[PATCH] D70852: [InstCombine] Guard maxnum/minnum conversions with a TTI query

[Ulrich Weigand via Phabricator via llvm-commits]

uweigand added a comment.

But in those cases like funnel shifts, common code (specifically SelectionDAGBuilder::visitIntrinsicCall) will handle the expansion if the target doesn't have an instruction.

So if we do need to do that for minnum/maxnum, that would probably be the better place.

However, this is still a bit different, because minnum/maxnum can also be the result of an explicit call to fmin/fmax in the source, and it would seem surprising to have that translated into a select sequence as well.  If we originally had an fmin/fmax call, which was translated into minnum/maxnum that we then couldn't further optimize, the current expansion (back to the fmin/fmax call) seems correct to me.

It's only the case where we originally had a conditional that was translated into minnum/maxnum that expansion into fmin/fmax is incorrect (because those routines are only available when linking with -lm which the user might not do since there's no actual call to a libm routine in the original source).

The problem really is IMO that there's no way to tell these two cases apart at instruction selection time.


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  https://reviews.llvm.org/D70852/new/

https://reviews.llvm.org/D70852