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
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