On Fri, Sep 6, 2019 at 3:03 PM Segher Boessenkool <seg...@kernel.crashing.org> wrote: > > On Fri, Sep 06, 2019 at 11:14:08AM -0700, Nick Desaulniers wrote: > > Here's the case that I think is perfect: > > https://developers.redhat.com/blog/2016/02/25/new-asm-flags-feature-for-x86-in-gcc-6/ > > > > Specifically the feature test preprocessor define __GCC_ASM_FLAG_OUTPUTS__. > > > > See exactly how we handle it in the kernel: > > - > > https://github.com/ClangBuiltLinux/linux/blob/0445971000375859008414f87e7c72fa0d809cf8/arch/x86/include/asm/asm.h#L112-L118 > > - > > https://github.com/ClangBuiltLinux/linux/blob/0445971000375859008414f87e7c72fa0d809cf8/arch/x86/include/asm/rmwcc.h#L14-L30 > > > > Feature detection of the feature makes it trivial to detect when the > > feature is supported, rather than brittle compiler version checks. > > Had it been a GCC version check, it wouldn't work for clang out of the > > box when clang added support for __GCC_ASM_FLAG_OUTPUTS__. But since > > we had the helpful __GCC_ASM_FLAG_OUTPUTS__, and wisely based our use > > of the feature on that preprocessor define, the code ***just worked*** > > for compilers that didn't support the feature ***and*** compilers when > > they did support the feature ***without changing any of the source > > code*** being compiled. > > And if instead you tested whether the actual feature you need works as > you need it to, it would even work fine if there was a bug we fixed that > breaks things for the kernel. Without needing a new compiler.
That assumes a feature is broken out of the gate and is putting the cart before the horse. If a feature is available, it should work. If you later find it to be unsatisfactory, sure go out of your way to add ugly compiler-specific version checks or upgrade your minimally supported toolchain; until then feature detection is much cleaner (see again __GCC_ASM_FLAG_OUTPUTS__). > > Or as another example, if we added support for some other flags. (x86 > has only a few flags; many other archs have many more, and in some cases > newer hardware actually has more flags than older). I think compiler flags are orthogonal to GNU C extensions we're discussing here. > > With the "macro" scheme we would need to add new macros in all these > cases. And since those are target-specific macros, that quickly expands > beyond reasonable bounds. I don't think so. Can you show me an example codebase that proves me wrong? > > If you want to know if you can do X in some environment, just try to do X. That's a very autoconf centric viewpoint. Why doesn't the kernel take that approach for __GCC_ASM_FLAG_OUTPUTS__? Why not repeatedly invoke $CC to find if every compiler __attribute__ is supported? Do you think it's faster for the C preprocessor to check for a few #ifdefs, or to repeatedly invoke $CC at build or compile time to detect new features? -- Thanks, ~Nick Desaulniers