[PATCH] D50725: [SystemZ] Replace subreg_r with subreg_h

[Ulrich Weigand via Phabricator via llvm-commits]

uweigand added a comment.

Well, the original rationale for using different subreg indices for float/vector registers is given here:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150504/274358.html

The main underlying issue is that the architecture states that writing to a floating-point register (which overlays the upper half of a vector register) may clobber the lower half of that vector register.  I believe the current processor implementations do not actually do that, but it still would be preferable to model this correctly.

On the other hand, just using the different subreg index names doesn't actually do that, it's just a cosmetic marker.  So I don't necessarily object to reverting that difference (as this patch would do).  Just wondering if there's a better way to actually express the underlying issue.


Repository:
  rL LLVM

https://reviews.llvm.org/D50725