[ARM] Make InstrEmitter mark CPSR defs dead for Thumb1.

The "dead" markings allow existing target-independent optimizations,
like MachineSink, to trigger more frequently. The CPSR defs would have
eventually been marked dead by LiveVariables, so this only affects
optimizations before regalloc.

The ARMBaseInstrInfo.cpp change is fixing a bug which is only visible
with this change: the transform adds a use to an otherwise dead def
of CPSR. This is covered by existing regression tests.

thumb2-tbh.ll breaks for Thumb1 due to MachineLICM changing the
generated code; I'll fix it in D53452.

Differential Revision: https://reviews.llvm.org/D53453

llvm-svn: 345420

2 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp b/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
index 9f57df87fb2..fc9c227e4df 100644
--- a/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
@@ -959,7 +959,7 @@ EmitMachineNode(SDNode *Node, bool IsClone, bool IsCloned,
   }
 
   // Finally mark unused registers as dead.
-  if (!UsedRegs.empty() || II.getImplicitDefs())
+  if (!UsedRegs.empty() || II.getImplicitDefs() || II.hasOptionalDef())
     MIB->setPhysRegsDeadExcept(UsedRegs, *TRI);
 
   // Run post-isel target hook to adjust this instruction if needed.
diff --git a/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp b/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
index 0d1908ada7f..c9d78df4b37 100644
--- a/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
@@ -2963,6 +2963,8 @@ bool ARMBaseInstrInfo::optimizeCompareInstr(
   for (unsigned i = 0, e = OperandsToUpdate.size(); i < e; i++)
     OperandsToUpdate[i].first->setImm(OperandsToUpdate[i].second);
 
+  MI->clearRegisterDeads(ARM::CPSR);
+
   return true;
 }