[llvm] [Codegen] Remove redundant instruction using machinelateCleanup (PR #139716)

[Jay Foad via llvm-commits]

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@@ -189,7 +189,13 @@ static bool isCandidate(const MachineInstr *MI, Register &DefedReg,
       if (MO.isDef()) {
         if (i == 0 && !MO.isImplicit() && !MO.isDead())
           DefedReg = MO.getReg();
-        else
+        else if (i != 0 && DefedReg != MCRegister::NoRegister) {
+          if (MO.isDead() && MO.isImplicit())
+            continue;
+          if (MO.isImplicit() && TRI->regsOverlap(MO.getReg(), DefedReg))
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jayfoad wrote:

Example:
// If a later implicit def overlaps with the first def, ignore it.
"Overlaps" seem like the wrong condition here. Don't you need to check that its a subset of the first def?

What I'd really like to see is an explanation of _exactly_ what kind of instruction we're looking for, e.g. "must have at least one explicit def and any other def must either be dead or a subset of the first def". And then make sure that the code matches the comment.

https://github.com/llvm/llvm-project/pull/139716