Conditional analysis before inliner

Hi,

I have pass phase ordering question.

I am interested in performing a transformation that matches several LLVM IR instructions and converts them to a single builtin. This sort of transformation must occur prior to inlining because inlining needs to analyze cost of the builtin. However, the LLVM IR → builtin transformation requires scalar evolution which typically doesn’t run until after the inliner.

My question is, is there a way to run scalar evolution conditionally prior to inlining? I’d like to leave compile time unaffected if my code doesn’t contain any of these builtins.

Thanks,

Stan

Hi,

I have pass phase ordering question.

I am interested in performing a transformation that matches several LLVM IR instructions and converts them to a single builtin. This sort of transformation must occur prior to inlining because inlining needs to analyze cost of the builtin. However, the LLVM IR -> builtin transformation requires scalar evolution which typically doesn't run until after the inliner.

This isn't right, or at least, not the complete picture; loop optimization passes run interleaved with the inliner. (Try looking at the output of "-debug-pass=Executions" to see exactly what's happening.)

My question is, is there a way to run scalar evolution conditionally prior to inlining? I'd like to leave compile time unaffected if my code doesn't contain any of these builtins.

ScalarEvolution is lazy, so it's essentially free if you don't query it.

-Eli

Thanks for the answer Eli.

I think I was a little too vague in my question. The program has the following structure:
void foo() {
bar();
}

void bar() {
for (…) {
__builtin_compute()
}
}

We would like to use scalar evolution + loopinfowrapperpass inside instcombine only if __builtin_compute is present. instcombine can convert the loop into a single builtin ie.

void bar() {
__builtin_wide_compute();
}

When the inliner runs on foo(), we want bar() to already have the transformation to reflect the cost of the inlining decision more accurately.

I do see that scalar evolution is a lazy but loopwrapinfopass is not. Is there a way to run these two analysis passes conditionally before inlining?

Thanks,

Stan

I probably wouldn't worry about the cost of LoopInfo; it's relatively cheap to compute. But if you're really concerned, you can run your pass after a pass which preserves LoopInfo (so it's free because it was already computed). Or you can stick your code into an existing pass which requires LoopInfo anyway, like LoopIdiomRecognize.

-Eli

I do not think we have this framework of conditionally kicking in analysis passes. But one can always do “intelligent” prior setting of attribute over function which can be checked in getAnalysisUsage for a perticular pass to conditionally enable needed analysis.

Cheers

Jatin