[llvm] r187139 - Allocate local registers in order for optimal coloring.

[Jakob Stoklund Olesen]

On Jul 25, 2013, at 11:35 AM, Andrew Trick <atrick at apple.com> wrote:

> Author: atrick
> Date: Thu Jul 25 13:35:14 2013
> New Revision: 187139
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=187139&view=rev
> Log:
> Allocate local registers in order for optimal coloring.

Linear scan forever!

Besides the better coloring, I like how this enables more ldm/ldrd formation on ARM.

> @@ -411,9 +413,21 @@ void RAGreedy::enqueue(LiveInterval *LI)
>     // everything else has been allocated.
>     Prio = Size;
>   } else {
> -    // Everything is allocated in long->short order. Long ranges that don't fit
> -    // should be spilled (or split) ASAP so they don't create interference.
> -    Prio = (1u << 31) + Size;
> +    if (ExtraRegInfo[Reg].Stage == RS_Assign && !LI->empty() &&
> +        LIS->intervalIsInOneMBB(*LI)) {
> +      // Allocate original local ranges in linear instruction order. Since they
> +      // are singly defined, this produces optimal coloring in the absence of
> +      // global interference and other constraints.
> +      Prio = LI->beginIndex().distance(Indexes->getLastIndex());

The distance here could be divided by SlotIndexes::Slot_Count to prevent it from interfering with the high bits of Prio.

Thanks,
/jakob