svn commit: r274489 - in head/sys/amd64: amd64 include

[Scott Long]

> On Nov 20, 2014, at 11:33 PM, Rui Paulo <rpaulo at me.com> wrote:
> 
> On Nov 13, 2014, at 14:11, Scott Long <scottl at 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.

Scott