This is the mail archive of the
gcc-help@gcc.gnu.org
mailing list for the GCC project.
Re: Do not spill variables/registers on the stack
Hi,
What if you try looking at the output of '-S -fverbose-asm'? Doesn't
that include all of the -f<thing> optimizery flags that are actually
passed to cc1? Maybe one of those will suggest something helpful?
Do the -O<x> flags only select sets of -f<thing>(s)? Or do they also
enable some things on their own? In other words if I did -O1 and then
did -fno-<thing> for everything that -O1 turned on would I effectively
disable -O1?
kevin
On Wed, Feb 2, 2011 at 12:39 PM, Stefan Schulze Frielinghaus
<stefan@seekline.net> wrote:
> On Mi, 2011-02-02 at 19:05 +0100, Drasko DRASKOVIC wrote:
>> On Wed, Feb 2, 2011 at 6:55 PM, Stefan Schulze Frielinghaus
>> <stefan@seekline.net> wrote:
>> >> But have you tried comparing the outputs of what -O0 outputs to say -O2 ?
>> >
>> > The problem with using anything else then -O0 is that it enables other
>> > optimization techniques, e.g. constant propagation (the example of the
>> > first mail would be scaled down to a simple "return 0;") which I do not
>> > want. Therefore, I would like to compile my code without any
>> > optimizations except register allocation.
>>
>> Hi Stefan,
>> keep in mind that "register" keyword is only a **hint** given to
>> compiler to do register optimization. Compiler is not obliged to
>> listen to your hints, and it probably does not do so without
>> optimization turned on (I am not sure if it can be forced).
>
> Yes that's true. I only used it in my examples to demonstrate what
> result I actually wanted.
>
>> BTW, you should also keep in mind that ANSI C does not allow for
>> taking the address of a register object; this restriction does not
>> apply to C++. However, if the address-of operator (&) is used on an
>> object, the compiler must put the object in a location for which an
>> address can be represented. In practice, this means in memory instead
>> of in a register. Because of this restriction GCC will ignore the
>> register keyword on variables whos address is taken at any point in
>> the program. So, ensure that somewhere in your code you are not using
>> the addresses operator on these variables.
>
> This wouldn't be a problem for me because in ANSI C, as you already
> mentioned, the address-of operator is not allowed for variables which
> reside in a register, ergo the variable cannot stay in a register and
> needs to be spilled on the stack. That is perfectly fine for me. I'm
> more interested in all other variables.
>
> Kind regards,
> Stefan
>
>