all: merge branch dev.regabi (d3cd483) into master

This CL merges the dev.regabi branch to the master branch.

In the dev.regabi branch we have refactored the compiler, and laid
some preliminary work for enabling a register-based ABI (issue #40724),
including improved late call/return lowering, improved ABI wrapper
generation, reflect call prepared for the new ABI, and reserving
special registers in the internal ABI. The actual register-based ABI
has not been enabled for the moment. The ABI-related changes are behind
GOEXPERIMENT=regabi and currently off by default.

Updates #40724, #44222.
Fixes #44224.

Change-Id: Id5de9f734d14099267ab717167aaaeef31fdba70

src/cmd/asm/internal/arch/arch.go

@@ -109,6 +109,10 @@ func archX86(linkArch *obj.LinkArch) *Arch {
 	register["SB"] = RSB
 	register["FP"] = RFP
 	register["PC"] = RPC
+	if linkArch == &x86.Linkamd64 {
+		// Alias g to R14
+		register["g"] = x86.REGG
+	}
 	// Register prefix not used on this architecture.
 
 	instructions := make(map[string]obj.As)

src/cmd/asm/internal/asm/operand_test.go

@@ -259,6 +259,7 @@ var amd64OperandTests = []operandTest{
 	{"R15", "R15"},
 	{"R8", "R8"},
 	{"R9", "R9"},
+	{"g", "R14"},
 	{"SI", "SI"},
 	{"SP", "SP"},
 	{"X0", "X0"},

src/cmd/asm/internal/asm/parse.go

@@ -305,7 +305,7 @@ func (p *Parser) pseudo(word string, operands [][]lex.Token) bool {
 // references and writes symabis information to w.
 //
 // The symabis format is documented at
-// cmd/compile/internal/gc.readSymABIs.
+// cmd/compile/internal/ssagen.ReadSymABIs.
 func (p *Parser) symDefRef(w io.Writer, word string, operands [][]lex.Token) {
 	switch word {
 	case "TEXT":