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Re: REVISED PATCH: adjust init_set_costs to account for call_used_regs (PR42505)


Zdenek Dvorak wrote:

what about simply increasing the value of target_res_regs, instead? After all, target_res_regs is intended to acount for the scratch registers etc.

What about performance results on some mainstream architecture, say x86_64?

I'm out of time for further experimentation and benchmarking; my manager told me 2 weeks ago already that I'd spent enough time on this bug. At this point, I just want to get something checked in that prevents ivopts from thinking it has 9 registers available when it only has 4 because of the call in the loop. If adding the additional register pressure parameter is unacceptable without further experiments, I'll remove it entirely.


This needs to be a bit more careful -- you should exclude builtin calls that will
not be expanded to real function calls (e.g., builtin_prefetch),

Is there code elsewhere in the compiler that knows which builtins are guaranteed not to expand into a real function call? I looked quickly and it seems other middle-end passes do not attempt to differentiate builtins from regular calls. If there is not already such code I can borrow, see my comment above about being out of time. ;-) I'll argue again here that, in the absence of more specific analysis, it is more conservatively correct for the register pressure computation to assume that the call-clobbered registers are unavailable than to assume that they are, as the costs of being wrong are much higher in the latter case.


-Sandra


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