Add a param to decide stack slot sharing at -O0

[Easwaran Raman]

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?

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

cfgexpand.patch
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