On Wed, Aug 15, 2007 at 11:05:35PM +0200, Segher Boessenkool wrote:
> >>No; compilation units have nothing to do with it, GCC can optimise
> >>across compilation unit boundaries just fine, if you tell it to
> >>compile more than one compilation unit at once.
> >
> >Last I checked, the Linux kernel build system did compile each .c file
> >as a separate compilation unit.
>
> I have some patches to use -combine -fwhole-program for Linux.
> Highly experimental, you need a patched bleeding edge toolchain.
> If there's interest I'll clean it up and put it online.
>
> David Woodhouse had some similar patches about a year ago.
Sounds exciting... ;-)
> >>>In many cases, the compiler also has to assume that
> >>>msleep_interruptible()
> >>>might call back into a function in the current compilation unit, thus
> >>>possibly modifying global static variables.
> >>
> >>It most often is smart enough to see what compilation-unit-local
> >>variables might be modified that way, though :-)
> >
> >Yep. For example, if it knows the current value of a given such local
> >variable, and if all code paths that would change some other variable
> >cannot be reached given that current value of the first variable.
>
> Or the most common thing: if neither the address of the translation-
> unit local variable nor the address of any function writing to that
> variable can "escape" from that translation unit, nothing outside
> the translation unit can write to the variable.
But there is usually at least one externally callable function in
a .c file.
> >At least given that gcc doesn't know about multiple threads of
> >execution!
>
> Heh, only about the threads it creates itself (not relevant to
> the kernel, for sure :-) )
;-)
Thanx, Paul
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