>> No; compilation units have nothing to do with it, GCC can optimise
quoted text
>> 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.

quoted text
>>> 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.

quoted text
> 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 :-) )


Segher

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