WRT http://gbenson.livejournal.com/2007/12/21/
GCC is being overzealous because of a default that was local to one file was
made global on 2003-10-07, and this changed the behavior of the #if statement
in explow.c's allocate_dynamic_stack_space():
#if defined (STACK_DYNAMIC_OFFSET) || defined (STACK_POINTER_OFFSET)
#define MUST_ALIGN 1
#else
#define MUST_ALIGN (PREFERRED_STACK_BOUNDARY < BIGGEST_ALIGNMENT)
#endif
Unfortunately, STACK_POINTER_OFFSET isn't a preprocessor constant on all ports.
We could change the above to:
#if defined (STACK_DYNAMIC_OFFSET)
#define MUST_ALIGN 1
#else
#define MUST_ALIGN (STACK_POINTER_OFFSET || PREFERRED_STACK_BOUNDARY <
BIGGEST_ALIGNMENT)
#endif
but on at least one port (pa), STACK_POINTER_OFFSET depends on the size of the
outgoing arguments of a function, which we don't necessarily know yet at the
point we expand alloca builtins. For pa, it's never zero, but for other ports
it might be, and then this would break.
BTW, function.c still provides a no-longer-necessary default for
STACK_POINTER_OFFSET.
--
Summary: GCC generates too many alignment adds for alloca
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: regression
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: aoliva at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34548
