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Extra 8 bytes on stack in gcc 4.3 (64-bit)
- From: Bob Plantz <plantz at cds1 dot net>
- To: gcc-help <gcc-help at gcc dot gnu dot org>
- Date: Thu, 09 Apr 2009 20:56:05 -0700
- Subject: Extra 8 bytes on stack in gcc 4.3 (64-bit)
When I use gcc -S in an x86-64 environment running in 64-bit mode on the
following C function:
int main( void )
{
long sum = foo( 3, 4, 5, 6, 7 );
}
gcc-4.1 allocates 16 bytes on the stack for the local variable, sum (my
comments added):
main:
.LFB3:
pushq %rbp # save caller base pointer
.LCFI2:
movq %rsp, %rbp # establish our base pointer
.LCFI3:
subq $16, %rsp # space for local variable
.LCFI4:
movl $7, %r8d # pass 5th argument
.........
The ABI specifies that the stack pointer will be "16-byte aligned at
process entry." So even though sum only takes 8 bytes (longs in 64-bit
mode are 8 bytes), I assume that the compiler allocates 16 bytes to
maintain the 16-byte alignment.
But with gcc-4.3 I get 24 bytes:
main:
.LFB3:
pushq %rbp # save caller base pointer
.LCFI2:
movq %rsp, %rbp # establish our base pointer
.LCFI3:
subq $24, %rsp # space for local variable
.LCFI4:
movl $7, %r8d # pass 5th argument
.........
Now the stack is no longer 16-byte aligned. Furthermore, this stack
alignment could have been accomplished by allocating only the 8 bytes
needed for the local variable, sum.
What am I missing here?
--Bob