Allow coalescing with reserved physregs in certain cases:

When a virtual register has a single value that is defined as a copy of a
reserved register, permit that copy to be joined. These virtual register are
usually copies of the stack pointer:

  %vreg75<def> = COPY %ESP; GR32:%vreg75
  MOV32mr %vreg75, 1, %noreg, 0, %noreg, %vreg74<kill>
  MOV32mi %vreg75, 1, %noreg, 8, %noreg, 0
  MOV32mi %vreg75<kill>, 1, %noreg, 4, %noreg, 0
  CALLpcrel32 ...

Coalescing these virtual registers early decreases register pressure.
Previously, they were coalesced by RALinScan::attemptTrivialCoalescing after
register allocation was completed.

The lower register pressure causes the mcinst-lowering-cmp0.ll test case to fail
because it depends on linear scan spilling a particular register.

I am deleting 2008-08-05-SpillerBug.ll because it is counting the number of
instructions emitted, and its revision history shows the 'correct' count being
edited many times.

llvm-svn: 128845

3 files changed, 14 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
index 68bd0a60c98..07f83ea7cef 100644
--- a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
+++ b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -572,7 +572,7 @@ void LiveIntervals::handleRegisterDef(MachineBasicBlock *MBB,
   if (TargetRegisterInfo::isVirtualRegister(MO.getReg()))
     handleVirtualRegisterDef(MBB, MI, MIIdx, MO, MOIdx,
                              getOrCreateInterval(MO.getReg()));
-  else if (allocatableRegs_[MO.getReg()]) {
+  else {
     MachineInstr *CopyMI = NULL;
     if (MI->isCopyLike())
       CopyMI = MI;
diff --git a/llvm/lib/CodeGen/RegAllocLinearScan.cpp b/llvm/lib/CodeGen/RegAllocLinearScan.cpp
index 966570f3f33..ef78949c54e 100644
--- a/llvm/lib/CodeGen/RegAllocLinearScan.cpp
+++ b/llvm/lib/CodeGen/RegAllocLinearScan.cpp
@@ -572,7 +572,7 @@ void RALinScan::initIntervalSets()
 
   for (LiveIntervals::iterator i = li_->begin(), e = li_->end(); i != e; ++i) {
     if (TargetRegisterInfo::isPhysicalRegister(i->second->reg)) {
-      if (!i->second->empty()) {
+      if (!i->second->empty() && allocatableRegs_.test(i->second->reg)) {
         mri_->setPhysRegUsed(i->second->reg);
         fixed_.push_back(std::make_pair(i->second, i->second->begin()));
       }
diff --git a/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp b/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
index 93aa0a31207..c621726a03f 100644
--- a/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
+++ b/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
@@ -919,13 +919,23 @@ SimpleRegisterCoalescing::ShortenDeadCopySrcLiveRange(LiveInterval &li,
 /// are not spillable! If the destination interval uses are far away, think
 /// twice about coalescing them!
 bool SimpleRegisterCoalescing::shouldJoinPhys(CoalescerPair &CP) {
+  bool Allocatable = li_->isAllocatable(CP.getDstReg());
+  LiveInterval &JoinVInt = li_->getInterval(CP.getSrcReg());
+
+  /// Always join simple intervals that are defined by a single copy from a
+  /// reserved register. This doesn't increase register pressure, so it is
+  /// always beneficial.
+  if (!Allocatable && CP.isFlipped() && JoinVInt.containsOneValue())
+    return true;
+
   if (DisablePhysicalJoin) {
     DEBUG(dbgs() << "\tPhysreg joins disabled.\n");
     return false;
   }
 
-  // Only coalesce to allocatable physreg.
-  if (!li_->isAllocatable(CP.getDstReg())) {
+  // Only coalesce to allocatable physreg, we don't want to risk modifying
+  // reserved registers.
+  if (!Allocatable) {
     DEBUG(dbgs() << "\tRegister is an unallocatable physreg.\n");
     return false;  // Not coalescable.
   }
@@ -944,8 +954,6 @@ bool SimpleRegisterCoalescing::shouldJoinPhys(CoalescerPair &CP) {
   // FIXME: Why are we skipping this test for partial copies?
   //        CodeGen/X86/phys_subreg_coalesce-3.ll needs it.
   if (!CP.isPartial()) {
-    LiveInterval &JoinVInt = li_->getInterval(CP.getSrcReg());
-
     const TargetRegisterClass *RC = mri_->getRegClass(CP.getSrcReg());
     unsigned Threshold = allocatableRCRegs_[RC].count() * 2;
     unsigned Length = li_->getApproximateInstructionCount(JoinVInt);