[dev.ssa] cmd/compile: add instrumentation to regalloc

Change-Id: Ice206f7e94af4a148d9dd9a7570f5ed21722bedc
Reviewed-on: https://go-review.googlesource.com/14075Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/regalloc.go

@@ -105,6 +105,7 @@ import (
 )
 
 const regDebug = false
+const logSpills = false
 
 // regalloc performs register allocation on f.  It sets f.RegAlloc
 // to the resulting allocation.
@@ -402,6 +403,9 @@ func (s *regAllocState) allocValToReg(v *Value, mask regMask, nospill bool) *Val
 		// Instead, we regenerate the flags register by issuing the same instruction again.
 		// This requires (possibly) spilling and reloading that instruction's args.
 		case v.Type.IsFlags():
+			if logSpills {
+				fmt.Println("regalloc: regenerating flags")
+			}
 			ns := s.nospill
 			// Place v's arguments in registers, spilling and loading as needed
 			args := make([]*Value, 0, len(v.Args))
@@ -429,9 +433,15 @@ func (s *regAllocState) allocValToReg(v *Value, mask regMask, nospill bool) *Val
 		// Load v from its spill location.
 		// TODO: rematerialize if we can.
 		case vi.spill2 != nil:
+			if logSpills {
+				fmt.Println("regallog: load spill2")
+			}
 			c = s.curBlock.NewValue1(v.Line, OpLoadReg, v.Type, vi.spill2)
 			vi.spill2used = true
 		case vi.spill != nil:
+			if logSpills {
+				fmt.Println("regalloc: load spill")
+			}
 			c = s.curBlock.NewValue1(v.Line, OpLoadReg, v.Type, vi.spill)
 			vi.spillUsed = true
 		default: