IRA_COVER_CLASSES In gcc47

[Paulo J. Matos]

Vladimir,

Thanks for for the explanation.

Cheers,

Paulo Matos

On 23/03/12 16:08, Vladimir Makarov wrote:
> On 03/23/2012 11:04 AM, Paulo J. Matos wrote:
>> Hello,
>>
>> I am trying to find exactly what happened to IRA_COVER_CLASSES in
>> gcc47. From what I have seen it seems that it was simply removed. Does
>> the register allocator now automatically computes the cover classes?
>>
> No. Before gcc4.7 we use coloring on non-intersected classes (so called
> cover classes). That was a classical approach with well known
> disadvantages for irregular register class architectures.
>
> Since 4.7 we use more sophisticated trivial coloring criteria which work
> well even on intersected register classes. To be more accurate, we
> calculate an approximation of an profitable hard regs for each pseudo.
> These approximations form a tree. The tree is used for find trivial
> colorability of the pseudos. It was a surprise that such approach is
> profitable even for architectures with regular register files like ppc.
> Here is an excerpt from comments on the top ira.c file:
>
>
> We also use a modification of Chaitin-Briggs algorithm which
> works for intersected register classes of allocnos. To
> figure out trivial colorability of allocnos, the mentioned
> above tree of hard register sets is used. To get an idea how
> the algorithm works in i386 example, let us consider an
> allocno to which any general hard register can be assigned.
> If the allocno conflicts with eight allocnos to which only
> EAX register can be assigned, given allocno is still
> trivially colorable because all conflicting allocnos might be
> assigned only to EAX and all other general hard registers are
> still free.
>
> To get an idea of the used trivial colorability criterion, it
> is also useful to read article "Graph-Coloring Register
> Allocation for Irregular Architectures" by Michael D. Smith
> and Glen Holloway. Major difference between the article
> approach and approach used in IRA is that Smith's approach
> takes register classes only from machine description and IRA
> calculate register classes from intermediate code too
> (e.g. an explicit usage of hard registers in RTL code for
> parameter passing can result in creation of additional
> register classes which contain or exclude the hard
> registers). That makes IRA approach useful for improving
> coloring even for architectures with regular register files
> and in fact some benchmarking shows the improvement for
> regular class architectures is even bigger than for irregular
> ones. Another difference is that Smith's approach chooses
> intersection of classes of all insn operands in which a given
> pseudo occurs. IRA can use bigger classes if it is still
> more profitable than memory usage.
>
>


-- 
PMatos