Messages in this thread | | | Subject | Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible | From | Christophe Leroy <> | Date | Wed, 14 Apr 2021 14:42:51 +0200 |
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Le 14/04/2021 à 14:24, Segher Boessenkool a écrit : > On Wed, Apr 14, 2021 at 12:01:21PM +1000, Nicholas Piggin wrote: >> Would be nice if we could let the compiler deal with it all... >> >> static inline unsigned long lr(unsigned long *mem) >> { >> unsigned long val; >> >> /* >> * This doesn't clobber memory but want to avoid memory operations >> * moving ahead of it >> */ >> asm volatile("ldarx %0, %y1" : "=r"(val) : "Z"(*mem) : "memory"); >> >> return val; >> } > > (etc.) > > That can not work reliably: the compiler can put random instructions > between the larx and stcx. this way, and you then do not have guaranteed > forward progress anymore. It can put the two in different routines > (after inlining and other interprocedural optimisations), duplicate > them, make a different number of copies of them, etc. > > Nothing of that is okay if you want to guarantee forward progress on all > implementations, and also not if you want to have good performance > everywhere (or anywhere even). Unfortunately you have to write all > larx/stcx. loops as one block of assembler, so that you know exactly > what instructions will end up in your binary. > > If you don't, it will fail mysteriously after random recompilations, or > have performance degradations, etc. You don't want to go there :-) >
Could the kernel use GCC builtin atomic functions instead ?
https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
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