Re: [AMBER-Developers] questions about compiler flags

From: Volodymyr Babin <>
Date: Mon, 1 Feb 2010 22:12:33 -0500 (EST)

> 1. Is anyone still familiar with the Large File support in Linux? My
> questions are about the flags
> added by configure. Specifically:
> a. Aren't these only needed for x86 (32-bit) compiles? Do we need them
> for x86-64 (em64t)?
> b. My Googling seems to indicate that only the first flag is required, not
> the second one; might this be true? Does anyone have a test of this
> functionality?

I am not familiar, but googling/looking into /usr/include/features.h
makes me think that both flags may have effect on libc on both 32 and
64 bit platforms. These #defines seem to activate different libc features,
and, assuming that both are needed, I would keep both flags set.
(64-bit offset and functions that take off_t argument [fseeko, ftello]).
It is possible that in some cases one flag implies the other, but I
did not see this clearly stated anywhere.

> 2. Do we really need the "-ip -O3 -xHost" flags for optimized ifort
> compiles?
> Is this better/worse/the same as "-fast" (used for pmemd)? Does sander
> need
> anything more than -O3? Is there a good compromise between "fastest of
> all
> 2^N possible combinations of flags on this particular machine" and
> "something
> that sacrifices a few percent in performance but which is likely to both
> work
> now and continue working after Intel puts out its next release"?

Why not just have -O2 there and a README file containing more
aggressive combinations for the curious.



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Received on Mon Feb 01 2010 - 19:30:03 PST
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