Re: stack allocation and gcc

From: Ihar 'Philips' Filipau
Date: Wed Mar 10 2004 - 09:28:00 EST

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).


0x68 != 68.

[ All known to me assemblers on ix86 use hexadecimal number by default. I was sort of surprised to find out that ppc gas uses decimal numbers only. ]

0x68 == 104 == 32*3 + 4 + 4.

As I have said, keep adding to function 'do { char buf[32]; printf(buf); } while(0)' increse this number. As in my logic it is supposed to be unchanged - all renundant buffers do have same size - 32 bytes.

Ihar 'Philips' Filipau / with best regards from Saarbruecken.
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