[Webkit-unassigned] [Bug 251261] New: Large allocations in a tight loop perform poorly when using the JavaScriptCore C and Objective-C APIs

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=251261

            Bug ID: 251261
           Summary: Large allocations in a tight loop perform poorly when
                    using the JavaScriptCore C and Objective-C APIs
           Product: WebKit
           Version: WebKit Local Build
          Hardware: Mac (Apple Silicon)
                OS: macOS 13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: kasperisager at hey.com

Consider the following C and Objective-C programs that perform 64 KiB allocations in a tight loop:

```
#include <JavaScriptCore/JavaScriptCore.h>

int
main () {
  JSContextRef context = JSGlobalContextCreate(NULL);

  JSStringRef source = JSStringCreateWithUTF8CString(
    "for (let i = 0; i < 1e6; i++) new Uint8Array(1024 * 64)"
  );

  JSEvaluateScript(context, source, NULL, NULL, 1, NULL);
}

```

```
#include <JavaScriptCore/JavaScriptCore.h>

int
main () {
  JSContext *context = [[JSContext alloc] init];

  [context evaluateScript:@"for (let i = 0; i < 1e6; i++) new Uint8Array(1024 * 64)"];
}
```

When running these on my machine, I'm observing an 8x slowdown compared to running the same allocation loop in either Safari or using the jsc helper from the builtin JavaScriptCore framework (/System/Library/Frameworks/JavaScriptCore.framework/Versions/Current/Helpers/jsc) and I cannot seem to figure out why. Both programs were compiled with clang using -O3.

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