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

On Thu, Aug 16, 2007 at 03:23:28AM +0200, Segher Boessenkool wrote:

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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...  ;-)

Yeah, the breakage is *quite* spectacular :-)

I bet!!!  ;-)

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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.

Of course, but often none of those will (indirectly) write a certain
static variable.

But there has to be some path to the static functions, assuming that
they are not dead code.  Yes, there can be cases where the compiler
knows enough about the state of the variables to rule out some of code
paths to them, but I have no idea how often this happens in kernel
code.

						Thanx, Paul