Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 7 Sep 2021 10:10:10 -0700 | Subject | Re: [PATCH] Enable '-Werror' by default for all kernel builds |
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On Tue, Sep 7, 2021 at 2:11 AM Arnd Bergmann <arnd@kernel.org> wrote: > > > x86_64-alpine.log:drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:452:13: error: stack frame size (1800) exceeds limit (1280) in function 'dcn_bw_calc_rq_dlg_ttu' [-Werror,-Wframe-larger-than] > > x86_64-alpine.log:drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn21/display_rq_dlg_calc_21.c:1657:6: error: stack frame size (1336) exceeds limit (1280) in function 'dml21_rq_dlg_get_dlg_reg' [-Werror,-Wframe-larger-than] > > x86_64-alpine.log:drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn30/display_rq_dlg_calc_30.c:1831:6: error: stack frame size (1352) exceeds limit (1280) in function 'dml30_rq_dlg_get_dlg_reg' [-Werror,-Wframe-larger-than] > > x86_64-alpine.log:drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn31/display_rq_dlg_calc_31.c:1676:6: error: stack frame size (1336) exceeds limit (1280) in function 'dml31_rq_dlg_get_dlg_reg' [-Werror,-Wframe-larger-than] > > x86_64-alpine.log:drivers/vhost/scsi.c:1831:12: error: stack frame size (1320) exceeds limit (1280) in function 'vhost_scsi_release' [-Werror,-Wframe-larger-than] > > > > Another instance where distros lower CONFIG_FRAME_WARN below the 2048 > > default. Again, none look particularly scary but should still probably > > be dealt with. > > I would argue that they are still scary and should be addressed in the > code, it's just that we don't see them on build bots that use the 2048 byte default.
No, they are scary for another reason entirely: clang is clearly doing a *HORRIBLE* job with stack usage.
To take that dml30_rq_dlg_get_dlg_reg() function as an example: yes, it has a few structures on the stack, but gcc allocates 512-720 bytes of stack space depending on my config.
Not 1280 bytes.
So it's not even *close* to the 1024 byte limit with gcc, much less the 2kB one.
I don't know why clang basically decides to use almost double the stack space. Maybe it's some other config option that does it, I tried a fairly normal one and a "almost everythign enabled" one, and couldn't get close to the reported stack frame size with gcc.
Just to try to make things as close as possible, I tried with the exact same normal non-debug config (apart from obvious compiler-dependent things), and picked that dml30_rq_dlg_get_dlg_reg() function to look at (for no real reason other than that the stack frame was biggest above.
Gcc did a 720-byte stack frame for that case. Not great, but whatever.
clang did a 1136-byte stack frame for the same thing.
Do I know why? No. I do note that that code is disgusting.
It's passing one of those structs around by value, for example. That's a 72-byte structure that is copied on the stack due to stupid calling conventions. Maybe clang generates a few extra temporaries for it as part of the function call stack setup? Who knows..
Linus
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