[all-commits] [llvm/llvm-project] 2ec71e: [RegisterCoalescer] Fix the creation of subranges ...

[qcolombet via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 2ec71ea7c74df20983031c6e1be07b14da0e9109
      https://github.com/llvm/llvm-project/commit/2ec71ea7c74df20983031c6e1be07b14da0e9109
  Author: Quentin Colombet <qcolombet at apple.com>
  Date:   2019-12-05 (Thu, 05 Dec 2019)

  Changed paths:
    M llvm/lib/CodeGen/RegisterCoalescer.cpp
    A llvm/test/CodeGen/SystemZ/regcoal-subranges-update-remat.mir

  Log Message:
  -----------
  [RegisterCoalescer] Fix the creation of subranges when rematerialization is used

* Context *

During register coalescing, we use rematerialization when coalescing is not
possible. That means we may rematerialize a super register when only a smaller
register is actually used.
E.g.,
0B v1 = ldimm 0xFF
1B v2 = COPY v1.low8bits
2B   = v2
=>
0B v1 = ldimm 0xFF
1B v2 = ldimm 0xFF
2B   = v2.low8bits

Where xB are the slot indexes.
Here v2 grew from a 8-bit register to a 16-bit register.

When that happens and subregister liveness is enabled, we create subranges for
the newly created value.
E.g., before remat, the live range of v2 looked like:
main range: [1r, 2r)
(Reads v2 is defined at index 1 slot register and used before the slot register
of index 2)

After remat, it should look like:
main range: [1r, 2r)
low 8 bits: [1r, 2r)
high 8 bits: [1r, 1d) <-- dead def

I.e., the unsused lanes of v2 should be marked as dead definition.

* The Problem *

Prior to this patch, the live-ranges from the previous exampel, would have the
full live-range for all subranges:
main range: [1r, 2r)
low 8 bits: [1r, 2r)
high 8 bits: [1r, 2r) <-- too long

* The Fix *

Technically, the code that this patch changes is not wrong:
When we create the subranges for the newly rematerialized value, we create only
one subrange for the whole bit mask.
In other words, at this point v2 live-range looks like this:
main range: [1r, 2r)
low & high: [1r, 2r)

Then, it gets wrong when we call LiveInterval::refineSubRanges on low 8 bits:
main range: [1r, 2r)
low 8 bits: [1r, 2r)
high 8 bits: [1r, 2r) <-- too long

Ideally, we would like LiveInterval::refineSubRanges to be able to do the right
thing and mark the dead lanes as such. However, this is not possible, because by
the time we update / refine the live ranges, the IR hasn't been updated yet,
therefore we actually don't have enough information to do the right thing.

Another option to fix the problem would have been to call
LiveIntervals::shrinkToUses after the IR is updated. This is not desirable as
this may have a noticeable impact on compile time.

Instead, what this patch does is when we create the subranges for the
rematerialized value, we explicitly create one subrange for the lanes that were
used before rematerialization and one for the lanes that were not used. The used
one inherits the live range of the main range and the unused one is just created
empty. The existing rematerialization code then detects that the unused one are
not live and it correctly sets dead def intervals for them.

https://llvm.org/PR41372