llvm dev - Apr 2014 - [LLVMdev] How should I update LiveIntervals after removing a use of a register?

Hi,

I am working on a simple copy propagation pass for the R600 backend that
propagates immediates rather than registers.  For example, I want to
transform:

...
%vreg1 = V_MOV_B32 1
%vreg2 = V_ADD_I32 %vreg1, %vreg0
...

into:

%vreg1 = V_MOV_B32 1 ; <- Only delete this if it is dead
%vreg2 = V_ADD_I32 1, %vreg0

For best results, I am trying to run this pass after the
TwoAddressInstruction pass, which means I need to preserve
the LiveIntervals analysis.

My question is: How do I update the LiveIntervals to reflect the fact
that V_ADD_I32 no longer uses %vreg0?

Thanks,
Tom

On Apr 3, 2014, at 6:29 PM, Tom Stellard <tom at stellard.net> wrote:
> Hi,
> 
> I am working on a simple copy propagation pass for the R600 backend that
> propagates immediates rather than registers.  For example, I want to
> transform:
> 
> ...
> %vreg1 = V_MOV_B32 1
> %vreg2 = V_ADD_I32 %vreg1, %vreg0
> ...
> 
> into:
> 
> %vreg1 = V_MOV_B32 1 ; <- Only delete this if it is dead
> %vreg2 = V_ADD_I32 1, %vreg0
> 
> For best results, I am trying to run this pass after the
> TwoAddressInstruction pass, which means I need to preserve
> the LiveIntervals analysis.
> 
> My question is: How do I update the LiveIntervals to reflect the fact
> that V_ADD_I32 no longer uses %vreg0?
Hi Tom,

Hopefully you figured this out ;) IIRC, the safe thing to do is call
removeInterval and createAndComputeVirtRegInterval for both vreg0 and vreg1.

-Andy

On Sat, Apr 05, 2014 at 12:36:19AM -0700, Andrew Trick
wrote:
> 
> On Apr 3, 2014, at 6:29 PM, Tom Stellard <tom at stellard.net> wrote:
> 
> > Hi,
> > 
> > I am working on a simple copy propagation pass for the R600 backend
that
> > propagates immediates rather than registers.  For example, I want to
> > transform:
> > 
> > ...
> > %vreg1 = V_MOV_B32 1
> > %vreg2 = V_ADD_I32 %vreg1, %vreg0
> > ...
> > 
> > into:
> > 
> > %vreg1 = V_MOV_B32 1 ; <- Only delete this if it is dead
> > %vreg2 = V_ADD_I32 1, %vreg0
> > 
> > For best results, I am trying to run this pass after the
> > TwoAddressInstruction pass, which means I need to preserve
> > the LiveIntervals analysis.
> > 
> > My question is: How do I update the LiveIntervals to reflect the fact
> > that V_ADD_I32 no longer uses %vreg0?
> 
> Hi Tom,
> 
> Hopefully you figured this out ;) IIRC, the safe thing to do is call
removeInterval and createAndComputeVirtRegInterval for both vreg0 and vreg1.
> 
I found a slightly different solution, which was to
call LiveIntervals::shrinkToUses(vreg1).  This also gives me a list of
dead instructions, which is nice.  Are there an disadvantages to doing
it this way?

Thanks,
Tom