On Fri, 21 Nov 2014 16:26:47 -0700 Scott Long <scott4l...@yahoo.com> wrote: > On Nov 20, 2014, at 11:33 PM, Rui Paulo <rpa...@me.com> wrote: >> On Nov 13, 2014, at 14:11, Scott Long <sco...@freebsd.org> wrote: >>> >>> Author: scottl >>> Date: Thu Nov 13 22:11:44 2014 >>> New Revision: 274489 >>> URL: https://svnweb.freebsd.org/changeset/base/274489 >>> >>> Log: >>> Extend earlier addition of stack frames to most of support.S. This makes >>> stack traces in KDB, HWPMC, and DTrace much more reliable and useful. >> >> No performance differences? The kernel enables/disables the compiler >> option to omit the frame pointer based on the kernel config file. If >> DDB, DTrace, or HWPMC is enabled, the frame pointer is always saved in >> C functions. >> >> Some of these functions are in the hot path, so if you didn't see any >> performance problem, I wonder if we should disable -fomit-frame-pointer >> always. > > That’s a good question to look further into. I didn’t see any measurable > differences with this change. I think that the cost of the function call > itself masks the cost of a few extra instructions, but I didn’t test with > switching it on/off for the entire kernel. That said, I purposely > implemented this as macros so it could be easily changed in the future. > If someone finds that this measurably impacts a certain workload, I > wouldn’t object to making it conditional, though it does complicate any > hand-written ASM code that tries to access the stack via %esp offsets. > We don’t have anything like that now, but Kip was concerned enough about > it in the future that I left it enabled unconditionally.
An alternative is to annotate the functions with .cfi directives. Stack unwinding doesn't need frame pointers then. http://www.logix.cz/michal/devel/gas-cfi/ https://sourceware.org/binutils/docs-2.24/as/CFI-directives.html _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
