llvm dev - Feb 2013 - [LLVMdev] Deleting LiveVariables

On Feb 8, 2013, at 4:37 PM, Cameron Zwarich <zwarich at apple.com> wrote:
> On Feb 8, 2013, at 4:20 PM, Jakob Stoklund Olesen <stoklund at
2pi.dk> wrote:
> 
>> On Feb 8, 2013, at 4:03 PM, Cameron Zwarich <zwarich at
apple.com> wrote:
>> 
>>> How much of the work is done here? I'd be happy to do the phi
elimination part, since I basically did that for StrongPhiElimination (RIP).
>> 
>> Any help would be appreciated.
>> 
>> I did a bit of the easy stuff in 2-addr, it has a LIS =
getAnalysisIfAvailable<LiveIntervals>() member that it sometimes updates.
It has a lot of weird transformations, though.
>> 
>> I haven't touched phi-elim yet because of the 'bubble-up'
approach.
>> 
>> If you want to help with phi-elim, you could hack -early-live-intervals
to insert LiveIntervals before phi-elim, and then run MF.verify(this) at the end
of runOnMachineFunction(). That should be enough to check that phi-elim updates
live intervals correctly even when 2-addr is going to break it later.
> 
> I'll try doing that. Did you ever add a way to update LiveIntervals
quickly after splitting an edge or will that have to finally be added? I can
skip the critical edge splitting for now.
That doesn't exist yet, we'll need that too.

My thinking was to keep a list of all global live ranges in LiveIntervals so we
at least don't have to go through all the local virtual registers when
splitting a critical edge. (That's how LiveVariables is updated now, and
it's slow).
> IIRC you run into a lot of problems with NEON subregister defs, which might
be fixed by your new direct LiveIntervals implementation.
>> 
>> The direct live intervals work on anything that has virtual registers -
it can handle all the sub-register fun.
>> 
>> I don't think you will see any problems with sub-registers in
phi-elim because PHI instructions never have sub-register operands. (And this
version of phi-elim just inserts a million copies).
> 
> Jogging my memory some more, the problem wasn't with values in phis, it
was just that the change in pass ordering broke subregister liveness in some
other ways. StrongPhiElimination goes after the 2-address pass rather than
before it, but it looks like you are preserving the existing ordering.
I see. Yes, things get a lot more complicated after 2-addr has removed
INSERT_SUBREG and REG_SEQUENCE instructions. Sub-registers everywhere.

/jakob

On Feb 8, 2013, at 4:45 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>
wrote:
> On Feb 8, 2013, at 4:37 PM, Cameron Zwarich <zwarich at apple.com>
wrote:
> 
>> I'll try doing that. Did you ever add a way to update LiveIntervals
quickly after splitting an edge or will that have to finally be added? I can
skip the critical edge splitting for now.
> 
> That doesn't exist yet, we'll need that too.
> 
> My thinking was to keep a list of all global live ranges in LiveIntervals
so we at least don't have to go through all the local virtual registers when
splitting a critical edge. (That's how LiveVariables is updated now, and
it's slow).
I checked in LiveIntervals support to PHIElimination in r174831. I encountered
all of the usual edge cases in 'make check', so hopefully it has no
latent bugs.

I'll add a splitEdge() method to LiveIntervalAnalysis (got a better name?)
that does things the slow way first, and then we can speed it up.

Cameron

On Feb 9, 2013, at 10:51 PM, Cameron Zwarich <zwarich at apple.com> wrote:
> On Feb 8, 2013, at 4:45 PM, Jakob Stoklund Olesen <stoklund at
2pi.dk> wrote:
> 
>> On Feb 8, 2013, at 4:37 PM, Cameron Zwarich <zwarich at
apple.com> wrote:
>> 
>>> I'll try doing that. Did you ever add a way to update
LiveIntervals quickly after splitting an edge or will that have to finally be
added? I can skip the critical edge splitting for now.
>> 
>> That doesn't exist yet, we'll need that too.
>> 
>> My thinking was to keep a list of all global live ranges in
LiveIntervals so we at least don't have to go through all the local virtual
registers when splitting a critical edge. (That's how LiveVariables is
updated now, and it's slow).
> 
> I checked in LiveIntervals support to PHIElimination in r174831. I
encountered all of the usual edge cases in 'make check', so hopefully it
has no latent bugs.
Very cool. Thanks, Cameron.
> I'll add a splitEdge() method to LiveIntervalAnalysis (got a better
name?) that does things the slow way first, and then we can speed it up.
Sounds good.

/jakob