http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51491
--- Comment #4 from vries at gcc dot gnu.org 2011-12-12 21:25:58 UTC --- f1.c vs. f.c: ... int g(int*); int g(int*); int f1(void) | int f(void) { { int tt = 0; int tt = 0; int t = 4; int t = 4; { { int a[4]; | int a[t]; tt = g(a); tt = g(a); tt += a[0]; tt += a[0]; } } { { int a[4]; int a[4]; tt += g(a); tt += g(a); tt += a[0]; tt += a[0]; } } return tt; return tt; } } ... f1.c and f.c are compiled like this (for x86_64): ... $ gcc -O2 -S f.c f1.c -fdump-tree-all ... f1.c.149t.optimized has 2 clobbers: ... f1 () { int a[4]; int a[4]; int tt; int D.1712; int D.1711; int D.1710; <bb 2>: tt_3 = g (&a); D.1710_4 = a[0]; tt_5 = D.1710_4 + tt_3; a ={v} {CLOBBER}; D.1711_6 = g (&a); tt_7 = D.1711_6 + tt_5; D.1712_8 = a[0]; tt_9 = D.1712_8 + tt_7; a ={v} {CLOBBER}; return tt_9; } ... f.c.149t.optimized has only 1 clobbers: ... f () { <unnamed-unsigned:8> D.1726[16]; int a[4]; int tt; int D.1722; int D.1721; void * saved_stack.2; int D.1719; <bb 2>: saved_stack.2_3 = __builtin_stack_save (); tt_18 = g (&D.1726); D.1719_19 = MEM[(int[0:D.1713] *)&D.1726][0]; tt_20 = D.1719_19 + tt_18; __builtin_stack_restore (saved_stack.2_3); D.1721_21 = g (&a); tt_22 = D.1721_21 + tt_20; D.1722_23 = a[0]; tt_24 = D.1722_23 + tt_22; a ={v} {CLOBBER}; return tt_24; } ... f1 uses only 16 bytes stack space, since the 2 'a[4]' arrays share the stack space: ... grep 'subq.*rsp' f1.s f.s f1.s: subq $16, %rsp f.s: subq $32, %rsp ...