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Re: [PATCH][GCC][mid-end] Add a hook to support telling the mid-end when to probe the stack [patch (2/6)]
- From: Jeff Law <law at redhat dot com>
- To: Tamar Christina <tamar dot christina at arm dot com>, gcc-patches at gcc dot gnu dot org
- Cc: nd at arm dot com, rguenther at suse dot de, ian at airs dot com
- Date: Wed, 11 Jul 2018 12:52:42 -0600
- Subject: Re: [PATCH][GCC][mid-end] Add a hook to support telling the mid-end when to probe the stack [patch (2/6)]
- References: <20180711112058.GA15773@arm.com>
On 07/11/2018 05:21 AM, Tamar Christina wrote:
> Hi All,
>
> This patch adds a hook to tell the mid-end about the probing requirements of the
> target. On AArch64 we allow a specific range for which no probing needs to
> be done. This same range is also the amount that will have to be probed up when
> a probe is needed after dropping the stack.
>
> Defining this probe comes with the extra requirement that the outgoing arguments
> size of any function that uses alloca and stack clash be at the very least 8
> bytes. With this invariant we can skip doing the zero checks for alloca and
> save some code.
>
> A simplified version of the AArch64 stack frame is:
>
> +-----------------------+
> | |
> | |
> | |
> +-----------------------+
> |LR |
> +-----------------------+
> |FP |
> +-----------------------+
> |dynamic allocations | -\ probe range hook effects these
> +-----------------------+ --\ and ensures that outgoing stack
> |padding | -- args is always > 8 when alloca.
> +-----------------------+ ---/ Which means it's always safe to probe
> |outgoing stack args |-/ at SP
> +-----------------------+
>
>
> This allows us to generate better code than without the hook without affecting
> other targets.
>
> With this patch I am also removing the stack_clash_protection_final_dynamic_probe
> hook which was added specifically for AArch64 but that is no longer needed.
>
> Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu and no issues.
> Both targets were tested with stack clash on and off by default.
>
> Ok for trunk?
>
> Thanks,
> Tamar
>
> gcc/
> 2018-07-11 Tamar Christina <tamar.christina@arm.com>
>
> PR target/86486
> * explow.c (anti_adjust_stack_and_probe_stack_clash): Support custom
> probe ranges.
> * target.def (stack_clash_protection_alloca_probe_range): New.
> (stack_clash_protection_final_dynamic_probe): Remove.
> * targhooks.h (default_stack_clash_protection_alloca_probe_range) New.
> (default_stack_clash_protection_final_dynamic_probe): Remove.
> * targhooks.c: Likewise.
> * doc/tm.texi.in (TARGET_STACK_CLASH_PROTECTION_ALLOCA_PROBE_RANGE): New.
> (TARGET_STACK_CLASH_PROTECTION_FINAL_DYNAMIC_PROBE): Remove.
> * doc/tm.texi: Regenerate.
>
The control flow is a bit convoluted here, but after a few false starts
where I thought this was wrong, I think it's OK.
Jeff