Mark typed-array header loads invariant (#2016)

packages/compiler/src/backend/llvm/bytes-alias.ts

@@ -7,9 +7,15 @@ import type { LlvmEmitterContext } from "./expr-context.js";
  * buffer. Neither overlaps the other, a native field, array storage or a
  * module global's slot, so element stores leave header loads and global
  * receiver slots loop-invariant. Untagged accesses and calls still alias
- * every category. */
+ * every category.
+ *
+ * Header loads are also `!invariant.load`: no store or call between the
+ * construction of a view and its release changes them. Without it, a data
+ * pointer load ahead of a bounds branch is sunk into the in-bounds block,
+ * and LICM keeps a conditionally executed load inside the loop. */
 export function bytesHeaderAlias(host: LlvmEmitterContext): string {
-  return host.memoryAliasAttachment("bytes:header");
+  const tag = host.memoryAliasAttachment("bytes:header");
+  return tag === "" ? "" : `${tag}, !invariant.load !{}`;
 }
 
 export function bytesElementAlias(host: LlvmEmitterContext): string {

packages/compiler/src/backend/llvm/bytes-loop-versions.test.ts

@@ -249,6 +249,11 @@ test("emitted loops version the body and tag typed-array memory", () => {
   const elements = /store double [^\n]*!tbaa !(\d+)/.exec(body)![1];
   const header = /load ptr, ptr [^\n]*!tbaa !(\d+)/.exec(body)![1];
   expect(elements).not.toBe(header);
+  // Header loads are invariant for the view's lifetime; element accesses
+  // are not.
+  expect(body).toMatch(/load ptr, ptr [^\n]*, !tbaa !\d+, !invariant\.load !\{\}$/m);
+  expect(body).toMatch(/load i64, ptr [^\n]*, !tbaa !\d+, !invariant\.load !\{\}$/m);
+  expect(body).not.toMatch(/(load double|store double)[^\n]*invariant\.load/);
 });
 
 test("loops to a typed array's length version the other typed arrays they index", () => {

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

@@ -64,6 +64,7 @@ import {
   emitPairedLoopSafety,
 } from "./counted-loops.js";
 import { emitBytesLoopVersionGuard, type BytesLoopVersion } from "./bytes-loop-versions.js";
+import { bytesHeaderAlias } from "./bytes-alias.js";
 import { mangleLocal } from "../mangle.js";
 import { LlvmUnsupportedError } from "./unsupported.js";
 import type { LlValue } from "./expr-context.js";
@@ -1229,10 +1230,7 @@ export function emitStmtBody(emitter: LlEmitter, s: IrStmt): void {
             }
             B.line(`${inBounds} = icmp ult i32 ${narrow}, ${len}`);
           } else {
-            emitter.markMemoryPointer(lenPtr, "bytes:header");
-            B.line(
-              `${len} = load ${emitter.sizeType}, ptr ${lenPtr}${emitter.fieldAliasAttachment(lenPtr)}`,
-            );
+            B.line(`${len} = load ${emitter.sizeType}, ptr ${lenPtr}${bytesHeaderAlias(emitter)}`);
             B.line(`${inBounds} = icmp ult ${emitter.sizeType} ${index}, ${len}`);
           }
           B.condBr(inBounds, lb, exit);