Hi Matthew, > /* LRA will allocate an FPR for an integer mode pseudo instead of spilling > to memory if an FPR is present in the allocno class. It is rare that > we actually need to place an integer mode value in an FPR so where > possible limit the allocation to GR_REGS. This will slightly pessimize > code that involves integer to/from float conversions as these will have > to reload into FPRs in LRA. Such reloads are sometimes eliminated and > sometimes only partially eliminated. We choose to take this penalty > in order to eliminate usage of FPRs in code that does not use floating > point data. > > This change has a similar effect to increasing the cost of FPR->GPR > register moves for integer modes so that they are higher than the cost > of memory but changing the allocno class is more reliable. > > This is also similar to forbidding integer mode values in FPRs entirely > but this would lead to an inconsistency in the integer to/from float > instructions that say integer mode values must be placed in FPRs. */ > > I'm keen to get the description of this right so please feel free to change > it further if it isn't clear (or correct).
This description is definitely more accurate. At first glance, I wasn't sure whether to include the bit about partial elimination since it caused by post reload passes but the introduction of reloads should be avoided in the first place. > I don't know if this change will lead to classic reload being unusable for > MIPS. I'm not worried about that but I think it is probably wise to > remove classic reload support for MIPS now; we are dependent on LRA for > several features already. Indeed. I think it's the right time to drop the classic reload and resolve LRA issues if they come up. > Do you have any details on when we are left with suboptimal code for > int->float conversions? I'd like to keep a record of them in this thread > or in the comment so we know what is left to fix. I opened a bug report https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66204 and just realised that I had never CCed Vlad. Hopefully, this is the last remaining issue related to integers and floating-point registers. > > > + if (INTEGRAL_MODE_P (PSEUDO_REGNO_MODE (regno)) && allocno_class == > > ALL_REGS) > > + return GR_REGS; > > + return allocno_class; > > +} > > + > > Trim the extra trailing newline. > > OK to commit if you are happy with the comment. I'll update the comment, do a quick regression and commit it. Regards, Robert
