RS: [Beowulf] Sempron compile optimization

Greg Lindahl lindahl at pathscale.com
Thu Nov 24 00:23:08 EST 2005


On Wed, Nov 23, 2005 at 04:31:43PM -0800, Martin Siegert wrote:

> Actually, people may think differently about that: 32bit code may
> run slightly faster. Thus, if you do not need the address space you
> just as well can compile in 32 bit mode.

This is true for just about any 64-bit mode. Fortunately, it's the
case that 64-bit modes which have extra registers (and hence generally
higher performance) generally have the right compiler default. MIPS is
another example where n32 is never slower than n64, and so n32 is the
default. On Opteron and EM64T, the 64-bit mode is almost always faster
due to the extra registers and better calling convention. Except for
things like compilers. Which is why our compiler is a 32-bit app.

> Defaults are more or less a matter of taste ...

Until they get in the way of performance.

> What is really disappointing is that most (?) configure scripts (and
> libtool, etc.) break when you compile in 64bit mode (xlc -q64 ...
> or gcc -m64 ...):

It's true that if you want to build 64-bit AMD64 from rpms, you still
need to make a few symlinks, like /usr/X11R6/lib -> lib64.

-- greg
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