FreeBSD 5.2 v/s FreeBSD 4.9 MFLOPS performance (gcc3.3.3 v/sgcc2.9.5)

[Tim Kientzle]

Matthew Dillon wrote:
>     I'm surprised Bruce hasn't chimed in here yet.  I guess he's tired of
>     repeating himself.
> 
>     In 4.9, libcsu, which generates crt1.o (which is the start code for
>     C programs which the linker links in automatically) has this line in it:
> 
> 	    andl    $~0xf, %%esp            # align stack to 16-byte boundary
> 
>     So anything linked with 4.9 is going to align the stack on a 
>     16 byte boundary no matter WHAT the kernel does.
> 
>     FreeBSD-5 does not have this alignment in its crt1.o because GCC3
>     automatically aligns the stack on a per-procedure basis.  Or at least
>     it is supposed to.  Maybe it's broke?  :-)

I've not looked at 3.3, but I seem to recall that GCC 3.2
did not actually align the stack within each function, but
preserved the alignment.  (That is, each function assumed the stack
had a certain alignment on entry and ensured that alignment
was preserved for any subsequent function calls.)

I had my doubts about this, as it meant there was a LOT
of stack fiddling before and after every function call.
(The alignment was handled at caller, not callee.  A lot of
functions don't need any alignment at all, really, so
it seems like it could be wasted effort in a lot of cases.)

If I'm remembering this correctly, then aligning
the stack in crt1.o would be pretty much essential.

Tim Kientzle