stack allocation and gcc

Ihar 'Philips' Filipau
2004-03-10T12:28:18Z

Hello All!

[ please cc: me ]

I have observed funny behaviour of both gcc 2.95/322 on ppc32 and
i686 platforms.

Have written this routine and compiled it with 'gcc -O2':

int a(int v)
{
char buf[32];

if (v > 5) {
char buf2[32];
printf( buf, buf2 );
} else {
char buf2[32];
printf( buf, buf2 );
}
return 1;
}

I expected that stack on every branch of 'if(v>5)' will be allocated
later - but seems that gcc allocate stack space once and in this case it
will 'overallocate' 32 bytes - 'char buf2' will be allocated twice for
every branch. On i686 gcc allocates 108 bytes, on ppc32 it allocates 116
bytes. (additional space seems to be induced by printf() call)
Adding to this routine something like 'do { char a[32]; } while(0);'
several times shows that stack buffers are not reused - and allocated
for every this kind of context separately.

As to my understanding - since this buffers do live in different
mutually exclusive contextes - they can be reused. But this seems to be
not case. Waste of precious kernel stack space - and waste of d-cache.

I have read 'info gcc' - but found nothing relevant to this.
I've checked ppc abi - but found no limitations to reuse of stack space.

Is it expected behaviour of compiler? gcc feature?

[ I have created macro which opens into inline function call which
utilizes va_list - on ppc32 va_list adds at least 32 bytes to stack use.
Seems to be bad idea for kernel-space, since every use if macro adds to
stack use (10 macro calls == 320 bytes). Easy to rewrite to not to use
va_list - but have I *NO* stack allocation check script in place - this
stuff could easily get into production release. Not nice. ]

disassembling outputs:

--- objdump/ix86 -------------------
00000000 :
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 ec 68 sub $0x68,%esp
6: 83 7d 08 05 cmpl $0x5,0x8(%ebp)
a: 7e 1c jle 28 <a+0x28>
c: 83 ec 08 sub $0x8,%esp
f: 8d 45 b8 lea 0xffffffb8(%ebp),%eax
12: 50 push %eax
13: 8d 45 d8 lea 0xffffffd8(%ebp),%eax
16: 50 push %eax
17: e8 fc ff ff ff call 18 <a+0x18>
18: R_386_PC32 printf
1c: 83 c4 10 add $0x10,%esp
1f: b8 01 00 00 00 mov $0x1,%eax
24: c9 leave
25: c3 ret
26: 89 f6 mov %esi,%esi
28: 83 ec 08 sub $0x8,%esp
2b: 8d 45 98 lea 0xffffff98(%ebp),%eax
2e: eb e2 jmp 12 <a+0x12>
------------------------------------

--- objdump/ppc82xx ----------------
00000000 :
0: 94 21 ff 90 stwu r1,-112(r1)
4: 7c 08 02 a6 mflr r0
8: 90 01 00 74 stw r0,116(r1)
c: 2c 03 00 05 cmpwi r3,5
10: 40 81 00 18 ble 28 <a+0x28>
14: 38 61 00 08 addi r3,r1,8
18: 38 81 00 28 addi r4,r1,40
1c: 4c c6 31 82 crclr 4*cr1+eq
20: 48 00 00 01 bl 20 <a+0x20>
20: R_PPC_REL24 printf
24: 48 00 00 14 b 38 <a+0x38>
28: 38 61 00 08 addi r3,r1,8
2c: 38 81 00 48 addi r4,r1,72
30: 4c c6 31 82 crclr 4*cr1+eq
34: 48 00 00 01 bl 34 <a+0x34>
34: R_PPC_REL24 printf
38: 38 60 00 01 li r3,1
3c: 80 01 00 74 lwz r0,116(r1)
40: 7c 08 03 a6 mtlr r0
44: 38 21 00 70 addi r1,r1,112
48: 4e 80 00 20 blr
------------------------------------