fix(asm): treat the arm64 $-8 frame as frameless and encode the RET forms

Assisted-by: GLM 5.3 Flash

asm/arm64_assemble.go

@@ -396,7 +396,7 @@ func encodeARM64Instr(instr *ast.Instr, pc int, offsets map[string]int, fi arm64
 	// Pseudo-instructions and special cases first.
 	switch mnem {
 	case "RET":
-		return arm64Return(fi), nil
+		return arm64RetInstr(fi, ops, relocs), nil
 	case "NOP", "NOOP":
 		return a64wordLE(a64NOP), nil
 	case "UNDEF":

asm/arm64_frame.go

@@ -87,7 +87,10 @@ func arm64ComputeFrame(t *ast.Text) arm64FrameInfo {
 	if fi.frame != 0 || !fi.leaf {
 		fi.autosize = fi.frame + 8 // space for the saved LR
 		// The toolchain always adds an extrasize: 8 when the total leaves a
-		// 16-byte alignment gap, another 16 when already aligned.
+		// 16-byte alignment gap, another 16 when already aligned.  An
+		// autosize of zero (the $-8 convention included) is frameless and
+		// takes neither.
+		if fi.autosize != 0 {
 			switch fi.autosize % 16 {
 			case 8:
 				fi.autosize += 8
@@ -99,6 +102,13 @@ func arm64ComputeFrame(t *ast.Text) arm64FrameInfo {
 				fi.autosize += 16 - (fi.autosize % 16)
 			}
 		}
+	}
+	if fi.autosize == 0 {
+		// The NOFRAME shape: the toolchain forces the leaf mark on any
+		// autosize-zero function (calls included), so nothing saves LR and
+		// no stack-split guard runs.
+		fi.leaf = true
+	}
 	switch {
 	case fi.noSplit:
 	case fi.autosize < stackSmall && fi.leaf:
@@ -301,6 +311,37 @@ func arm64Return(fi arm64FrameInfo) []byte {
 	return a64WordsLE(ws...)
 }
 
+// arm64RetInstr encodes a RET.  The plain form runs the frame epilogue and
+// branches to LR; RET Rn runs the epilogue and branches to the register
+// (asm7.go case 78); RET sym(SB) runs the epilogue and branches to the
+// symbol with the call relocation, the toolchain's retJMP tail call.
+func arm64RetInstr(fi arm64FrameInfo, ops []*ast.Operand, relocs *[]Reloc) []byte {
+	out := arm64Return(fi)
+	if len(ops) != 1 {
+		return out
+	}
+	op := ops[0]
+	switch {
+	case op.Addr.Sym != nil && op.Addr.Sym.Pseudo == "" && op.Addr.Base == "" && op.Addr.Index == "":
+		if rn := arm64RegNum(op.Addr.Sym.Name); rn >= 0 {
+			// The operand form replaces the default BR LR word.
+			return append(out[:len(out)-4], a64wordLE(0xd65f0000|uint32(rn)<<5)...)
+		}
+	case op.Addr.Sym != nil && op.Addr.Sym.Pseudo == "SB" && op.Addr.Base == "" && op.Addr.Index == "":
+		if relocs != nil {
+			*relocs = append(*relocs, Reloc{
+				Off:    0,
+				After:  4,
+				Name:   op.Addr.Sym.Name,
+				Addend: op.Addr.Sym.Offset,
+				Kind:   RelArm64Branch,
+			})
+		}
+		return append(out[:len(out)-4], a64wordLE(0x14000000)...) // B sym(SB)
+	}
+	return out
+}
+
 // arm64PrologueSpadjPC returns the function-relative byte offset where the
 // prologue has finished decrementing SP (the delta becomes autosize).
 func arm64PrologueSpadjPC(fi arm64FrameInfo) int {