Question about SSA-transformation

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1

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

2

    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