Marcel Weiher mpw

Twitter thread: https://twitter.com/theshawwn/status/1456925974919004165
Hacker News thread: https://news.ycombinator.com/item?id=29128998

November 6, 2021

How does JAX allocate memory on a TPU?

jnp.device_put(1) is deceptively simple to write in JAX. But on a TPU, what actually happens? How does a tensor containing the value 1 actually get onto a TPU?

Turns out, the answer is "C++", and a lot of it.

Diff output of two lldb commands

Setup

Copy the contents of the last snippet (lldbinit) from the gist page, and paste into your .lldbinit file. This makes the ksdiff macro known inside lldb.
Put file ksdiff.py in ~/.lldb/
sudo pip install temp
Restart Xcode debug session

Example

(lldb) ksdiff ;

libdispatch efficiency tips

The libdispatch is one of the most misused API due to the way it was presented to us when it was introduced and for many years after that, and due to the confusing documentation and API. This page is a compilation of important things to know if you're going to use this library. Many references are available at the end of this document pointing to comments from Apple's very own libdispatch maintainer (Pierre Habouzit).

My take-aways are:

You should create very few, long-lived, well-defined queues. These queues should be seen as execution contexts in your program (gui, background work, ...) that benefit from executing in parallel. An important thing to note is that if these queues are all active at once, you will get as many threads running. In most apps, you probably do not need to create more than 3 or 4 queues.
Go serial first, and as you find performance bottle necks, measure why, and if concurrency helps, apply with care, always validating under system pressure. Reuse

DEPRECATED for Xcode 10 🎈

(check out What's New in Swift at 11:40, slide 42)

Whole Module Compilation Optimizations: Why these terms are sometimes misleading

When you look up how to compile swift faster for debug builds, people very earnestly give advice that seems contradictory: you should "try using the whole module optimization flag," and also "never use whole module optimization for debugging". [^1]

This is confusing because some of us are using these two general words:

compilation: "turning text into an executable program"

	//
	// FreeSpaceViewController.swift
	// Free Space
	//
	// Created by Kyle Howells on 04/05/2022.
	//

	import UIKit

	class FreeSpaceViewController: UIViewController {

	// Created by Marcin Krzyzanowski

	import Foundation

	public protocol JSONEncodable: Encodable { }

	public extension JSONEncodable {
	func toJSON(using encoder: @autoclosure () -> JSONEncoder = JSONEncoder()) throws -> String {
	try String(decoding: encoder().encode(self), as: UTF8.self)
	}

	frame {
	items: [int] # implicit input
	value: int # implicit input
	min_index: int
	max_index: int
	mid_index: int
	<- found_index: int # explicit output
	}

	# sel is selector

	/**
	HACK around FB6940492: Mac Catalyst: Crash when presenting view controllers

	'NSRangeException', reason: 'Cannot remove an observer <UINSSceneViewController 0x600003558790> for the key path "view.window.screen.contentLayoutRect" from <UINSSceneViewController 0x600003558790> because it is not registered as an observer.'

	0 CoreFoundation 0x00007fff3b20f183 __exceptionPreprocess + 250
	1 libobjc.A.dylib 0x00007fff71719b64 objc_exception_throw + 48
	2 Foundation 0x00007fff3d8ca2aa -[NSObject(NSKeyValueObserverRegistration) _removeObserver:forProperty:] + 578
	3 Foundation 0x00007fff3d8ca01b -[NSObject(NSKeyValueObserverRegistration) removeObserver:forKeyPath:] + 74
	4 Foundation 0x00007fff3d8e2898 -[NSObject(NSKeyValueObserverRegistration) removeObserver:forKeyPath:context:] + 190

	//
	// AppDelegate.swift
	// SwiftUITestApp
	//
	// Created by Matt Gallagher on 4/6/24.
	// Copyright © 2019 Matt Gallagher. All rights reserved.
	//

	import Cocoa
	import SwiftUI

	static void PSPDFFixiOS12AccessibilityTestCrash(void) {
	let accessibilityLoaderClass = NSClassFromString(@"UIAccessibilityInformationLoader");
	let accessibilitySEL = NSSelectorFromString(@"_loadAccessibilityInformationOnMainThread:");
	__block IMP originalIMP = pspdf_swizzleSelectorWithBlock(accessibilityLoaderClass, accessibilitySEL, ^(id _self, BOOL onMainThread) {
	@try {
	((void (*)(id, SEL, BOOL))originalIMP)(_self, accessibilitySEL, onMainThread);
	} @catch (NSException *exception) {
	NSLog(@"Exception received: %@", exception);
	if ([exception.name isEqualToString:NSInvalidArgumentException] && [exception.reason containsString:@"_accessibilityLoadAccessibilityInformation"]) {
	NSLog(@"Ignoring IOS 12b5 weirdness...");