[SystemZ] Use BRCT and BRCTG to eliminate add-&-compare sequences

This patch just uses a peephole test for "add; compare; branch" sequences
within a single block.  The IR optimizers already convert loops to
decrement-and-branch-on-nonzero form in some cases, so even this
simplistic test triggers many times during a clang bootstrap and
projects/test-suite run.  It looks like there are still cases where we
need to more strongly prefer branches on nonzero though.  E.g. I saw a
case where a loop that started out with a check for 0 ended up with a
check for -1.  I'll try to look at that sometime.

I ended up adding the Reference class because MachineInstr::readsRegister()
doesn't check for subregisters (by design, as far as I could tell).

llvm-svn: 187723

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
index 5dd8d98d27d..c2a6a7f1018 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
@@ -684,6 +684,14 @@ SystemZInstrInfo::getBranchInfo(const MachineInstr *MI) const {
                              MI->getOperand(0).getImm(),
                              MI->getOperand(1).getImm(), &MI->getOperand(2));
 
+  case SystemZ::BRCT:
+    return SystemZII::Branch(SystemZII::BranchCT, SystemZ::CCMASK_ICMP,
+                             SystemZ::CCMASK_CMP_NE, &MI->getOperand(2));
+
+  case SystemZ::BRCTG:
+    return SystemZII::Branch(SystemZII::BranchCTG, SystemZ::CCMASK_ICMP,
+                             SystemZ::CCMASK_CMP_NE, &MI->getOperand(2));
+
   case SystemZ::CIJ:
   case SystemZ::CRJ:
     return SystemZII::Branch(SystemZII::BranchC, SystemZ::CCMASK_ICMP,