Re: [PATCH] Enable '-Werror' by default for all kernel builds

Harry Wentland
2021-09-07T21:07:38Z

On 2021-09-07 1:33 p.m., Linus Torvalds wrote:
> On Tue, Sep 7, 2021 at 10:10 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> 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..
>
> Ooh, yes.
>
> This attached patch is crap - it converts the helper functions to use
> const pointers instead of passing the whole structure, but it then
> only converts that one file that *uses* them.
>
> So the end result will not compile in general, but you can do
>
> make drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn30/display_rq_dlg_calc_30.o
>
> and it compiles for me.
>
> And while gcc doesn't care that much - it will apparently either
> generate the argument stack every call - clang cares deeply.
>
> The nasty 720-byte stack frame that clang generates turns into just a
> 320-byte one, and code generation in general looks a *lot* better.
>
> Now, as mentioned, this patch is broken and incomplete. But I really
> think the AMD GPU people need to do this. It makes those functions go
> from practically unusable to not horribly disgusting.
>
> So Harry/Leo/Alex/Christian and amd-gfx list - can you look into
> making this ugly "make one file compile better" patch actually work
> properly?
>

Yes, will take a look at this tonight. We definitely shouldn't be passing
large structs by value.

Harry

> It *looks* lto me ike that code was perhaps written for a C++ compiler
> and the helpers have been written as a "pass by reference", and the
> arguments used to be
>
> const display_data_rq_misc_params_st& rq_misc_param
>
> and then the compiler will pass the argument as a pointer. And then it
> was converted to C, and the "pass by reference" in the function
> declaration was turned into "pass by value", to avoid changing "." to
> "->" in the use.
>
> But a '&arg' thing in C++ really is a '*arg' pointer in C, and should
> have been done as that.
>
> Of course, it's also possible that that code was simply written by
> somebody who didn't understand just *how* horrible it is to pass
> structures bigger than a word or two by value.
>
> Do we have a compiler warning for passing big structures by value?
>
> Linus
>