[LLVMdev] X86: copyConstantToRegister

[Chris Lattner]

On Mon, 7 Jun 2004, Vladimir Prus wrote:

>
> Hello,
> looking at X86 codegen, I see this:
>
> void ISel::copyConstantToRegister(MachineBasicBlock *MBB,
>                                   MachineBasicBlock::iterator IP,
>                                   Constant *C, unsigned R) {
>   if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
> 	.....
>        case Instruction::Add:
>           emitSimpleBinaryOperation(MBB, IP, CE->getOperand(0),
>                                                   CE->getOperand(1),
>                                                   Class, R);
>
> I'm not sure I understand this logic. If we have "1 + 2" as constant
> expression, then why emit the code to perform addition? It should be possible
> to just fold the expression and copy immediate "3" into a register.
>
> I must be missing something, but what?

LLVM constant expressions are used when it is not possible to fold the
constant.  In particular expressions like "10/0" are not constant foldable
(so it must be evaluated at runtime), and link-time constants are not
foldable... stuff like:

int X;
int Y;
int Z = (int)&X-(int)&Y;

Yes, this is gross and disturbing, but we do support it. Constant
expressions are the reason why there is a separation in the X86 isel
between the "visit" methods and the "emit" methods.

> Another question is about TargetInstrInfo::CreateCodeToLoadConst. This seems
> like a perfect fit for the task, but it's not used in X86 backend at all.
> Further, the destination register is represented by Instruction*, and not by
> integer number as elsewhere. What does that Instruction* mean?

This is a deprecated method that is only used by the SparcV9 backend.  It
used an extremely different way of representing virtual registers based on
Value*'s.  It turns out that doing this has some serious problems, so we
are slowly migrating away from it.

-Chris

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