On Tue, May 05, 2020 at 05:40:21PM +0200, Christophe Leroy wrote: > >>+#define __put_user_asm_goto(x, addr, label, op) \ > >>+ asm volatile goto( \ > >>+ "1: " op "%U1%X1 %0,%1 # put_user\n" \ > >>+ EX_TABLE(1b, %l2) \ > >>+ : \ > >>+ : "r" (x), "m<>" (*addr) \ > > > >The "m<>" here is breaking GCC 4.6.3, which we allegedly still support. > > > >Plain "m" works, how much does the "<>" affect code gen in practice? > > > >A quick diff here shows no difference from removing "<>". > > It was recommended by Segher, there has been some discussion about it on > v1 of this patch, see > https://patchwork.ozlabs.org/project/linuxppc-dev/patch/4fdc2aba6f5e51887d1cd0fee94be0989eada2cd.1586942312.git.christophe.le...@c-s.fr/ > > As far as I understood that's mandatory on recent gcc to get the > pre-update form of the instruction. With older versions "m" was doing > the same, but not anymore.
Yes. How much that matters depends on the asm. On older CPUs (6xx/7xx, say) the update form was just as fast as the non-update form. On newer or bigger CPUs it is usually executed just the same as an add followed by the memory access, so it just saves a bit of code size. > Should we ifdef the "m<>" or "m" based on GCC > version ? That will be a lot of churn. Just make 4.8 minimum? Segher