Re: [PATCH] ifcvt.cc: Prevent excessive if-conversion for conditional moves

[Robin Dapp <rdapp@linux.ibm.com>]

Hi,
 
> On optimizing for speed, default_noce_conversion_profitable_p() allows
> plenty of headroom, so this patch has little impact.
> 
> Also, if the target-specific cost estimate is accurate or allows for
> margins, the impact should be similarly small.
I believe this part of ifcvt does/did not use the costing on purpose.
It will generally convert more sequences than other paths that compare
before and after costs since we just count the number of converted
insns comparing them against the "branch costs".  Similar to rtx costs
they are kind of relative to a single insn but AFAIK it's not used
consistently everywhere.  All the major platforms have low branch costs
nowadays (0 or 1?) thus we won't emit too many conditional moves here.

In general I agree that we should compare costs everywhere and not just
count (the costing should include the branch costs as well) but this would
be a major overhaul.  For your case (assuming xtensa), could you not
tune xtensa_branch_cost?  It is currently 3 allowing up to 4 conditional
moves to be generated.  optimize_function_for_speed_p is already being
passed to the hook so you could make use of that and decrease branch
costs when optimizing for size only.

Regards
 Robin