>>> On 21.02.18 at 22:39, <keesc...@chromium.org> wrote: > On Mon, Feb 19, 2018 at 6:49 AM, Jan Beulich <jbeul...@suse.com> wrote: >> Commit df3405245a ("x86/asm: Add suffix macro for GEN_*_RMWcc()") >> introduced "suffix" RMWcc operations, adding bogus clobber specifiers: >> For one, on x86 there's no point explicitly clobbering "cc". In fact, > > Do you have more details on this? It seems from the GCC clobbers > docs that "cc" is needed for asm that changes the flags register. > Given the explicit subl and decl being used for these macros, it seems > needed to me. > >> with gcc properly fixed, this results in an overlap being detected by >> the compiler between outputs and clobbers. Further more it seems bad > > Do you mean the flags register is already being included as an > implicit clobber because there is an output of any kind? I can't find > documentation that says this is true. If anything it looks like it > could be "improved" from a full "cc" clobber to an output operand > flag, like =@ccs.
As hpa has already said, "cc" has been automatically included as a clobber forever on x86 (until the condition code outputs appeared, at which point the clobber became added only if no such output was present). >> --- 4.16-rc2/arch/x86/include/asm/rmwcc.h >> +++ 4.16-rc2-x86-rmwcc-clobbers/arch/x86/include/asm/rmwcc.h >> @@ -2,8 +2,7 @@ >> #ifndef _ASM_X86_RMWcc >> #define _ASM_X86_RMWcc >> >> -#define __CLOBBERS_MEM "memory" >> -#define __CLOBBERS_MEM_CC_CX "memory", "cc", "cx" >> +#define __CLOBBERS_MEM(clb...) "memory", ## clb > > This leaves a trailing comma in the non-cx case. I thought that caused > me problems in the past, but maybe that's GCC version-specific? No, it's formally specified to drop the comma. Note this is using a gcc extension, not the C99 way of having a macro with variable number of arguments (where - without again some gcc special handling - this indeed would be a problem). Jan