[RAGreedy] Add an experimental deferred spilling feature.

The idea of deferred spilling is to delay the insertion of spill code until the
very end of the allocation. A "candidate" to spill variable might not required
to be spilled because of other evictions that happened after this decision was
taken. The spirit is similar to the optimistic coloring strategy implemented in
Preston and Briggs graph coloring algorithm.

For now, this feature is highly experimental. Although correct, it would require
much more modification to properly model the effect of spilling.

Anyway, this early patch helps prototyping this feature.

Note: The test case cannot unfortunately be reduced and is probably fragile.
llvm-svn: 242585

Author: Quentin Colombet

llvm/lib/CodeGen/RegAllocGreedy.cpp

@@ -86,6 +86,14 @@ static cl::opt<bool> EnableLocalReassignment(
              "may be compile time intensive"),
     cl::init(false));
 
+static cl::opt<bool> EnableDeferredSpilling(
+    "enable-deferred-spilling", cl::Hidden,
+    cl::desc("Instead of spilling a variable right away, defer the actual "
+             "code insertion to the end of the allocation. That way the "
+             "allocator might still find a suitable coloring for this "
+             "variable because of other evicted variables."),
+    cl::init(false));
+
 // FIXME: Find a good default for this flag and remove the flag.
 static cl::opt<unsigned>
 CSRFirstTimeCost("regalloc-csr-first-time-cost",
@@ -157,6 +165,11 @@ class RAGreedy : public MachineFunctionPass,
     /// Live range will be spilled.  No more splitting will be attempted.
     RS_Spill,
 
+
+    /// Live range is in memory. Because of other evictions, it might get moved
+    /// in a register in the end.
+    RS_Memory,
+
     /// There is nothing more we can do to this live range.  Abort compilation
     /// if it can't be assigned.
     RS_Done
@@ -414,6 +427,7 @@ const char *const RAGreedy::StageName[] = {
     "RS_Split",
     "RS_Split2",
     "RS_Spill",
+    "RS_Memory",
     "RS_Done"
 };
 #endif
@@ -536,6 +550,13 @@ void RAGreedy::enqueue(PQueue &CurQueue, LiveInterval *LI) {
     // Unsplit ranges that couldn't be allocated immediately are deferred until
     // everything else has been allocated.
     Prio = Size;
+  } else if (ExtraRegInfo[Reg].Stage == RS_Memory) {
+    // Memory operand should be considered last.
+    // Change the priority such that Memory operand are assigned in
+    // the reverse order that they came in.
+    // TODO: Make this a member variable and probably do something about hints.
+    static unsigned MemOp = 0;
+    Prio = MemOp++;
   } else {
     // Giant live ranges fall back to the global assignment heuristic, which
     // prevents excessive spilling in pathological cases.
@@ -2512,13 +2533,23 @@ unsigned RAGreedy::selectOrSplitImpl(LiveInterval &VirtReg,
     return PhysReg;
 
   // Finally spill VirtReg itself.
-  NamedRegionTimer T("Spiller", TimerGroupName, TimePassesIsEnabled);
-  LiveRangeEdit LRE(&VirtReg, NewVRegs, *MF, *LIS, VRM, this);
-  spiller().spill(LRE);
-  setStage(NewVRegs.begin(), NewVRegs.end(), RS_Done);
+  if (EnableDeferredSpilling && getStage(VirtReg) < RS_Memory) {
+    // TODO: This is experimental and in particular, we do not model
+    // the live range splitting done by spilling correctly.
+    // We would need a deep integration with the spiller to do the
+    // right thing here. Anyway, that is still good for early testing.
+    setStage(VirtReg, RS_Memory);
+    DEBUG(dbgs() << "Do as if this register is in memory\n");
+    NewVRegs.push_back(VirtReg.reg);
+  } else {
+    NamedRegionTimer T("Spiller", TimerGroupName, TimePassesIsEnabled);
+    LiveRangeEdit LRE(&VirtReg, NewVRegs, *MF, *LIS, VRM, this);
+    spiller().spill(LRE);
+    setStage(NewVRegs.begin(), NewVRegs.end(), RS_Done);
 
-  if (VerifyEnabled)
-    MF->verify(this, "After spilling");
+    if (VerifyEnabled)
+      MF->verify(this, "After spilling");
+  }
 
   // The live virtual register requesting allocation was spilled, so tell
   // the caller not to allocate anything during this round.