Allow <undef> flags on def operands as well as uses.

The <undef> flag says that a MachineOperand doesn't read its register,
or doesn't depend on the previous value of its register.

A full register def never depends on the previous register value.  A
partial register def may depend on the previous value if it is intended
to update part of a register.

For example:

  %vreg10:dsub_0<def,undef> = COPY %vreg1
  %vreg10:dsub_1<def> = COPY %vreg2

The first copy instruction defines the full %vreg10 register with the
bits not covered by dsub_0 defined as <undef>.  It is not considered a
read of %vreg10.

The second copy modifies part of %vreg10 while preserving the rest.  It
has an implicit read of %vreg10.

This patch adds a MachineOperand::readsReg() method to determine if an
operand reads its register.

Previously, this was modelled by adding a full-register <imp-def>
operand to the instruction.  This approach makes it possible to
determine directly from a MachineOperand if it reads its register.  No
scanning of MI operands is required.

llvm-svn: 141124

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
index cacbd0a61b6..3d95827226a 100644
--- a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
+++ b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -304,8 +304,15 @@ void LiveIntervals::handleVirtualRegisterDef(MachineBasicBlock *mbb,
 
     // Make sure the first definition is not a partial redefinition. Add an
     // <imp-def> of the full register.
-    if (MO.getSubReg())
+    if (MO.getSubReg()) {
       mi->addRegisterDefined(interval.reg);
+      // Mark all defs of interval.reg on this instruction as reading <undef>.
+      for (unsigned i = MOIdx, e = mi->getNumOperands(); i != e; ++i) {
+        MachineOperand &MO2 = mi->getOperand(i);
+        if (MO2.isReg() && MO2.getReg() == interval.reg && MO2.getSubReg())
+          MO2.setIsUndef();
+      }
+    }
 
     MachineInstr *CopyMI = NULL;
     if (mi->isCopyLike()) {