[dev.regabi] cmd/compile: add code to support register ABI spills around morestack calls

This is a selected copy from the register ABI experiment CL, focused
on the files and data structures that handle spilling around morestack.
Unnecessary code from the experiment was removed, other code was adapted.

Would it make sense to leave comments in the experiment as pieces are
brought over?

Experiment CL (for comparison purposes)
https://go-review.googlesource.com/c/go/+/28832

Change-Id: I92136f070351d4fcca1407b52ecf9b80898fed95
Reviewed-on: https://go-review.googlesource.com/c/go/+/279520
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

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

@@ -58,6 +58,9 @@ type Func struct {
 	// of keys to make iteration order deterministic.
 	Names []LocalSlot
 
+	// RegArgs is a slice of register-memory pairs that must be spilled and unspilled in the uncommon path of function entry.
+	RegArgs []ArgPair
+
 	// WBLoads is a list of Blocks that branch on the write
 	// barrier flag. Safe-points are disabled from the OpLoad that
 	// reads the write-barrier flag until the control flow rejoins

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

@@ -87,3 +87,29 @@ func (t LocPair) String() string {
 	}
 	return fmt.Sprintf("<%s,%s>", n0, n1)
 }
+
+type ArgPair struct {
+	reg *Register
+	mem LocalSlot
+}
+
+func (ap *ArgPair) Reg() int16 {
+	return ap.reg.objNum
+}
+
+func (ap *ArgPair) Type() *types.Type {
+	return ap.mem.Type
+}
+
+func (ap *ArgPair) Mem() *LocalSlot {
+	return &ap.mem
+}
+
+func (t ArgPair) String() string {
+	n0 := "nil"
+	if t.reg != nil {
+		n0 = t.reg.String()
+	}
+	n1 := t.mem.String()
+	return fmt.Sprintf("<%s,%s>", n0, n1)
+}

src/cmd/internal/obj/link.go

@@ -766,6 +766,17 @@ type Auto struct {
 	Gotype  *LSym
 }
 
+// RegArg provides spill/fill information for a register-resident argument
+// to a function.  These need spilling/filling in the safepoint/stackgrowth case.
+// At the time of fill/spill, the offset must be adjusted by the architecture-dependent
+// adjustment to hardware SP that occurs in a call instruction.  E.g., for AMD64,
+// at Offset+8 because the return address was pushed.
+type RegArg struct {
+	Addr           Addr
+	Reg            int16
+	Spill, Unspill As
+}
+
 // Link holds the context for writing object code from a compiler
 // to be linker input or for reading that input into the linker.
 type Link struct {
@@ -796,10 +807,11 @@ type Link struct {
 	DebugInfo          func(fn *LSym, info *LSym, curfn interface{}) ([]dwarf.Scope, dwarf.InlCalls) // if non-nil, curfn is a *gc.Node
 	GenAbstractFunc    func(fn *LSym)
 	Errors             int
+	RegArgs            []RegArg
 
-	InParallel    bool // parallel backend phase in effect
+	InParallel      bool // parallel backend phase in effect
-	UseBASEntries bool // use Base Address Selection Entries in location lists and PC ranges
+	UseBASEntries   bool // use Base Address Selection Entries in location lists and PC ranges
-	IsAsm         bool // is the source assembly language, which may contain surprising idioms (e.g., call tables)
+	IsAsm           bool // is the source assembly language, which may contain surprising idioms (e.g., call tables)
 
 	// state for writing objects
 	Text []*LSym
@@ -844,6 +856,32 @@ func (ctxt *Link) Logf(format string, args ...interface{}) {
 	ctxt.Bso.Flush()
 }
 
+func (ctxt *Link) SpillRegisterArgs(last *Prog, pa ProgAlloc) *Prog {
+	// Spill register args.
+	for _, ra := range ctxt.RegArgs {
+		spill := Appendp(last, pa)
+		spill.As = ra.Spill
+		spill.From.Type = TYPE_REG
+		spill.From.Reg = ra.Reg
+		spill.To = ra.Addr
+		last = spill
+	}
+	return last
+}
+
+func (ctxt *Link) UnspillRegisterArgs(last *Prog, pa ProgAlloc) *Prog {
+	// Unspill any spilled register args
+	for _, ra := range ctxt.RegArgs {
+		unspill := Appendp(last, pa)
+		unspill.As = ra.Unspill
+		unspill.From = ra.Addr
+		unspill.To.Type = TYPE_REG
+		unspill.To.Reg = ra.Reg
+		last = unspill
+	}
+	return last
+}
+
 // The smallest possible offset from the hardware stack pointer to a local
 // variable on the stack. Architectures that use a link register save its value
 // on the stack in the function prologue and so always have a pointer between

src/cmd/internal/obj/x86/obj6.go

@@ -1114,7 +1114,8 @@ func stacksplit(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, newprog obj.ProgA
 	spfix.Spadj = -framesize
 
 	pcdata := ctxt.EmitEntryStackMap(cursym, spfix, newprog)
-	pcdata = ctxt.StartUnsafePoint(pcdata, newprog)
+	spill := ctxt.StartUnsafePoint(pcdata, newprog)
+	pcdata = ctxt.SpillRegisterArgs(spill, newprog)
 
 	call := obj.Appendp(pcdata, newprog)
 	call.Pos = cursym.Func().Text.Pos
@@ -1139,7 +1140,8 @@ func stacksplit(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, newprog obj.ProgA
 		progedit(ctxt, callend.Link, newprog)
 	}
 
-	pcdata = ctxt.EndUnsafePoint(callend, newprog, -1)
+	pcdata = ctxt.UnspillRegisterArgs(callend, newprog)
+	pcdata = ctxt.EndUnsafePoint(pcdata, newprog, -1)
 
 	jmp := obj.Appendp(pcdata, newprog)
 	jmp.As = obj.AJMP
@@ -1147,9 +1149,9 @@ func stacksplit(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, newprog obj.ProgA
 	jmp.To.SetTarget(cursym.Func().Text.Link)
 	jmp.Spadj = +framesize
 
-	jls.To.SetTarget(call)
+	jls.To.SetTarget(spill)
 	if q1 != nil {
-		q1.To.SetTarget(call)
+		q1.To.SetTarget(spill)
 	}
 
 	return end