Date: Sun, 23 Nov 2014 19:02:06 +0100 From: Tijl Coosemans <tijl@FreeBSD.org> To: Scott Long <scott4long@yahoo.com> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Scott Long <scottl@FreeBSD.org>, Rui Paulo <rpaulo@me.com> Subject: Re: svn commit: r274489 - in head/sys/amd64: amd64 include Message-ID: <20141123190206.37942760@kalimero.tijl.coosemans.org> In-Reply-To: <13EC3116-6146-42FC-8941-2C7C009224B3@yahoo.com> References: <201411132211.sADMBjP3009246@svn.freebsd.org> <35E5EAD8-99C1-43C0-8D01-B3B5B86ECA25@me.com> <13EC3116-6146-42FC-8941-2C7C009224B3@yahoo.com>
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On Fri, 21 Nov 2014 16:26:47 -0700 Scott Long <scott4long@yahoo.com> wrote: > On Nov 20, 2014, at 11:33 PM, Rui Paulo <rpaulo@me.com> wrote: >> On Nov 13, 2014, at 14:11, Scott Long <scottl@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.htmlhelp
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