llvm dev - Feb 2007 - [LLVMdev] Question about SSA-transformation

Dear LLVM'ers,

     I'm producing code for the PowerPC, and many often times it happens 
that the bytecodes generated by LLVM have phi-functions like:

A4 = phi(A1, A2)
A5 = phi(A1, A3)

Where the 'A1' parameter appears in two different phi-functions. Could 
someone give me an example in "pseudo-assembly" that would produce
code
like that after being transformed into SSA form? Does it happen because of 
some optimization, like in:

V = 1000;
if(...) {
     A1 = 1;
     A2 = 2;
} else {
     A1 = V;
     A2 = V;
}
use(A1);
use(A2);

Where one can remove the assignments that use 'V' by placing it directly
in the phi function, like in:
V = 1000;
if(...) {
     A1 = 1;
     A2 = 2;
}
A3 = phi(V, A1);
A4 = phi(V, A2);

Thanks,

Fernando

On Sat, 17 Feb 2007, Fernando Magno Quintao Pereira
wrote:
>     I'm producing code for the PowerPC, and many often times it happens
> that the bytecodes generated by LLVM have phi-functions like:
>
> A4 = phi(A1, A2)
> A5 = phi(A1, A3)
ok
> Where the 'A1' parameter appears in two different phi-functions.
Could
> someone give me an example in "pseudo-assembly" that would
produce code
> like that after being transformed into SSA form? Does it happen because of
> some optimization, like in:
Right, SSA construction with copy propagation (which we do, because 
llvm has no copy instruction) can cause this.
> V = 1000;
> if(...) {
>     A1 = 1;
>     A2 = 2;
> } else {
>     A1 = V;
>     A2 = V;
> }
> use(A1);
> use(A2);
Yep, exactly.

-Chris

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