feat(asm): add RISC-V ELF relocatable object emission and SB relocation

support

verify/groundtruth.go

@@ -19,7 +19,20 @@ import (
 // keyed by the function's short name (the part after the middle dot).
 // This is the universal oracle: any file that `go tool asm` accepts can
 // be verified, with no hand-written reference.
+//
+// For RISC-V sources the assembler is invoked with GOARCH=riscv64;
+// the caller must set the architecture via GroundTruthArch.
 func GroundTruth(path string) (map[string][]byte, error) {
+	return groundTruthArch(path, "")
+}
+
+// GroundTruthRISCV assembles the given .s file with the Go toolchain in
+// RISC-V cross-assembly mode (GOARCH=riscv64).
+func GroundTruthRISCV(path string) (map[string][]byte, error) {
+	return groundTruthArch(path, "riscv64")
+}
+
+func groundTruthArch(path, goarch string) (map[string][]byte, error) {
 	goroot := runtime.GOROOT()
 	asmBin := filepath.Join(goroot, "pkg", "tool", runtime.GOOS+"_"+runtime.GOARCH, "asm")
 	if _, err := os.Stat(asmBin); err != nil {
@@ -27,7 +40,6 @@ func GroundTruth(path string) (map[string][]byte, error) {
 	}
 	includeDir := filepath.Join(goroot, "pkg", "include")
 
-	// Create a temp file for the object output.
 	tmpDir, err := os.MkdirTemp("", "gasm-verify-*")
 	if err != nil {
 		return nil, fmt.Errorf("verify: tempdir: %w", err)
@@ -35,14 +47,17 @@ func GroundTruth(path string) (map[string][]byte, error) {
 	defer os.RemoveAll(tmpDir)
 	objPath := filepath.Join(tmpDir, "out.o")
 
-	// Derive a package name from the file name (the assembler needs -p).
 	base := filepath.Base(path)
 	pkg := strings.TrimSuffix(base, ".s")
 	pkg = strings.TrimSuffix(pkg, "_amd64")
+	pkg = strings.TrimSuffix(pkg, "_riscv64")
 
 	cmd := exec.Command(asmBin, "-I", includeDir, "-p", pkg, "-o", objPath, path)
+	if goarch != "" {
+		cmd.Env = append(os.Environ(), "GOARCH="+goarch)
+	}
 	if out, err := cmd.CombinedOutput(); err != nil {
-		return nil, fmt.Errorf("verify: go tool asm: %w\n%s", err, out)
+		return nil, fmt.Errorf("verify: go tool asm (%s): %w\n%s", goarch, err, out)
 	}
 
 	objData, err := os.ReadFile(objPath)