On Wed, Mar 4, 2020 at 10:28 AM Brooks Davis <bro...@freebsd.org> wrote: > > On Wed, Mar 04, 2020 at 12:27:08PM +0100, Guido Falsi wrote: > > On 02/03/20 18:13, Ryan Libby wrote: > > > On Mon, Mar 2, 2020 at 12:45 AM Alexander V. Chernikov <melif...@ipfw.ru> > > > wrote: > > >> > > >> 28.02.2020, 18:32, "Ryan Libby" <rli...@freebsd.org>: > > >>> Author: rlibby > > >>> Date: Fri Feb 28 18:32:36 2020 > > >>> New Revision: 358439 > > >>> URL: https://svnweb.freebsd.org/changeset/base/358439 > > >>> > > >>> Log: > > >>> amd64 atomic.h: minor codegen optimization in flag access > > >>> > > >>> Previously the pattern to extract status flags from inline assembly > > >>> blocks was to use setcc in the block to write the flag to a register. > > >>> This was suboptimal in a few ways: > > >>> - It would lead to code like: sete %cl; test %cl; jne, i.e. a flag > > >>> would just be loaded into a register and then reloaded to a flag. > > >>> - The setcc would force the block to use an additional register. > > >>> - If the client code didn't care for the flag value then the setcc > > >>> would be entirely pointless but could not be eliminated by the > > >>> optimizer. > > >>> > > >>> A more modern inline asm construct (since gcc 6 and clang 9) allows > > >>> for > > >> This effectively restricts kernel builds by all older compilers. > > >> Is there any chance of making it conditional depending on the compiler > > >> version/features? > > > > > > Yes, it is possible to test for __GCC_ASM_FLAG_OUTPUTS__. It is more > > > maintenance effort going forward. If building current with an old cross > > > compiler is an important scenario, we can either revert this and the > > > following revision or work up a patch to make it conditional. I'll see > > > what that might look like. > > > > > > > Actually this causes emulators/virtualbox-ose port to fail to build: > > > > In file included from /usr/src/sys/sys/systm.h:44: > > /usr/include/machine/atomic.h:230:1: error: invalid output constraint > > '=@cce' in asm > > ATOMIC_CMPSET(char); > > ^ > > /usr/include/machine/atomic.h:205:4: note: expanded from macro > > 'ATOMIC_CMPSET' > > : "=@cce" (res), /* 0 */ \ > > ^ > > /usr/include/machine/atomic.h:230:1: error: invalid output constraint > > '=@cce' in asm > > > > (and so on) > > > > > > the virtualbox-ose port is forced to use an older clang version due to > > crashes when compiled with newer ones. > > > > Not sure whose responsibility is to fix this. > > I suspect that now that we don't care about gcc 4.2.1, we should > restructure machine/atomic.h to use __atomic compiler builtins in nearly > all cases. We could then conditionalize small sets of mircooptimized > assembly versions based on the availability of compiler features if they > add any value. > > On CheriBSD we've switched the RISC-V to use the C versions and are > overdue to do the same to MIPS. Reworking things to make this the > default would decrease our maintenance burden and it seems unlikely that > most of our platforms would benefit from handcode assembly (given the > general level of optimization in our lower-tier platforms). > > -- Brooks
There's further discussion on that topic in the original review (D23869) and in D23661. _______________________________________________ freebsd-emulation@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-emulation To unsubscribe, send any mail to "freebsd-emulation-unsubscr...@freebsd.org"
