
case Instruction::Load:
   ...
case Instruction::Store:
   ...
case Instruction::AllocaInst:
   ...

Which is something that is just not possible with RTTI.

-Chris

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