test(vexpdif): rewrite pto_vexpdif_fusion as vreg-level pto-test-opt case - #18

Summary

Rewrite test/lit/tile_fusion/pto_vexpdif_fusion.pto from an end-to-end ptoas pipeline test into a hand-written vreg-level case exercised via pto-test-opt -pto-vexpdif-fusion, matching the pattern of the mask/f16/multiuse siblings.

Problem

On a clean main-llvm19-build checkout (0003225f4), pto_vexpdif_fusion.pto failed:

CHECK: pto.vexpdif {{.*}} "ODD"   <- expected
actual: two separate local-boundary scf.for loops (template_tsub / template_texp),
        vsub result written via vsts then reloaded via vlds for vexp

The end-to-end RUN drove the test through TileLib template expansion, where tsub/texp fall back to ordinary PTODSL templates in separate local-boundary loops (boundary_reason = "non_vmi_local_boundary_fallback"). The intervening vsts/vlds broke the vsub -> vexp SSA chain that PTOVexpdifFusion matches (exp.getInput().getDefiningOp<VsubOp>()), so the pass never fired.

Fix

Rewrite as hand-written vreg IR, decoupled from the ptoas pipeline, TileLib templates, and local-boundary fallback:

func.func @vexpdif_fusion(%a : !pto.vreg<64xf32>, %b : !pto.vreg<64xf32>) -> !pto.vreg<64xf32> {
  %res = pto.fusion_region {
    %m = pto.pset_b32 "PAT_ALL" : !pto.mask<b32>
    %sub = pto.vsub %a, %b, %m : ...
    %exp = pto.vexp %sub, %m : ...
    pto.yield(%exp) : ...
  } : !pto.vreg<64xf32>
  ...
}
// CHECK: pto.vexpdif {{.*}} "ODD"
// CHECK-NOT: pto.vsub
// CHECK-NOT: pto.vexp {{.*}} :

• vsub single-use feeding vexp, sharing one mask SSA inside one fusion_region — exactly the shape canFuse requires.
• Drops the second LLVM RUN line; it depended on the full ptoas pipeline, and vexpdif LLVM lowering is already covered by test/lit/vmi_new/vmi_to_vpto_vexpdif_f16.pto and vmi_layout_assignment_vexpdif.pto.

Verification

All four vexpdif-fusion lit tests pass on main-llvm19-build:

pto_vexpdif_fusion        PASS
pto_vexpdif_fusion_mask   PASS
pto_vexpdif_fusion_f16    PASS
pto_vexpdif_fusion_multiuse PASS

Scope

Single file, test-only change.