cmd/compile: dense numbering for GP registers

For register maps, we need a dense numbering of registers that may
contain pointers of interest to the garbage collector. Add this to
Register and compute it from the GP register set.

For #24543.

Change-Id: If6f0521effca5eca4d17895468b1fc52d67e0f32
Reviewed-on: https://go-review.googlesource.com/109351
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -27,6 +27,7 @@ type Config struct {
 	gpRegMask       regMask       // general purpose integer register mask
 	fpRegMask       regMask       // floating point register mask
 	specialRegMask  regMask       // special register mask
+	GCRegMap        []*Register   // garbage collector register map, by GC register index
 	FPReg           int8          // register number of frame pointer, -1 if not used
 	LinkReg         int8          // register number of link register if it is a general purpose register, -1 if not used
 	hasGReg         bool          // has hardware g register
@@ -375,6 +376,21 @@ func NewConfig(arch string, types Types, ctxt *obj.Link, optimize bool) *Config
 		c.sparsePhiCutoff = uint64(v) // convert -1 to maxint, for never use sparse
 	}
 
+	// Create the GC register map index.
+	// TODO: This is only used for debug printing. Maybe export config.registers?
+	gcRegMapSize := int16(0)
+	for _, r := range c.registers {
+		if r.gcNum+1 > gcRegMapSize {
+			gcRegMapSize = r.gcNum + 1
+		}
+	}
+	c.GCRegMap = make([]*Register, gcRegMapSize)
+	for i, r := range c.registers {
+		if r.gcNum != -1 {
+			c.GCRegMap[r.gcNum] = &c.registers[i]
+		}
+	}
+
 	return c
 }
 

src/cmd/compile/internal/ssa/gen/main.go

@@ -301,6 +301,7 @@ func genOp() {
 			continue
 		}
 		fmt.Fprintf(w, "var registers%s = [...]Register {\n", a.name)
+		var gcRegN int
 		for i, r := range a.regnames {
 			pkg := a.pkg[len("cmd/internal/obj/"):]
 			var objname string // name in cmd/internal/obj/$ARCH
@@ -315,7 +316,18 @@ func genOp() {
 			default:
 				objname = pkg + ".REG_" + r
 			}
-			fmt.Fprintf(w, "  {%d, %s, \"%s\"},\n", i, objname, r)
+			// Assign a GC register map index to registers
+			// that may contain pointers.
+			gcRegIdx := -1
+			if a.gpregmask&(1<<uint(i)) != 0 {
+				gcRegIdx = gcRegN
+				gcRegN++
+			}
+			fmt.Fprintf(w, "  {%d, %s, %d, \"%s\"},\n", i, objname, gcRegIdx, r)
+		}
+		if gcRegN > 32 {
+			// Won't fit in a uint32 mask.
+			log.Fatalf("too many GC registers (%d > 32) on %s", gcRegN, a.name)
 		}
 		fmt.Fprintln(w, "}")
 		fmt.Fprintf(w, "var gpRegMask%s = regMask(%d)\n", a.name, a.gpregmask)

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

@@ -19,6 +19,7 @@ type Location interface {
 type Register struct {
 	num    int32 // dense numbering
 	objNum int16 // register number from cmd/internal/obj/$ARCH
+	gcNum  int16 // GC register map number (dense numbering of registers that can contain pointers)
 	name   string
 }
 
@@ -32,6 +33,12 @@ func (r *Register) ObjNum() int16 {
 	return r.objNum
 }
 
+// GCNum returns the runtime GC register index of r, or -1 if this
+// register can't contain pointers.
+func (r *Register) GCNum() int16 {
+	return r.gcNum
+}
+
 // A LocalSlot is a location in the stack frame, which identifies and stores
 // part or all of a PPARAM, PPARAMOUT, or PAUTO ONAME node.
 // It can represent a whole variable, part of a larger stack slot, or part of a