On 22/11/2019 11:38, Andrew Pinski wrote:
>> It is enabled in all optimization levels besides -Os (since besides possible
>> increasing the stack usage it also might increase code side).
> It is disabled at -Os because it is duplicating the loop header; which in
> turn is considered increasing code size (though sometimes that can have a
> side effect of decreasing the code size later on but that is a different
> story).
> The increase of stack usage is due to register pressure with respect to other
> optimizations that can now work with the copied loop header. If anything,
> the register pressure heuristics needs improvement for code motion passes or
> the ability to undo those code motion while doing register allocation. THIS
> IS a HUGE project and should not be taken lightly. It just happens this code
> happens here and causes issues. It is not the normal case really.
>
Thanks for the information, at least for the specific snippet it seems that
both -fno-tree-loop-im and -fno-tree-pre are the ones generating most spilling.
So the question I have it is worth to disable -free-ch when -fstack-conserve
is set (since it the flag idea to prevent such pessimizations) or the idea is
just to disable -ftree-ch for such cases.
> Thanks,
> Andrew
>
> ________________________________________
> From: linaro-toolchain <linaro-toolchain-boun...@lists.linaro.org> on behalf
> of Adhemerval Zanella <adhemerval.zane...@linaro.org>
> Sent: Friday, November 22, 2019 5:40 AM
> To: Arnd Bergmann
> Cc: Linaro Toolchain Mailman List
> Subject: [EXT] High stack usage due ftree-ch
>
> External Email
>
> ----------------------------------------------------------------------
> Hi Arnd,
>
> I took a look on the stack usage issue in the kernel snippet you provided [1],
> and as you have noted the most impact indeed come from -ftree-ch optimization.
> It is enabled in all optimization levels besides -Os (since besides possible
> increasing the stack usage it also might increase code side).
>
> I am still fulling grasping what free-ch optimization does, but my
> understanding
> so far is it tries to reorganize the loop for later loop optimization phases.
> More specifically, what it ends up doing on the specific snippet is create
> extra
> stack variables for the internal membber access in the inner loop (which in
> its
> turns increase stack usage).
>
> This is also why adding the compiler barrier inhibits the optimization, since
> it
> prevents the ftree-ch to optimize the internal loop reorganization and it is
> passed as is to later optimizations phases.
>
> It is also a generic pass that affects all architecture, albeit the resulting
> stack will depend on later passes. With GCC 9.2.1 I see the resulting stack
> usage using -fstack-usage along with -O2:
>
> arm 632
> aarch64 448
> powerpc 912
> powerpc64le 560
> s390 600
> s390x 632
> i386 1376
> x86_64 784
>
> Also, -fconserve-stack does not really help with this pass since ftree-ch does
> not check the flag usage. The fconserve-stack currently only seems to effect
> the inliner by setting both large-stack-frame and large-stack-frame-growth to
> some conservative values.
>
> The straightforward change I am checking is just to disable tree-ch
> optimization
> if fconserve-stack is also enabled:
>
> diff --git a/gcc/tree-ssa-loop-ch.c b/gcc/tree-ssa-loop-ch.c
> index b894a7e0918..b14dd66257c 100644
> --- a/gcc/tree-ssa-loop-ch.c
> +++ b/gcc/tree-ssa-loop-ch.c
> @@ -291,7 +291,8 @@ public:
> {}
>
> /* opt_pass methods: */
> - virtual bool gate (function *) { return flag_tree_ch != 0; }
> + virtual bool gate (function *) { return flag_tree_ch != 0
> + && flag_conserve_stack == 0; }
>
> /* Initialize and finalize loop structures, copying headers inbetween. */
> virtual unsigned int execute (function *);
>
> On powerpc64le with gcc master:
>
> $ /home/azanella/gcc/gcc-git-build/gcc/xgcc -B
> /home/azanella/gcc/gcc-git-build/gcc -O2 ../stack_usage.c -c -fstack-usage &&
> cat stack_usage.su
> ../stack_usage.c:157:6:mlx5e_grp_sw_update_stats 496 static
>
> $ /home/azanella/gcc/gcc-git-build/gcc/xgcc -B
> /home/azanella/gcc/gcc-git-build/gcc -O2 ../stack_usage.c -c -fstack-usage
> -fconserve-stack && cat stack_usage.su
> ../stack_usage.c:157:6:mlx5e_grp_sw_update_stats 176 static
>
> The reference for minimal stack usage is with -Os:
>
> $ /home/azanella/gcc/gcc-git-build/gcc/xgcc -B
> /home/azanella/gcc/gcc-git-build/gcc -Os ../stack_usage.c -c -fstack-usage
> && cat stack_usage.su
> ../stack_usage.c:157:6:mlx5e_grp_sw_update_stats 32 static
>
> I will try to check if also enable the same test for -fgcse and -free-ter
> do make sense.
>
> [1]
> https://urldefense.proofpoint.com/v2/url?u=https-3A__godbolt.org_z_WKa-2DBd&d=DwIGaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=L_uAQMgirzaBwiEk05NHY-AMcNfJzugOS_xTjrtS94k&m=ySgQrryO8OlXh50QdjZ81DXxOL3LLUd7ecrtnTWd8zA&s=FWfDuHQlXPrv4N6aGpxHBIR_9-0axgnkvWu5FKlMExU&e=
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