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Re: [PATCH] Improve alloca alignment
- From: Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>, Jeff Law <law at redhat dot com>
- Cc: Eric Botcazou <ebotcazou at adacore dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>, Rainer Orth <ro at CeBiTec dot Uni-Bielefeld dot DE>, nd <nd at arm dot com>
- Date: Tue, 17 Oct 2017 12:04:40 +0000
- Subject: Re: [PATCH] Improve alloca alignment
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Wilco Dijkstra wrote:
>
> Yes STACK_BOUNDARY applies to virtual_stack_dynamic_rtx and all other
> virtual frame registers. It appears it's main purpose is to enable alignment
> optimizations since PREFERRED_STACK_BOUNDARY is used to align
> local and outgoing argument area etc. So if you don't want the alignment
> optimizations it is feasible to set STACK_BOUNDARY to a lower value
> without changing the stack layout.
>
> There is also STACK_DYNAMIC_OFFSET which computes the total offset
> from the stack. It's not obvious whether the default version should align (since
> outgoing arguments are already aligned there is no easy way to record the
> extra padding), but we could assert if the offset isn't aligned.
Also there is something odd in the sparc backend:
/* Given the stack bias, the stack pointer isn't actually aligned. */
#define INIT_EXPANDERS \
do { \
if (crtl->emit.regno_pointer_align && SPARC_STACK_BIAS) \
{ \
REGNO_POINTER_ALIGN (STACK_POINTER_REGNUM) = BITS_PER_UNIT; \
REGNO_POINTER_ALIGN (HARD_FRAME_POINTER_REGNUM) = BITS_PER_UNIT; \
} \
} while (0)
That lowers the alignment for the stack and frame pointer. So assuming that works
and blocks alignment optimizations, why isn't this done for the dynamic offset as well?
Wilco