David Green 1cb12fa9ed [GlobalISel] Combine unmerge(unmerge()) if the result is legal. (#109606) ...

This attempts to fold:
```
  %1:_(<2 x s32>), %2:_(<2 x s32>) = G_UNMERGE_VALUES %0:_(<4 x s32>)
  %3:_(s32), %4:_(s32) = G_UNMERGE_VALUES %1
```
Into a single UNMERGE:
```
  %3:_(s32), %4:_(s32), %5:_(s32), %6:_(s32) = G_UNMERGE_VALUES %0
```

This transform already exists, this patch alters it to occur when the
result UNMERGE is considered legal.

It does not try to transform where the result would be extracting a
subelement from a vector at the moment, as the code is not setup to
handle it.
```
  %1:_(s32), %2:_(s32) = G_UNMERGE_VALUES %0:_(<2 x s32>)
  %3:_(s16), %4:_(s16) = G_UNMERGE_VALUES %1
```

This helps us reduce the amount of legalization artefacts, especially
from widened vectors padded with undef.

2024-09-25 07:04:09 +01:00

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