Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

[Segher Boessenkool <segher@kernel.crashing.org>]

> > 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.

> > > 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.

> 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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