> * Case II - User Guided Versioning where the function bodies for each
> version differ and is provided by the user.
>
> This case pertains to multi-versioning when the source bodies of the
> two or more versions are different and are provided by the user. Here
> too, I want to use a new attribute, “version”. Now, the user can
> specify versioning intent like this:
>
> int __attribute__((version(“popcnt”))
> find_popcnt(unsigned int i)
> {
> // inline assembly of the popcnt instruction, specialized version.
> asm(“popcnt ….”);
> }
>
> int
> find_popcnt(unsigned int i)
> {
> //generic code for doing this
> ...
> }
>
> This uses function overloading to specify versions. The compiler will
> understand that versioning is requested, since the functions have
> different attributes with "version", and will generate the code to
> execute the right function at run-time. The compiler should check for
> the existence of one body without the attribute which will be the
> default version.
The use of function overloading is discussed in
http://gcc.gnu.org/ml/gcc-patches/2011-05/msg00150.html
>
> * Case III - Versioning is done automatically by the compiler.
>
> I want to add a new compiler flag “-mversion” along the lines of “-m”.
> If the user specifies “-mversion=popcnt” then the compiler will
> automatically create two versions of any function that is impacted by
> the new instruction. The difference between “-m” and “-mversion” will
> be that while “-m” generates only the specialized version, “-mversion”
> will generate both the specialized and the generic versions. There is
> no need to explicity mark any function for versioning, no source
> changes.
-mversion=targ1,targ2,.. or -mversion=feature1,feature2 .. is a way to
tell the compiler the set of target hardwares the program is going be
running on. It is an extension of -mcpu=....
It is up to the compiler to decision whether MV is done or what
function to be MVed.
The __clone__ attribute proposed here
(http://gcc.gnu.org/wiki/FunctionSpecificOpt) can be used together
with -mversion=... to achieve the similar effect of version attribute
mentioned above -- the difference is that version attribute is
'mandatory'.
David
>
> The compiler will decide if it is beneficial to multi-version a
> function based on heuristics using hotness information, code size
> growth, etc.
>
>
> Runtime support
> ===============
>
> In order for the compiler to generate multi-versioned code, it needs
> to call functions that would test if a particular feature exists or
> not at run-time. For example, IsPopcntSupported() would be one such
> function. I have prepared a patch to do this which adds the runtime
> support in libgcc and supports new builtins to test the various
> features. I will send the patch separately to keep the dicussions
> focused.
>
>
> Thoughts?
>
> Thanks,
> -Sri.
>
