Better handling of dead super registers in LiveVariables. We used to do this:

   CALL ... %RAX<imp-def>
   ... [not using %RAX]
   %EAX = ..., %RAX<imp-use, kill>
   RET %EAX<imp-use,kill>

Now we do this:

   CALL ... %RAX<imp-def, dead>
   ... [not using %RAX]
   %EAX = ...
   RET %EAX<imp-use,kill>

By not artificially keeping %RAX alive, we lower register pressure a bit.

The correct number of instructions for 2008-08-05-SpillerBug.ll is obviously
55, anybody can see that. Sheesh.

llvm-svn: 97838

2 files changed, 22 insertions, 22 deletions

diff --git a/llvm/lib/CodeGen/LiveVariables.cpp b/llvm/lib/CodeGen/LiveVariables.cpp
index 68c85394d4c..519990e04a2 100644
--- a/llvm/lib/CodeGen/LiveVariables.cpp
+++ b/llvm/lib/CodeGen/LiveVariables.cpp
@@ -365,27 +365,7 @@ bool LiveVariables::HandlePhysRegKill(unsigned Reg, MachineInstr *MI) {
     }
   }
 
-  if (LastRefOrPartRef == PhysRegDef[Reg] && LastRefOrPartRef != MI) {
-    if (LastPartDef)
-      // The last partial def kills the register.
-      LastPartDef->addOperand(MachineOperand::CreateReg(Reg, false/*IsDef*/,
-                                                true/*IsImp*/, true/*IsKill*/));
-    else {
-      MachineOperand *MO =
-        LastRefOrPartRef->findRegisterDefOperand(Reg, false, TRI);
-      bool NeedEC = MO->isEarlyClobber() && MO->getReg() != Reg;
-      // If the last reference is the last def, then it's not used at all.
-      // That is, unless we are currently processing the last reference itself.
-      LastRefOrPartRef->addRegisterDead(Reg, TRI, true);
-      if (NeedEC) {
-        // If we are adding a subreg def and the superreg def is marked early
-        // clobber, add an early clobber marker to the subreg def.
-        MO = LastRefOrPartRef->findRegisterDefOperand(Reg);
-        if (MO)
-          MO->setIsEarlyClobber();
-      }
-    }
-  } else if (!PhysRegUse[Reg]) {
+  if (!PhysRegUse[Reg]) {
     // Partial uses. Mark register def dead and add implicit def of
     // sub-registers which are used.
     // EAX<dead>  = op  AL<imp-def>
@@ -419,6 +399,26 @@ bool LiveVariables::HandlePhysRegKill(unsigned Reg, MachineInstr *MI) {
       for (const unsigned *SS = TRI->getSubRegisters(SubReg); *SS; ++SS)
         PartUses.erase(*SS);
     }
+  } else if (LastRefOrPartRef == PhysRegDef[Reg] && LastRefOrPartRef != MI) {
+    if (LastPartDef)
+      // The last partial def kills the register.
+      LastPartDef->addOperand(MachineOperand::CreateReg(Reg, false/*IsDef*/,
+                                                true/*IsImp*/, true/*IsKill*/));
+    else {
+      MachineOperand *MO =
+        LastRefOrPartRef->findRegisterDefOperand(Reg, false, TRI);
+      bool NeedEC = MO->isEarlyClobber() && MO->getReg() != Reg;
+      // If the last reference is the last def, then it's not used at all.
+      // That is, unless we are currently processing the last reference itself.
+      LastRefOrPartRef->addRegisterDead(Reg, TRI, true);
+      if (NeedEC) {
+        // If we are adding a subreg def and the superreg def is marked early
+        // clobber, add an early clobber marker to the subreg def.
+        MO = LastRefOrPartRef->findRegisterDefOperand(Reg);
+        if (MO)
+          MO->setIsEarlyClobber();
+      }
+    }
   } else
     LastRefOrPartRef->addRegisterKilled(Reg, TRI, true);
   return true;
diff --git a/llvm/test/CodeGen/X86/2008-08-05-SpillerBug.ll b/llvm/test/CodeGen/X86/2008-08-05-SpillerBug.ll
index 67e14ffae5e..4c6493445a9 100644
--- a/llvm/test/CodeGen/X86/2008-08-05-SpillerBug.ll
+++ b/llvm/test/CodeGen/X86/2008-08-05-SpillerBug.ll
@@ -1,4 +1,4 @@
-; RUN: llc < %s -mtriple=i386-apple-darwin -disable-fp-elim -stats |& grep asm-printer | grep 58
+; RUN: llc < %s -mtriple=i386-apple-darwin -disable-fp-elim -stats |& grep asm-printer | grep 55
 ; PR2568
 
 @g_3 = external global i16		; <i16*> [#uses=1]