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

1 files changed, 12 insertions, 4 deletions

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);