On Sun, 11 Nov 2012, Dimitry Andric wrote:
On 2012-11-11 12:53, Bruce Evans wrote:
I'm not sure if either of us knows exactly what this does, but I like
this change. clang does stack alignment correctly, so it doesn't need
the gcc pessimization of aligning in the caller. clang aligns in the
Apparently we did know.
...
alignment. gcc's alignment in callers doesn't even work, because gcc
assumes that it is enough and never does it in callees even when it
is necessary:
auto int32_t foo[8] __aligned[32];
gcc fails to do the necessary alignment. clang does it.
auto int32_t foo[8] __aligned[16];
Both gcc and (hopefully only without the above fix) clang fail to do the
necessary alignment.
It works just fine now with clang. For the first example, I get:
pushl %ebp
movl %esp, %ebp
andl $-32, %esp
as prolog, and for the second:
pushl %ebp
movl %esp, %ebp
andl $-16, %esp
Good.
The andl executes very fast. Perhaps not as fast as subl on %esp,
because subl is normal so more likely to be optimized (they nominally
have the same speeds, but %esp is magic). Unfortunately, it seems to
be impossible to both align the stack and reserve some space on it in
1 instruction -- the andl might not reserve any.
special alignment in main(), but assumes that crtso did it. clang also
doesn't support -mpreferred-stack-boundary, so it is incompatible with
nonstandard ABI's like the one given by always using
-mpreferred-stack-boundary=32.
Apparently upstream never saw the need for this option. I strongly
doubt it is used very often outside FreeBSD...
It is most useful for working around gcc's behaviour. Hmm, the kernel
and boot blocks uses -mpreferred-stack-boundary=2 on i386 to save space.
This must have been turned off for clang, so the versions that did
16-bit alignment must have come close to blowing the kernel stack. In
boot blocks, I think there is plenty of stack but alignment wastes code
space.
I'd like to try -mpreferred-stack-boundary=3 in amd64 kernels, but this
is an i386-only option.
Yes, we need clang/our libraries to handle different alignments and
not assume that callers do more than the ABI requires and pessimize
on behalf of the libraries. Outside of libraries, the problem is small
provided -mpreferred-stack-boundary works, since you can compile
everything with the same -mpreferred-stack-boundary.
As far as I can see, with the 4 byte stack alignment that has been the
default, and is now also clang's default, our libraries should handle
any "incoming" stack alignment of >= 4 bytes. I don't think anybody
uses lower stack alignment, except maybe for the special case of boot
loaders, or extremely size-optimized code.
-Os could reasonably generate lower stack alignment, but that rarely
saves space. -Os generally doesn't give enough control over alignment
for space-time tradeoffs. I made some minor changes in FreeBSD's gcc
to make it _not_ give misaligned 32-bit variables (?) since -Os should
only optimize for space if it doesn't cost much time for small savings
in space. But if you only care about space, it would be useful to
minimize the alignment of everything.
Bruce
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