Skip the arena clear for record literals that write every field (#2470)

packages/compiler/src/backend/llvm/arenas.test.ts

@@ -67,6 +67,38 @@ console.log(total);
     expect(body).not.toContain("ScrArena.closed");
     expect(body).not.toContain("@scr_arena_disposal");
   }
+  // A record literal stores every field, and no dispose entry reads them:
+  // the record's constructor does not clear the object. A class constructor
+  // may leave fields unset, so the class's does.
+  const record = constructors.find((body) => body.includes("@sc_rnew_"))!;
+  const cls = constructors.find((body) => !body.includes("@sc_rnew_"))!;
+  expect(record).not.toContain("@llvm.memset");
+  expect(cls).toContain("@llvm.memset");
+});
+
+test("record literals in an arena that is not closed keep the clear their dispose entries need", async () => {
+  // The items hold outside strings, so their groups are not closed.
+  const llvm = await llvmFor(`
+interface Item {
+  label: string;
+  next: Item | null;
+}
+function chain(labels: string[], n: number): Item {
+  return n === 0 ? { label: labels[0]!, next: null } : { label: labels[n % labels.length]!, next: chain(labels, n - 1) };
+}
+function length(item: Item): number {
+  return item.next === null ? item.label.length : item.label.length + length(item.next);
+}
+const labels = ["a", "bb", "ccc"];
+let total = 0;
+for (let round = 0; round < 4; round++) total += length(chain(labels, 10 + round));
+console.log(total);
+`);
+  expect(llvm).toMatch(/@scr_arena_open\(ptr %[\w.]+, i32 0\)/);
+  const items = arenaConstructors(llvm).filter((body) => body.includes("@sc_rnew_"));
+  expect(items.length).toBe(1);
+  expect(items[0]).toContain(".dispose");
+  expect(items[0]).toContain("@llvm.memset");
 });
 
 test("an arena that is not closed keeps the dispose list in every arena-aware constructor", async () => {

packages/compiler/src/backend/llvm/arenas.ts

@@ -275,6 +275,12 @@ export function arenaCurrentAddress(host: ArenaHost, dest: string): string[] | n
  * the arena is full. The bump, the clear, the header and the dispose entry
  * are inline (scr_arena.c's ScrArena ABI); scr_arena_obj takes a new chunk
  * and scr_arena_disposal grows the list.
+ *
+ * `clear` is what the bump path zeroes: the whole object, or only the
+ * collector header, for callers that store every word of the object before
+ * anything reads it (a record literal of a shape with no dispose entries:
+ * `init` stores the count and the shape number, and the literal every
+ * field). scr_arena_obj and the heap constructor return zeroed objects.
  */
 export function arenaNewLines(
   host: ArenaHost,
@@ -284,6 +290,7 @@ export function arenaNewLines(
   dispose: string | null,
   init: string[],
   result: string,
+  clear: "object" | "header" = "object",
 ): string[] {
   const S = host.sizeType;
   // ScrArena's fields and dispose entries are pointer-sized (scr_arena.c).
@@ -328,8 +335,13 @@ export function arenaNewLines(
     `carve:`,
     `  %end = getelementptr inbounds i8, ptr %bp, ${S} %n`,
     `  store ptr %end, ptr %a`,
-    `  call void @llvm.memset.p0.${S}(ptr align 8 %bp, i8 0, ${S} %n, i1 false)`,
   );
+  if (clear === "object")
+    lines.push(`  call void @llvm.memset.p0.${S}(ptr align 8 %bp, i8 0, ${S} %n, i1 false)`);
+  else if (cyc !== null)
+    lines.push(
+      `  call void @llvm.memset.p0.${S}(ptr align 8 %bp, i8 0, ${S} ${CYC_HDR}, i1 false)`,
+    );
   if (cyc !== null)
     lines.push(
       `  %id = trunc i32 %tid to i16`,

packages/compiler/src/backend/llvm/shapes.ts

@@ -2387,8 +2387,20 @@ export function emitRecordShapes(
           `  %shp = getelementptr inbounds %${struct}, ptr %o, i64 0, i32 ${SHAPE_WORD_INDEX}`,
           `  store i32 ${shapeNumber}, ptr %shp ; shape number`,
         );
+      // A record literal covers every field of its shape (ir.ts recordLit),
+      // so without dispose entries, which read the fields at the drop, no
+      // field is read before the literal stores it.
       defs.push(
-        ...arenaNewLines(host, newSym, sizeOf, traced ? cycType : null, dispose, init, "%o"),
+        ...arenaNewLines(
+          host,
+          newSym,
+          sizeOf,
+          traced ? cycType : null,
+          dispose,
+          init,
+          "%o",
+          dispose === null ? "header" : "object",
+        ),
       );
       if (dispose !== null)
         defs.push(