On Fri, 2009-04-10 at 03:16 -0700, John (Eljay) Love-Jensen wrote:
> Hi Bob,
>
> > Now the stack is no longer 16-byte aligned.
>
> Leaf functions do not need to maintain stack alignment.
>
main is not a leaf function here. It calls foo.
> > Furthermore, this stack alignment could have been accomplished by allocating only the 8 bytes needed for the local variable, sum.
>
> Optimized code, or non-optimized?
>
> If it is non-optimized (-O0, which is the default if not specified), non-optimized code may be non-optimal.
>
It was non-optimized. But I still expected the compiler not to allocate
16 bytes on the local stack frame for no reason. Oddly (to me)
int main( void )
{
long sum = foo( 3, 4, 5, 6, 7 );
return 0;
}
produces:
main:
pushq %rbp
movq %rsp, %rbp
subq $16, %rsp
int main( void )
{
long sum = foo( 3, 4, 5, 6, 7 );
}
produces:
main:
pushq %rbp
movq %rsp, %rbp
subq $24, %rsp
and
void main( void )
{
long sum = foo( 3, 4, 5, 6, 7 );
}
produces:
main:
pushq %rbp
movq %rsp, %rbp
subq $16, %rsp
My "application" here is an x86-64 textbook I have written. The point of
my book is to show how the machine works at the instruction set level
when code is written at the C/C++ level. So I start my examples with C/C
++ code. The book serves as an introduction to architecture. It's not an
assembly language book, nor a compiler book. But I would like to be able
to explain things in the book that the student might see when he/she
replicates my examples. I do have a "your mileage may vary" due to
different gcc/as versions.
Bob
