On Thu, 2020-07-23 at 15:19 -0500, Segher Boessenkool wrote: > Hi! > > On Thu, Jul 23, 2020 at 01:42:59PM -0600, Jeff Law wrote: > > On Thu, 2020-07-23 at 14:25 -0500, Pat Haugen via Gcc-patches wrote: > > > Disable -fcaller-saves by default. > > > > > > This patch turns off -fcaller-saves by default so that IRA doesn't try > > > to use volatile regs for pseudos that are live across a call, which > > > would then require LRA to save/restore the reg around the call. > > > 2020-07-23 Pat Haugen <pthau...@linux.ibm.com> > > > > > > gcc/ > > > * common/config/rs6000/rs6000-common.c > > > (rs6000_option_optimization_table): Turn off -fcaller-saves. > > > > > > gcc/testsuite/ > > > * gcc.target/powerpc/caller-saves.c: New. > > What's driving this change? > > Peter noticed IRA allocates stuff to volatile registers while it is life > through a call, and then LRA has to correct that, not optimal. I can't recall if IRA or LRA handles the need to save them to the stack, but the whole point is you can do much better sometimes by saving into the stack with the caller-saves algorithm vs just giving up and spilling.
> > > IRA will do a cost/benefit analysis to see using call clobbered registers > > like > > this is profitable or not. You're just turning the whole thing off. > > '-fcaller-saves' > Enable allocation of values to registers that are clobbered by > function calls, by emitting extra instructions to save and restore > the registers around such calls. Such allocation is done only when > it seems to result in better code. > > This option is always enabled by default on certain machines, > usually those which have no call-preserved registers to use > instead. > > So, given what Peter saw, the analysis when to do or not do this isn't > as good as could be hoped for. Sure. That obviously happens. When it does the usual response is to dig deeper into why, not turn the entire option off. Jeff
