[ILUG] kernel programming curiosity

[Fergal Daly]

That's fair enough about gcc not inlining without -O, now that you mention
it, I think I'd heard that before, along with the fact that marking a
variable as register won't necessarily get it into a register, it's just
taken as a hint to the compiler.

The bit that's still really confusing me is where is gcc getting the capital
X from, I have grepped all the .h and .c files of 2.2.17 and 2.2.19 and
there is no mention of put_user_X, only put_user_x. Any idea what that's
about?

Fergal



On Sat, Apr 21, 2001 at 12:03:56PM +0100, Kenn Humborg wrote:
> In my experience, you _need_ to compile with -O.  With no optimisations
> enabled, GCC doesn't inline _any_ functions, even ones specifically marked
> __inline__.  Calls to __put_user_N and other inlined stuff gets translated
> to regular function calls.  Then either of these can happen:
> 
>    o  When compiled the kernel itself, this function may be declared
>       as static inline and no global symbol will be created for it,
>       so linking will fail
> 
>    o  When compiling a module, this symbol was never EXPORT_SYMBOL()ed
>       (or whatever the macro is called...), so module loading will fail.
> 
> 
> > What's the story? I thought -O wasn't meant to make any difference except 
> > speedwise. Is there something that says that all kernel code must be 
> > compiled with a -O switch? Where is gcc getting this capital X instead of a 
> > small x?
> 
> Take a look into the innards of include/asm/uaccess.h.  __put_user(addr,x) gets
> broken out to something like __put_user_X via some GCC-specific keywords
> and some pre-processor magic.  This, then, allows for byte, word and longword
> copies to be optimized individually.
> 
> The code is complex, but the generated code is simple.
> 
> Later,
> Kenn
> 
> 
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