On Thu, Nov 14, 2013 at 3:52 AM, Eric Botcazou <ebotca...@adacore.com> wrote: > Hi, > > we have a test in the gnat.dg testsuite (stack_usage1.adb) which checks that > the allocation of big temporaries created in non-overlapping blocks on the > stack is optimal, i.e. that they share a stack slot. It is run at -O0 and > passes. If you run it at -O2, it also passes. Now, if you run it at -O1, it > fails and that's a regression from the pre-TREE_CLOBBER_P era. > > The problem is that, when optimization is enabled, DECL_IGNORED_P variables > are removed from blocks by remove_unused_scope_block_p and moved to the > toplevel. Now defer_stack_allocation has: > > /* Variables in the outermost scope automatically conflict with > every other variable. The only reason to want to defer them > at all is that, after sorting, we can more efficiently pack > small variables in the stack frame. Continue to defer at -O2. */ > if (toplevel && optimize < 2) > return false; > > The comment is slightly obsolete in the TREE_CLOBBER_P era, since toplevel > variables don't necessarily conflict with each other, for example the above > variables moved to toplevel by remove_unused_scope_block_p. > > We don't think that we need to tweak again remove_unused_scope_block_p in the > TREE_CLOBBER_P era; instead we can defer the allocation of big DECL_IGNORED_P > variables at toplevel from defer_stack_allocation. > > Tested on x86_64-suse-linux, OK for the mainline? > > > 2013-11-14 Olivier Hainque <hain...@adacore.com> > > * cfgexpand.c (defer_stack_allocation): When optimization is enabled, > defer allocation of DECL_IGNORED_P variables at toplevel unless really > small. Factorize size threshold computation from the existing one. > (expand_used_vars): Refine comment. >
This caused: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65504 -- H.J.
