On Tue, Oct 1, 2019 at 2:28 AM Will Deacon <will@xxxxxxxxxx> wrote: > > Hi Nick, > > On Mon, Sep 30, 2019 at 02:50:10PM -0700, Nick Desaulniers wrote: > > So __attribute__((always_inline)) doesn't guarantee that code will be > > inlined. For instance in LLVM's inliner, it asks/answers "should I > > inline" and "can I inline." "Should" has to do with a cost model, and > > is very heuristic-y. "Can" has more to do with the transforms, and > > whether they're all implemented and safe. If you if you say > > __attribute__((always_inline)), the answer to "can I inline this" can > > still be *no*. The only way to guarantee inlining is via the C > > preprocessor. The only way to prevent inlining is via > > __attribute__((no_inline)). inline and __attribute__((always_inline)) > > are a heuristic laden mess and should not be relied upon. I would > > also look closely at code that *requires* inlining or the lack there > > of to be correct. That the kernel no longer compiles at -O0 is not a > > good thing IMO, and hurts developers that want a short > > compile/execute/debug cycle. > > > > 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. > > > 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. > gets the register allocations subtly wrong when passing a 'register' > variable into an inline asm. I would like to avoid this sort of thing > happening, since it can result in really nasty bugs that manifest at > runtime and are extremely difficult to debug, which is why I would much > prefer not to have this option on by default for arm64. I sent a patch > already: > > https://lkml.kernel.org/r/20190930114540.27498-1-will@xxxxxxxxxx > > and I'm happy to spin a v2 which depends on !CC_IS_CLANG as well. For small things like whether we mark a function always_inline or not, I think it's simpler to just keep the code consistent between compilers, even if it's to work around a bug in one compiler. A comment in the code would be sufficient. > > 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. -- Thanks, ~Nick Desaulniers
