Let LLVM merge the comparisons of a three-way string comparator

packages/compiler/src/backend/llvm/emit-module.ts

@@ -1601,8 +1601,8 @@ export function helperDefs(emitter: LlEmitter): string[] {
   // order; other differing words decide by their first differing byte,
   // and strings shorter than a word by their first byte. Otherwise a
   // proven-ASCII string on either side orders by bytes (scr_str_cmp), and
-  // other pairs call scr_str_cmp_u16_bytes, which only reads the two
-  // strings, so LLVM can merge repeated comparisons of one pair. Callers
+  // other pairs call scr_str_cmp_u16_bytes. Both only read the two strings,
+  // so LLVM can merge repeated comparisons of one pair. Callers
   // only test the sign of the result.
   if (emitter.decls.has(`declare i32 @scr_str_cmp_u16(ptr, ptr)`)) {
     const fn = "scr_str_cmp_u16";
@@ -1732,7 +1732,7 @@ export function helperDefs(emitter: LlEmitter): string[] {
       `  %anyascii = icmp slt i32 %cws, 0`,
       `  br i1 %anyascii, label %memorder, label %units`,
       `memorder:`,
-      `  %m = call i32 @scr_str_cmp(ptr %a, ptr %b)`,
+      `  %m = call i32 @scr_str_cmp(ptr %a, ptr %b) nounwind willreturn memory(argmem: read)`,
       `  ret i32 %m`,
       `units:`,
       `  %c = call i32 @scr_str_cmp_u16_bytes(ptr %a, ptr %b) nounwind willreturn memory(argmem: read)`,

packages/compiler/src/backend/llvm/string-lifetimes.test.ts

@@ -478,7 +478,9 @@ test("string comparisons decide identity, lengths and leading bytes before the r
     expect(units).toContain("%below = icmp ult i8 %bx, %by");
     expect(units).toContain("%tailable = and i1 %samelen, %short");
     expect(units).toContain("%anyascii = icmp slt i32 %cws, 0");
-    expect(units).toContain("call i32 @scr_str_cmp(ptr %a, ptr %b)");
+    expect(units).toContain(
+      "call i32 @scr_str_cmp(ptr %a, ptr %b) nounwind willreturn memory(argmem: read)",
+    );
     expect(units).toContain(
       "call i32 @scr_str_cmp_u16_bytes(ptr %a, ptr %b) nounwind willreturn memory(argmem: read)",
     );

tests/harness/string-compare-emission.test.ts

@@ -0,0 +1,47 @@
+/* String comparison emission: the runtime calls behind a relational string
+ * operator only read the two strings, so LLVM merges the comparisons of one
+ * pair. A three-way comparator (`a < b ? -1 : a > b ? 1 : 0`) then compares
+ * its strings once, as a comparator that tests one operator does. */
+import { createHash } from "node:crypto";
+import { mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import { expect, test } from "vitest";
+import { compile } from "@scriptc/compiler";
+
+const repoRoot = join(import.meta.dirname, "../..");
+const cacheDir = join(repoRoot, "node_modules/.cache/scriptc-tests");
+
+async function emittedAsm(name: string, source: string): Promise<string> {
+  const key = createHash("sha256").update(source).digest("hex").slice(0, 16);
+  const outDir = join(cacheDir, `string-compare-${key}`);
+  mkdirSync(outDir, { recursive: true });
+  const file = join(outDir, `${name}.ts`);
+  writeFileSync(file, source);
+  const result = await compile(file, {
+    outPath: join(outDir, name),
+    outDir,
+    backend: "llvm",
+    outputKind: "asm",
+  });
+  if (!result.ok)
+    throw new Error(result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"));
+  return readFileSync(result.artifact.path, "utf8");
+}
+
+const program = (body: string): string => `function order(a: string, b: string): number {
+  return ${body};
+}
+const words = process.argv.slice(2);
+words.sort(order);
+console.log(words.join(","));
+`;
+
+const compareCalls = (asm: string): number =>
+  asm.match(/\bcall[a-z]*\s+(?:sc_str_cmp_u16|scr_str_cmp_u16_bytes|scr_str_cmp)\b/g)?.length ?? 0;
+
+test("a three-way string comparator compares its pair once", async () => {
+  const once = compareCalls(await emittedAsm("once", program("a < b ? -1 : 1")));
+  const both = compareCalls(await emittedAsm("both", program("a < b ? -1 : a > b ? 1 : 0")));
+  expect(once).toBeGreaterThan(0);
+  expect(both).toBe(once);
+});