[GlobalISel] Is there a way to assign higher or lower priority to patterns?

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#1

Hi all,

I have two GIComplexOperandMatchers, one that takes [base(gpr)+offset(imm)], another that takes (global address inc offset) directly. It’s quite possible that I’m not doing things the right way™ by having these, but they seem to be making the transition easier.

However, because these patterns are look identical to the parser, tablegen seems to give them both equal preference - yet in all instances, the direct memory addressing variant ought be preferred over the one that requires a pointer register. I’m 99% sure that AddedComplexity is ignored by GlobalISel, and from quick look through GlobalISelEmitter.cpp I cannot find any alternative. Preferably, such an alternative would be applied to Operands, at that (maybe asking a bit much).

Before I go about renaming things such that alphabetical ordering gives me the results I want or hacking in a similar workaround, just want to check… is there a method I’ve missed for preferencing one pattern over another currently?

Thanks,
Alex Davies

#2

Hi Alex,

I can’t say I’m happy that this is the case* but the SelectionDAG importer does honour AddedComplexity. When it’s sorting the rules, the complexity score that SelectionDAG would have used has precedence over the priority that GlobalISel uses. I ended up having to give in on my opposition to using the complexity scores after we had a very hard time resolving ordering differences on an out of tree target that was using AddedComplexity a lot (pretty much everywhere) to fix up SelectionDAG’s scheme

*I dislike the way AddedComplexity tweaks an already dubious scoring system using magic numbers.