On Wed, Oct 9, 2013 at 4:11 AM, Richard Biener <richard.guent...@gmail.com> wrote: > On Tue, Oct 8, 2013 at 11:04 PM, Easwaran Raman <era...@google.com> wrote: >> In cfgexpand.c, variables in non-overlapping lexical scopes are >> assigned same stack locations at -O1 and above. At -O0, this is >> attempted only if the size of the stack objects is above a threshold >> (32). The rationale is at -O0, more variables are going to be in the >> stack and the O(n^2) stack slot sharing algorithm will increase the >> compilation time. This patch replaces the constant with a param which >> is set to 32 by default. We ran into a case where the presence of >> always_inline attribute triggered Wframe-larger-than warnings at -O0 >> but not at -O2 since the different inlined copies share the stack. We >> are ok with a slight increase in compilation time to get smaller stack >> frames even at -O0 and this patch would allow us do that easily. >> >> Bootstraps on x86_64/linux. Is this ok for trunk? > > Ok with > > +DEFPARAM (PARAM_MIN_SIZE_FOR_STACK_SHARING, > + "min-size-for-stack-sharing", > + "Attempt to share stack slots among variables in different > lexical blocks " > + "at O0 only if their sizes exceed this value", > + 32, 0, 0) > > changed to > > "The minimum size of variables taking part in stack slot sharing " > "when not optimizing" > 32, 0, 0) > > And with adding documentation for that param in doc/invoke.texi. > > Btw, I'm not sure the sharing algorithm is still quadratic - can you > investigate on that?
The partition_stack_vars is still quadratic in worst case (all live ranges overlap), but the expensive part is likely to be the add_scope_conflicts function. - Easwaran > Thanks, > Richard. > >> Thanks, >> Easwaran >> >> >> 2013-10-08 Easwaran Raman <era...@google.com> >> >> * params.def (PARAM_MIN_SIZE_FOR_STACK_SHARING): New param... >> * cfgexpand.c (defer_stack_allocation): ...use here
