On Tue, Oct 01, 2019 at 09:32:25AM -0700, Nick Desaulniers wrote: > On Tue, Oct 1, 2019 at 2:28 AM Will Deacon <will@xxxxxxxxxx> wrote: > > On Mon, Sep 30, 2019 at 02:50:10PM -0700, Nick Desaulniers wrote: > > > In this case, if there's a known codegen bug in a particular compiler > > > or certain versions of it, I recommend the use of either the C > > > preprocessor or __attribute__((no_inline)) to get the desired behavior > > > localized to the function in question, and for us to proceed with > > > Masahiro's cleanup. > > > > Hmm, I don't see how that would help. The problem occurs when things > > are moved out of line by the compiler (see below). > > It's being moved out of line because __attribute__((always_inline)) or > just inline provide no guarantees that outlining does not occur. It > would help to make functions that need to be inlined macros, because > the C preprocessor doesn't have that issue. Hmm, let me try to put it another way: enabling CONFIG_OPTIMIZE_INLINING has been shown to cause miscompilation with many versions of GCC on arm64 because the compiler moves things out of line that it otherwise doesn't appear to do. I don't see how __attribute__((no_inline)) helps with that, and replacing static functions with macros isn't great for type-checking and argument evaluation. > > > The comment above the use of CONFIG_OPTIMIZE_INLINING in > > > include/linux/compiler_types.h says: > > > * Force always-inline if the user requests it so via the .config. > > > Which makes me grimace (__attribute__((always_inline)) doesn't *force* > > > anything as per above), and the idea that forcing things marked inline > > > to also be __attribute__((always_inline)) is an "optimization" (re: > > > the name of the config; CONFIG_OPTIMIZE_INLINING) is also highly > > > suspect. Aggressive inlining leads to image size bloat, instruction > > > cache and register pressure; it is not exclusively an optimization. > > > > Agreed on all of this, but the fact remains that GCC has been shown to > > *miscompile* the arm64 kernel with CONFIG_OPTIMIZE_INLINING=y. Please, > > look at this thread: > > > > https://www.spinics.net/lists/arm-kernel/msg730329.html > > https://www.spinics.net/lists/arm-kernel/msg730512.html > > > > GCC decides to pull an atomic operation out-of-line and, in doing so, > > If the function is incorrect unless inlined, use a macro. The function is correct per the GCC documentation regarding register variables and inline asm. > > Reducing the instruction cache footprint is great, but not if the > > resulting code is broken! > > You don't have to convince compiler folks about correctness. ;) > Correctness trumps all, especially performance. Then why is this conversation so difficult? :( Will
