Hi, gcc 4 seems quite a bit smarter at keeping the stack smaller, and as you can see below allocates the 8 byte structure across two stack slots.
It did break something, which drew this to my attention, and James Wilson already some analsysis in http://www.gelato.unsw.edu.au/archives/linux-ia64/0507/1885.html. Quoting him > If gcc-3.4 and earlier, some structures (BLKmode structures) were being > over-aligned when allocated to stack slots. They always got the maximum > alignment (16 bytes for IA-64) instead of their natural alignment. It > isn't clear why. I think this was an accident of implementation. We > were basing variable alignment on modes instead of type alignment, and > since some BLKmode structures do require max alignment, we had to give > it to all of them. We came to the conclusion that we are unsure if this is a bug or a feature. I am unsure if the larger stack space required is a good trade off against possible better code. Thanks, -i --- sample --- [EMAIL PROTECTED]:/tmp$ cat test.c #include <stdio.h> struct disk_stat { int fd; unsigned count; }; int main(void) { int blah; struct disk_stat test; printf("%p\n", &blah); printf("%p\n", &test); } [EMAIL PROTECTED]:/tmp$ gcc-3.4 -o test test.c [EMAIL PROTECTED]:/tmp$ ./test 0x60000fffffcdf480 0x60000fffffcdf490 [EMAIL PROTECTED]:/tmp$ gcc-4.0 -o test test.c [EMAIL PROTECTED]:/tmp$ ./test 0x60000fffffd57490 0x60000fffffd57494 -- Summary: Allocation of structures across stack slots Product: gcc Version: 4.0.1 Status: UNCONFIRMED Severity: minor Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: ianw at gelato dot unsw dot edu dot au CC: gcc-bugs at gcc dot gnu dot org GCC target triplet: ia64-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22605
