[Mlir-commits] [mlir] [mlir][ArmSME] Use liveness information in the tile allocator (PR #90448)

[Benjamin Maxwell]

================
@@ -156,28 +234,123 @@ func.func @run_out_of_tiles_but_avoid_spill(%a: vector<[4]xf32>, %b: vector<[4]x
 
 // -----
 
-// Incorrect result! Everything other than zero assigned to tile 1 (which means values that are still live are overwritten).
-//
-// CHECK-BAD-LABEL: @avoidable_spill
-// CHECK-BAD: arm_sme.zero {tile_id = 0 : i32}
-// CHECK-BAD: arm_sme.get_tile {tile_id = 1 : i32}
-// CHECK-BAD-COUNT-4: arm_sme.move_vector_to_tile_slice {{.*}} {tile_id = 1 : i32}
+// We should be able to avoid spills like this, but logic handling this case is
+// not implemented yet. Note tile ID >= 16 means a spill/in-memory tile.
+
+//  CHECK-LIVE-RANGE-LABEL: @avoidable_spill
+//        CHECK-LIVE-RANGE: ========== Coalesced Live Ranges:
+//        CHECK-LIVE-RANGE: ^bb2:
+//   CHECK-LIVE-RANGE-NEXT: ||     test.some_use
+//   CHECK-LIVE-RANGE-NEXT: ||S    arm_sme.move_vector_to_tile_slice
+//   CHECK-LIVE-RANGE-NEXT: |||S   arm_sme.move_vector_to_tile_slice
+//   CHECK-LIVE-RANGE-NEXT: ||||S  arm_sme.move_vector_to_tile_slice
+//   CHECK-LIVE-RANGE-NEXT: |||||S arm_sme.move_vector_to_tile_slice
+//   CHECK-LIVE-RANGE-NEXT: ||E||| test.some_use
+//   CHECK-LIVE-RANGE-NEXT: || E|| test.some_use
+//   CHECK-LIVE-RANGE-NEXT: ||  E| test.some_use
+//   CHECK-LIVE-RANGE-NEXT: ||   E test.some_use
+//   CHECK-LIVE-RANGE-NEXT: ||     arith.addi
+//   CHECK-LIVE-RANGE-NEXT: EE     cf.br
+
+// Note in the live ranges (above) there is two constant live-ins (first two ranges),
+// which gives six overlapping live ranges. The allocator currently will spill the
+// first constant (which results in a real spill at it's use), however, this could
+// be avoided by using the knowledge that at the first "test.some_use" there's
+// actually only two live ranges (so we can fix this be duplicating the constant).
----------------
MacDue wrote:

> at the **first** "test.some_use" there's actually only two live ranges

```
//        CHECK-LIVE-RANGE: ^bb2:
//   CHECK-LIVE-RANGE-NEXT: ||     test.some_use
```
There's only two live ranges there (the others have not started yet).

https://github.com/llvm/llvm-project/pull/90448