x86-64 precompiled binary question

[bob 295]

We've been producing i386 precompiled binary versions of the SIMPL open source 
stuff for a while now.    This works well enough for those with 32 bit 
machines.   It doesn't work very well for 64 bit machines.

What is the equivalent GCC flag to produce x86-64 code to the lowest common 
denominator for 64 bit machines?   ie. what do Linux distributions use when 
they produce an x86-64 version? 

I looked up -mtune=generic on the GCC man page but it appears to produce some 
kind of hybrid 32bit/64bit code.  Very confusing.

Thanks in advance for your help.

bob
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