Hello, On Mon, 3 Feb 2025, H.J. Lu wrote:
> Author: Surya Kumari Jangala <jskum...@linux.ibm.com> > Date: Tue Jun 25 08:37:49 2024 -0500 > > ira: Scale save/restore costs of callee save registers with block > frequency > > scales the cost of saving/restoring a callee-save hard register in epilogue > and prologue with the entry block frequency, which, if not optimizing for > size, is 10000, for all targets. This merely represents the fact that the entry block is indeed entered exactly once per function invocation, i.e. 1.0 in fixed point with a scale of 1000. All costs in ira are (supposed to be) scaled by bb-frequency of the allocno/register occurence, and hence this add_cost to cater for xlogue-save/restore needs to be scaled by that as well, which is what Suryas patch was adding. Any fallout from that needs to be addressed on top of that, not by reverting it, or by introducing a hook to avoid that. Think of this scale as an arbitrary value to implement pseudo-fixed-point arithmetic for costs. All values need to be scaled by it. That its value is a seemingly large number of 1000 is not the worry, it represents 1.0 . If the issue is for instance that callee-saved registers aren't used because the prologue save/restore is now deemed too expensive relative to the around-call-save-restore when a call-clobbered register is used, then either the around-call-save-restore instructions aren't correctly costed (perhaps also missing the scale factor?), or because ties aren't broken nicely, in which case adding a 1 at one or the other place might be needed. Ciao, Michael.
