[PATCH] D88060: [GISel]: Few InsertVecElt combines

[Aditya Nandakumar via Phabricator via llvm-commits]

aditya_nandakumar added inline comments.


================
Comment at: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:2415
+  auto Index = getConstantVRegVal(MI.getOperand(3).getReg(), MRI);
+  if (!Index || *Index != (NumElts - 1))
+    return false;
----------------
arsenm wrote:
> This could start at any index? Why give up at NumElts - 1?
This is poorly attempting to match the highest index (assuming insert_vec_elts are found with increasing indices) so we can collect as many components as possible.
For eg if we traverse top down (like we currently do)
```
a = insert_vec_elt undef, x, 0
b = insert_vec_elt a, y, 1
```
We'll turn a into
```
a = build_vector x, undef
b = insert_vec_elt a, y, 1
```
Subsequently the insert_vec_elt(build_vec) combine would turn this into
```
b = build_vector x, y
```

Instead starting with b would give the build_vector in one shot.

Maybe the better option to do is check if the current insert_vec_elt is used by insert_vec_elts and bail out.


================
Comment at: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:2437
+
+  std::sort(Inserts.begin(), Inserts.end(),
+            [](const std::pair<Register, unsigned> &P1,
----------------
arsenm wrote:
> llvm::sort
Will do.


================
Comment at: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:2449
+                Inserts.end());
+  Optional<Register> UndefReg;
+  auto GetUndef = [&]() {
----------------
arsenm wrote:
> Optional doesn't really buy you anything here? You can just use Register()?
Will update.


================
Comment at: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:2453
+      return *UndefReg;
+    Builder.setInstr(MI);
+    UndefReg = Builder.buildUndef(DstTy.getScalarType()).getReg(0);
----------------
arsenm wrote:
> I don't think you need to reset the insert point here
I'm not resetting. I'm making sure it's set correctly (MI) so I can insert IMPLICIT_DEFs.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D88060/new/

https://reviews.llvm.org/D88060