Do not spill variables/registers on the stack

[David Brown]

On 03/02/2011 23:36, Drasko DRASKOVIC wrote:
> Back to the topic, few things crossed my mind :
>
> 1) Register keyword is a "hint" to the compiler, and thus not
> obligatory. We saw that for -O1 and above it is "useless hint", as it
> is always ignored. Is it however always respected by GCC when -O0 ? If
> yes - why is it a hint ? If no, in which cases it is not respected ?
> Only when you assign more vars than you have reg, or GCC might ignore
> it some other times even with -O0 ?
>
> 2) Is this behavior documented anywhere ? Can we find docs that note :
>      a) that "regiser" keyword is not ignored only with -O0
>      b) when -O0 in which cases it is taken into account and in which cases not.
>

<http://gcc.gnu.org/onlinedocs/gcc/Hints-implementation.html>


To quote:

4.8 Hints

     * The extent to which suggestions made by using the register 
storage-class specifier are effective (C90 6.5.1, C99 6.7.1).

       The register specifier affects code generation only in these ways:
           o When used as part of the register variable extension, see 
Explicit Reg Vars.
           o When -O0 is in use, the compiler allocates distinct stack 
memory for all variables that do not have the register storage-class 
specifier; if register is specified, the variable may have a shorter 
lifespan than the code would indicate and may never be placed in memory.
           o On some rare x86 targets, setjmp doesn't save the registers 
in all circumstances. In those cases, GCC doesn't allocate any variables 
in registers unless they are marked register.