[PATCH] D23172: IfConversion: Add implicit uses for live subregisters

[Krzysztof Parzyszek via llvm-commits]

kparzysz added inline comments.

================
Comment at: lib/CodeGen/IfConversion.cpp:1456-1460
@@ -1455,2 +1455,7 @@
       MIB.addReg(Reg, RegState::Implicit);
+    else {
+      for (MCSubRegIterator S(Reg, TRI); S.isValid(); ++S)
+        if (LiveBeforeMI.count(*S))
+          MIB.addReg(*S, RegState::Implicit);
+    }
   }
----------------
kparzysz wrote:
> MatzeB wrote:
> > Would it be more reasonable to check if any subreg is alive and then only add a single implicit use for `Reg`. Otherwise I'd be concerned that we end up with a large number of implicit-use operands for deep register hierarchies.
> I've been thinking about it.  What about this case?
> 
> ```
> Live-in: %H
> if (...) {
>   // Define/use super-reg.
>   %H:L = ...
>   ... = %H:L
> }
> // %H is still live here
> ... = %H
> ```
> 
> After if-conversion we would end up with something like
> ```
> %H:L = predicated ..., %H:L<imp-use>
> ... = predicated %H:L
> ```
> 
> Wouldn't the implicit use be invalid (since %L is not defined)?
The `%H:L` above is meant to represent a super-register consisting of two halves: %H and %L respectively.


Repository:
  rL LLVM

https://reviews.llvm.org/D23172