gasm-sdk/asm/loong64_frame.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

package asm

import (
	"strings"

	"sourcedock.dev/petrbalvin/gasm-devkit/ast"
)

// Loong64 frame mapping, matching the Go toolchain's loong64 backend.
//
// Go's loong64 functions have no frame pointer: FP and SP are synthetic
// registers resolved against the hardware stack pointer (R3) and the frame
// size.  The return address lives in R1 (the link register).
//
// The autosize is the real stack adjustment: the declared local frame plus
// the 8 bytes for the saved link register, rounded up to a multiple of 8
// (the toolchain aligns frames with `if autosize&4 != 0 { autosize += 4 }`).
// A leaf function (no calls) with a zero frame gets no prologue at all.
//
// Prologue (autosize > 0), byte-identical to the toolchain:
//
//	MOVV R1, -autosize(R3)   // save LR below the new SP (traceback-safe)
//	ADDV $-autosize, R3      // open the frame
//	MOVV R1, 0(R3)           // save LR again at SP (signal-safety)
//
// Epilogue: MOVV 0(R3), R1; ADDV $autosize, R3 (non-leaf only for the LR
// restore); the RET's jirl r0, r1, 0 follows.

// loong64FrameInfo holds the frame layout derived from a TEXT directive.
type loong64FrameInfo struct {
	autosize int  // the real SP adjustment (locals + saved LR, aligned)
	frame    int  // the declared $framesize
	args     int  // the declared -argsize
	noSplit  bool // the NOSPLIT flag
	leaf     bool // no call instructions in the body
}

// loong64ComputeFrame derives the frame layout for a TEXT function.
func loong64ComputeFrame(t *ast.Text) loong64FrameInfo {
	fi := loong64FrameInfo{
		frame: frameSize(t),
		args:  argsSize(t),
	}
	for _, f := range t.Flags {
		if f == "NOSPLIT" {
			fi.noSplit = true
		}
	}
	fi.leaf = loong64IsLeaf(t)
	if fi.frame != 0 {
		fi.autosize = fi.frame + 8 // space for the saved LR
		if fi.autosize&4 != 0 {
			fi.autosize += 4
		}
	} else if !fi.leaf {
		// A zero-frame non-leaf function still opens an 8-byte frame for LR.
		fi.autosize = 8
	}
	return fi
}

// loong64IsLeaf reports whether a function contains no call instructions
// (JAL/BL/CALL), matching the toolchain's LEAF mark, which drives the frame
// and the epilogue shape.
func loong64IsLeaf(t *ast.Text) bool {
	for _, stmt := range t.Body {
		in, ok := stmt.(*ast.Instr)
		if !ok {
			continue
		}
		switch strings.ToUpper(in.Mnemonic.Text) {
		case "JAL", "CALL", "BL":
			return false
		}
	}
	return true
}

// loong64Prologue returns the prologue bytes for a loong64 function.
func loong64Prologue(fi loong64FrameInfo) []byte {
	if fi.autosize == 0 {
		return nil
	}
	addiD := l64DualTable["ADDV"].imm
	return l64WordsLE(
		l64irr(l64loadStoreTable["MOVV"].st, -fi.autosize, 3, 1), // MOVV R1, -autosize(R3)
		l64irr(addiD, -fi.autosize, 3, 3),                        // ADDV $-autosize, R3
		l64irr(l64loadStoreTable["MOVV"].st, 0, 3, 1),            // MOVV R1, 0(R3)
	)
}

// loong64Return returns the bytes for a RET: the epilogue (restore LR and
// deallocate the frame when present) followed by jirl r0, r1, 0.
func loong64Return(fi loong64FrameInfo) []byte {
	var ws []uint32
	if fi.autosize != 0 {
		if !fi.leaf {
			// MOVV 0(R3), R1 — restore the link register.
			ws = append(ws, l64irr(l64loadStoreTable["MOVV"].ld, 0, 3, 1))
		}
		// ADDV $autosize, R3 — close the frame.
		ws = append(ws, l64irr(l64DualTable["ADDV"].imm, fi.autosize, 3, 3))
	}
	// jirl r0, r1, 0 — return.
	ws = append(ws, l64irr16(l64branchTable["JIRL"], 0, 1, 0))
	return l64WordsLE(ws...)
}

// loong64ResolvePseudo translates a pseudo-register memory reference into a
// hardware base register and offset.  x+N(FP) → (N + autosize + 8)(SP);
// x-N(SP) → (autosize - N)(SP).  Returns base = -1 for an unresolvable
// reference (SB: static data, handled by the relocation path).
func loong64ResolvePseudo(sym *ast.Symbol, fi loong64FrameInfo) (base int, off int32) {
	if sym == nil {
		return -1, 0
	}
	switch sym.Pseudo {
	case "FP":
		return 3, int32(sym.Offset) + int32(fi.autosize) + 8
	case "SP":
		return 3, int32(fi.autosize) + int32(sym.Offset)
	case "SB":
		return -1, int32(sym.Offset)
	}
	return -1, 0
}