[llvm] [LiveIntervals] Ignore artificial regs when adding kill flags (PR #116963)

[Jay Foad via llvm-commits]

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@@ -730,7 +730,16 @@ void LiveIntervals::addKillFlags(const VirtRegMap *VRM) {
     // Find the regunit intervals for the assigned register. They may overlap
     // the virtual register live range, cancelling any kills.
     RU.clear();
-    for (MCRegUnit Unit : TRI->regunits(PhysReg)) {
+    LaneBitmask ArtificialLanes = LaneBitmask::getNone();
+    for (MCRegUnitMaskIterator UI(PhysReg, TRI); UI.isValid(); ++UI) {
+      auto [Unit, Bitmask] = *UI;
+      // Record lane mask for all artificial RegUnits for this physreg.
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jayfoad wrote:

> It's actually not clear to me why there would ever be more than one root? What's the use-case for this?

I think the second root is only ever an aritificial register used to represent ad hoc aliasing (the `Aliases = [...]` field in .td files), but I could be misremembering the details.

https://github.com/llvm/llvm-project/pull/116963