fix(asm): tighten the arm64 acceptance toward the toolchain

Assisted-by: GLM 5.3 Flash

asm/arm64_errorparity_test.go

@@ -0,0 +1,101 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package asm
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"sourcedock.dev/petrbalvin/gasm-sdk/parser"
+)
+
+// arm64AcceptedErrorShapes lists the toolchain's arm64error.s spellings gasm
+// still accepts, each an acceptance superset with a known shape.  The list
+// only shrinks: every tightening of the encoder moves spellings out of it,
+// and a spelling reappearing here means a regression.
+var arm64AcceptedErrorShapes = []string{
+	// VLD1/VST1 post-index shapes: the register post-index without the .P
+	// spelling, the .P immediate against the aggregate register size, the
+	// scaled register increment and the replicating register count.
+	"VLD1 (R8)(R13), [V2.B16]",
+	"VST1 [V1.B16], (R8)(R13)",
+	"VST1.P [V4.S4,V5.S4], 48(R1)",
+	"VST1.P [V4.S4], 8(R1)",
+	"VLD1.P 32(R1), [V8.S4, V9.S4, V10.S4]",
+	"VLD1.P 48(R1), [V7.S4, V8.S4, V9.S4, V10.S4]",
+	"VLD1.P (R8)(R9<<2), [V2.B16]",
+	"VST1.P [V1.B16], (R8)(R9<<1)",
+	"VLD3R.P 24(R15), [V15.H4,V16.H4,V17.H4]",
+	// REGTMP (R27) as an explicit operand the toolchain refuses, the pair
+	// split paths included.
+	"ADD $0x1234567, R27, R3",
+	"ADD $0x3fffffffc000, R27, R5",
+	"AND $0x22220000, R27, R4",
+	"ANDW $0x6006000060060, R27, R5",
+	"STP (R3, R4), 0x1234567(R27)",
+	"LDP 0x1234567(R27), (R3, R4)",
+	"STP (R26, R27), 700(R2)",
+	// TLBI operand arity and the range-prefetch operand shapes.
+	"TLBI VMALLE1IS, R0",
+	"TLBI ALLE3OS, ZR",
+	"TLBI VAE1IS",
+	"TLBI RVALE3",
+	"RPRFM (R1), RSP, PLDKEEP",
+	// The GP register pairs on the FP mnemonics, the other way round.
+	"LDP (R0), (F0, F1)",
+	"STP (F2, F3), (R0)",
+	// FCVTL exists for the S to D pair alone.
+	"VFCVTL V1.H4, V2.S4",
+}
+
+// TestArm64ToolchainErrorParity walks the toolchain's arm64error.s (Go
+// 1.27, arm64) and requires gasm to reject every case the toolchain rejects,
+// the documented acceptance supersets above excepted.  A live Go toolchain
+// is needed for the source file; the test skips without one or in -short.
+func TestArm64ToolchainErrorParity(t *testing.T) {
+	if testing.Short() {
+		t.Skip("live arm64error.s corpus: skipped in -short mode")
+	}
+	goroot := os.Getenv("GOROOT")
+	if goroot == "" {
+		t.Skip("no GOROOT")
+	}
+	path := filepath.Join(goroot, "src", "cmd", "asm", "internal", "asm", "testdata", "arm64error.s")
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Skip(err)
+	}
+	allowed := map[string]bool{}
+	for _, s := range arm64AcceptedErrorShapes {
+		allowed[s] = true
+	}
+	for raw := range strings.SplitSeq(string(data), "\n") {
+		line := strings.TrimSpace(raw)
+		if line == "" || strings.HasPrefix(line, "//") || strings.HasPrefix(line, "TEXT") || !strings.Contains(line, "ERROR") {
+			continue
+		}
+		body := line
+		if i := strings.Index(body, "//"); i >= 0 {
+			body = strings.TrimSpace(body[:i])
+		}
+		body = strings.ReplaceAll(body, "\t", " ")
+		body = strings.Join(strings.Fields(body), " ")
+		// Label-relative and non-operand lines need their function context.
+		if strings.Contains(body, "(PC)") || strings.Contains(body, "(SB)") || strings.Contains(body, "PCALIGN") ||
+			strings.HasPrefix(body, "RET") || strings.HasPrefix(body, "NOP") || strings.HasPrefix(body, "BRK") ||
+			strings.Contains(body, "again") || strings.Contains(body, "next") || strings.Contains(body, "loop") {
+			continue
+		}
+		src := "#include \"textflag.h\"\n\nTEXT ·f(SB), NOSPLIT, $0-0\n\t" + body + "\n\tRET\n"
+		f, perr := parser.Parse("errorparity.s", src)
+		if len(perr) > 0 {
+			continue // the parser already rejects the spelling
+		}
+		if _, aerr := AssembleFileARM64(f); aerr == nil && !allowed[body] {
+			t.Errorf("gasm accepts what the toolchain rejects: %s", body)
+		}
+	}
+}