TargetRegisterInfo: Make the concept of imprecise lane masks explicit

LaneMasks as given by getSubRegIndexLaneMask() have a limited number of
of bits, so for targets with more than 31 disjunct subregister there may
be cases where:

   getSubReg(Reg,A) does not overlap getSubReg(Reg,B)

but we still have

   (getSubRegIndexLaneMask(A) & getSubRegIndexLaneMask(B)) != 0.

I had hoped to keep this an implementation detail of the tablegen but as
my next commit shows we can avoid unnecessary imp-defs operands if we
know that the lane masks in use are precise.

This is in preparation to http://reviews.llvm.org/D10470.

llvm-svn: 239837

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index e513a4f1ccf..7b7c7502172 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -2633,7 +2633,8 @@ bool RegisterCoalescer::joinVirtRegs(CoalescerPair &CP) {
       // "overflow bit" 32. As a workaround we drop all subregister ranges
       // which means we loose some precision but are back to a well defined
       // state.
-      assert((CP.getNewRC()->getLaneMask() & 0x80000000u)
+      assert(TargetRegisterInfo::isImpreciseLaneMask(
+             CP.getNewRC()->getLaneMask())
              && "SubRange merge should only fail when merging into bit 32.");
       DEBUG(dbgs() << "\tSubrange join aborted!\n");
       LHS.clearSubRanges();