Re: stack allocation and gcc

Bart Hartgers
2004-03-10T13:15:25Z

0x68 (hex) == 104 (dec)

Regards,
Bart

On 10 Mar, Jaco Kroon wrote:
> Hi Ihar,
>
> In your code you have 3 buffers, one in the mani branch of 32 bytes
> and one of 32 bytes in each of the sub branches. This adds up to a
> total of 64 bytes since as you say, the two buffers named buf2[32]
> can be shared. Thus it is 32 bytes for buf and 32 bytes for the
> shared buffer. Now if you look at the function startup code:
>
> 0: 55 push %ebp
> 1: 89 e5 mov %esp,%ebp
> 3: 83 ec 68 sub $0x68,%esp
>
> THe push saves the frame pointer (ebp). The mov sets up the new stack
> frame and the sub allocates space of 68 bytes on the stack, 4 bytes
> more than the expected 64, this is probably for temporary storage
> required somewhere in the function. As such, gcc does not allocate
> 32 bytes too many (at least not on i386, but probably not on other
> architectures either).
>
> Jaco
>
> Ihar 'Philips' Filipau wrote:
>
>> 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:
>>
> ===========================================
> This message and attachments are subject to a disclaimer. Please refer
> to www.it.up.ac.za/documentation/governance/disclaimer/ for full
> details. Hierdie boodskap en aanhangsels is aan 'n vrywaringsklousule
> onderhewig. Volledige besonderhede is by
> www.it.up.ac.za/documentation/governance/disclaimer/ beskikbaar.
> ===========================================
>

--
Bart Hartgers - TUE Eindhoven