I agree that it isn't our job.  I explicitly stated that we should
only be interested in this if it turned out that it indeed does what I
think it does.  Certainly, if it hurts more than it helps, why use it?

My experience thusfar--for what it's worth, which isn't much of any
probative value--is that -march=nocona AND gcc 4.1/NetBSD's default
are significantly worse than -march=x86-64.  I switched to the i386
port in part to get DRI, but mostly because the performance of my
machine (A Pentium D 930/Intel D945GTP motherboard) was HORRID using
the amd64 port.  One needed no benchmarks to see how much slower it
was in many cases.  I'm still researching this amidst a paucity of
information, but I've garnered thusfar that -march=nocona may actually
disable the instruction scheduler.  Moreover, there are bug reports of
gcc ( both 4.0 and 4.1) generating 3dnow prefetch instructions by
default on x86_64.

My point here is that such a flag isn't necessarily a no-op at all: it
tells the compiler only to use instructions which AMD and Intel
processors *have in common*.  In fact, gcc 4.2 and above DO have this
feature documented, although it is -march=generic.

Disparate instruction sets are even more of a problem for the Core 2 chips, etc.

I only have an EMT64 processor because I got it through a dealer I did
some work for at a great price, and the intel board has a 3-year parts
and labor warranty.  Barring these circumstances, I would've simply
bought an AMD CPU and saved myself the headache, as AMD is clearly the
better choice here, IMHO.

I'm also interested in -march=x86-64 because there are severe linux
compatability problems on NetBSD/amd64 _ONLY_ with Intel chips.

Again, I agree with the general spirit of your message.  The choices I
have with gcc 4.1, though, are like the US presidential elections as
of late: I'd go with a third party just to avoid the complications of
the other two. ;)

I rebuilt my whole tree and kernel with -march=x86_64 and now my usual
workloads seem a lot more on par with i386.  This could be a
confirmation bias of mine, though, so I'll have to run benchmarks at
some point.

Regards,

--Blair




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