On Mon, 2014-06-09 at 15:16 -0700, Junio C Hamano wrote:
> David Turner <dtur...@twopensource.com> writes:
> > Since Junio has picked up the first patch from previous versions of
> > this series, I'm just going to send the second (SSE) one. I decided
> > not to s/NO_SSE42/!HAVE_SSE42/ because it looks like git mostly uses
> > the former convention (for instance, that's what GIT_PARSE_WITH
> > generates).
> Yeah but NO_FROTZ is used only when FROTZ is something everybody is
> expected to have (e.g. it's in posix, people ought to have it, but
> we do support those who don't), isn't it? For a very arch specific
> stuff like sse42, I'd feel better to make it purely opt-in by
> forcing people to explicitly say HAVE_SSE42 to enable it.
The patch now has two kinds of autodetection:
1. At build-time, we check for the compiler supporting -msse4.2. If it
does, and if the user has not explicitly done --without-sse, then we
build with SSE support. This does not mean that the SSE code will
necessarily be used because:
2. At run-time, if we have built with SSE support, we check cpuid to
choose a version of the function that will run on the current CPU.
So I think we never hit a case where we try to use SSE and fail, which
is the major reason I see to make it non-default.
To me, this means that we should not require people to explicitly
request SSE, because we generally want to try to provide the
most-efficient version of git that will work everywhere. In fact, I am
not sure we need a --without-sse option at all, since all it saves is a
cpuid instruction. But I don't need to remove the option, in case
there's a use for it I'm not thinking of.
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