On Sat, Jun 27, 2020 at 12:43 PM Vladimir Oltean <olteanv@xxxxxxxxx> wrote: > > Hi Nick, > > On Mon, 22 Jun 2020 at 19:50, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > > > > > I really don't get what's the problem here. The listing of > > > ld9040_prepare at the given commit and with the given .config is: > > > > I wrote a tool to help debug these. > > https://github.com/ClangBuiltLinux/frame-larger-than > > If you fetch the randconfig and rebuild with debug info, that tool > > will help show you which variables are used in which stack frames and > > what their sizes are. Also note that strange things get dug up from > > randconfigs. > > > > > > -- > > Thanks, > > ~Nick Desaulniers > > I ran your tool and it basically told me that all 11 calls to Cool? No bugs running it? (I still need to extend support for many architectures) > ld9040_dcs_write from within ld9040_init are inlined by the compiler. > Each of these ld9040_dcs_write functions calls ld9040_spi_write_word > twice, so 22 inline calls to that. Now, sizeof(struct > spi_transfer)=136 and sizeof(struct spi_message)=104, so, no wonder we > run out of stack pretty quickly. I'd expect these to have distinct lifetimes resulting in stack slot reuse. When the compiler inlines functions, it introduces a lexical scope. You can imagine it inlining the body, but within a new `{}` delineated compound statement. Then the compiler knows that the variables local to those scopes can't outlive each other, and can reuse their stack slots in the containing function. Escape analysis comes into play, too, but I'm not sure that's an issue here. > > But my question is: what's wrong with the code, if anything at all? The general case we try to find+fix with this warning is excessively large stack allocations that probably should be heap allocated, percpu, or static. Also, the `noinline_for_stack` function annotation is used frequently for this. One known case of issues are the sanitizers, which can generally prevent the reuse of stack slots. Were any of those set in this config, since this was a randconfig? I'd check this first, then consider if `noinline_for_stack` is appropriate on any of the related functions. > Why does the compiler try to inline it, and then complain that it's > using too much stack The flag -Wframe-larger-than= is a warning on semantics, not really an optimization flag controlling the maximum stack depth of the function being inlined into. > when basically nobody appears to have asked it to > inline it? That's not really how inlining works. If you don't specify compiler attributes, then the compiler can decide to inline or not at its discretion. The `inline` keyword or its absence doesn't really affect this. __attribute__((always_inline)) and __attribute__((noinline)) can give you more control, but there are hyper obscure edge cases where even those don't work as advertised. -- Thanks, ~Nick Desaulniers _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
