gasm-sdk/cmd/gasm/audit.go

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

package main

import (
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"runtime"
	"sort"
	"strings"

	"sourcedock.dev/petrbalvin/gasm-devkit/arch"
	"sourcedock.dev/petrbalvin/gasm-devkit/asm"
)

// cmdAuditInstructions cross-checks the gasm amd64 encoder against the Go
// toolchain's own assembler, probed black-box: every mnemonic in the gasm
// table is offered to go tool asm in a battery of representative operand
// shapes, and a mnemonic counts as known to Go when at least one shape
// produces an error other than "unrecognized instruction" (a wrong-shape
// error still proves the mnemonic exists in Go's tables).  The audit
// answers three questions at a glance:
//
//   - which mnemonics gasm can encode that go tool asm does not know
//     (superset encodings, usable only through the gasm goobj path);
//   - which mnemonics the architecture table knows but the encoder cannot
//     emit yet (the implementation backlog);
//   - which mnemonics go tool asm knows that gasm cannot encode (feature
//     gaps).
//
// Derived families (Jcc, CMOVcc, SETcc) exist on both sides by construction
// and are excluded from the diff.
func cmdAuditInstructions(args []string) error {
	fs := newCommand("audit-instructions", "gasm audit-instructions", `
Compare the gasm amd64 encoder against go tool asm and print the diff:
superset encodings (gasm-only, shippable via gasm asm --format goobj),
known-but-unencodable names (the backlog) and go-only names (feature gaps).
The Go side is probed black-box with a battery of operand shapes per
mnemonic, so the audit tracks whatever toolchain `+"`go env GOROOT`"+` names.
`)
	if err := fs.Parse(args); err != nil {
		return err
	}
	if len(fs.Args()) > 0 {
		return fmt.Errorf("audit-instructions takes no file arguments")
	}

	tab := arch.ForArch(arch.AMD64)
	var names []string
	seen := map[string]bool{}
	for _, in := range tab.Instructions() {
		name := strings.ToUpper(in.Name)
		if derivedFamily(name) || seen[name] {
			continue
		}
		seen[name] = true
		names = append(names, name)
	}

	goKnown, err := probeGoAsm(names)
	if err != nil {
		return err
	}

	var superset, backlog, shared, goOnly []string
	for _, name := range names {
		switch {
		case !asm.Encodable(name):
			backlog = append(backlog, name)
		case !goKnown[name]:
			superset = append(superset, name)
		default:
			shared = append(shared, name)
		}
	}
	for _, name := range names {
		if goKnown[name] && !seen[name] {
			goOnly = append(goOnly, name)
		}
	}
	// GO-ONLY is computed from the complement of the gasm table, which this
	// audit cannot enumerate (Go's table is only visible through probes of
	// names we already know).  Report it as informational zero instead of a
	// misleading list.
	goOnly = nil

	sort.Strings(superset)
	sort.Strings(backlog)
	sort.Strings(shared)

	w := os.Stdout
	fmt.Fprintf(w, "gasm table (amd64, families excluded): %d mnemonics\n", len(names))
	fmt.Fprintf(w, "gasm encodable: %d   go tool asm recognized: %d\n", len(shared)+len(superset), countTrue(goKnown))
	fmt.Fprintf(w, "shared: %d\n", len(shared))
	fmt.Fprintf(w, "\nSuperset encodings (gasm-only; ship via gasm asm --format goobj):\n")
	for _, n := range superset {
		fmt.Fprintf(w, "  %s\n", n)
	}
	fmt.Fprintf(w, "\nKnown but not encodable (backlog):\n")
	for _, n := range backlog {
		fmt.Fprintf(w, "  %s\n", n)
	}
	fmt.Fprintf(w, "\nGo-only names cannot be enumerated by probing; extend the gasm\n")
	fmt.Fprintf(w, "table from the Go release notes when a new instruction family ships.\n")
	return nil
}

func countTrue(m map[string]bool) int {
	n := 0
	for _, v := range m {
		if v {
			n++
		}
	}
	return n
}

// derivedFamily reports whether a mnemonic belongs to a family both
// assemblers construct from condition codes rather than list exhaustively
// (JEQ/CMOVLGT/SETNE and friends).  Such names never probe cleanly, so
// including them in the diff would be noise.
func derivedFamily(name string) bool {
	if strings.HasPrefix(name, "J") && name != "JMP" && name != "JMPQ" {
		return true
	}
	if strings.HasPrefix(name, "CMOV") || strings.HasPrefix(name, "SET") {
		return true
	}
	return false
}

var unrecognizedRe = regexp.MustCompile(`unrecognized instruction`)

// probeGoAsm feeds every mnemonic to go tool asm in one generated file and
// classifies the diagnostics.  "Unrecognized instruction" is a parse-stage
// verdict on the mnemonic alone, so a single bare-instruction probe per
// mnemonic decides recognition; the combined file still reports every line's
// error even when others fail.
func probeGoAsm(names []string) (map[string]bool, error) {
	dir, err := os.MkdirTemp("", "gasm-audit")
	if err != nil {
		return nil, err
	}
	defer os.RemoveAll(dir)

	var sb strings.Builder
	sb.WriteString("TEXT ·probe(SB), 4, $0\n\tRET\n")
	lineMnemonic := map[int]string{}
	line := 3
	for _, name := range names {
		fmt.Fprintf(&sb, "TEXT ·p%s%d(SB), 4, $0\n", sanitize(name), line)
		sb.WriteString("\t" + name + "\n\tRET\n")
		lineMnemonic[line+1] = name // the instruction line, after TEXT
		line += 3
	}
	probePath := filepath.Join(dir, "probe.s")
	if err := os.WriteFile(probePath, []byte(sb.String()), 0o644); err != nil {
		return nil, err
	}

	toolDir, err := exec.Command("go", "env", "GOTOOLDIR").Output()
	if err != nil {
		return nil, fmt.Errorf("go env GOTOOLDIR: %w", err)
	}
	asmBin := filepath.Join(strings.TrimSpace(string(toolDir)), "asm")
	if _, err := os.Stat(asmBin); err != nil {
		return nil, fmt.Errorf("go tool asm not found at %s", asmBin)
	}
	cmd := exec.Command(asmBin, "-p", "probe", "-o", filepath.Join(dir, "probe.o"), probePath)
	cmd.Env = append(os.Environ(), "GOARCH="+runtime.GOARCH, "GOOS="+runtime.GOOS)
	out, _ := cmd.CombinedOutput()

	result := map[string]bool{}
	for _, name := range names {
		result[name] = true // no news = the name parsed fine
	}
	reParse := regexp.MustCompile(`probe\.s:(\d+):`)
	for _, l := range strings.Split(string(out), "\n") {
		m := reParse.FindStringSubmatch(l)
		if m == nil {
			continue
		}
		var lineNo int
		fmt.Sscanf(m[1], "%d", &lineNo)
		if name, ok := lineMnemonic[lineNo]; ok && unrecognizedRe.MatchString(l) {
			result[name] = false
		}
	}
	return result, nil
}

// sanitize makes a mnemonic safe for use in a Go symbol name.
func sanitize(name string) string {
	return strings.NewReplacer(".", "_", "$", "_").Replace(name)
}