Re: PATCH: Turn on -fomit-frame-pointer by default for 32bit Linux/x86

[Mark Mitchell <mark at codesourcery dot com>]

Mark Mitchell wrote:

> I will review the thread today.  Please give me some time to comment.

I can certainly see both sides of this issue.

On the one hand, most users use "-O2" or "-O3", and we want those users
to see the best performance possible.  It's not reasonable to expect
that most users will figure out that they can use -fomit-frame-pointer
to improve performance and add it in.  My understanding is that some
distributions (e.g., OpenSuSE, IIUC) are now building the vast majority
of the distribution with -fomit-frame-pointer, but end users will not
necessarily know that they can/should use this option.  The bottom line
is that -O2 should maximize performance.

On the other, changing this default will undoubtedly cause pain.  As
Daniel said, we know that ARM and MIPS users have wrestled with the lack
of a frame pointer for some time.  There is a surprising amount of code
that has built-in unwinders of various kinds.  Being able to get
accurate backtraces from systems deployed in the field is essential for
many users.  This code will break, silently, and people will be unhappy.

If I could, I'd go back in time and eliminate ABIs that use frame
pointers.  The use of unwind tables is, IMHO, a clearly superior
solution.  It provides for superior performance, and, for users
uninterested in backtraces, smaller code.  (Even for users who do need
backtraces, it may result in a smaller cache footprint, since the unwind
tables will remain paged out most of the time.)  But, we are where we
are, and we can't go back.  So, the question is whether to make a
transition now, and, if so, how.

On balance, I think it's appropriate to make the change.  I think
there's an overall move towards ABIs without frame pointers, and that if
x86 stakeholders want to change, it's not our place as GCC maintainers
to do this.  The fact that major distributions are already building the
distribution itself (including its run-time libraries) without frame
pointers suggests that users must be adapting.  And the fact that ARM
and MIPS users are still developing software suggests that people are
learning how to deal with unwind tables.  So, it's a significant
transition, and there will be fallout, but it seems reasonable to me.

Ultimately, however, I don't think this is an RM question.  I think it
should be determined by the x86 back-end maintainers.  I'm happy to give
my input, and to help moderate a discussion if appropriate, but this
change does not affect any other architecture, so I think it's properly
a question for the x86 back-end maintainers.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713