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Why do compilers even bother with exploiting undefinedness signed overflow? And what are those
mysterious cases where it helps?

A lot of people (myself included) are against transforms that aggressively exploit undefined behavior, but
I think it's useful to know what compiler writers are accomplishing by this.

TL;DR: C doesn't work very well if int!=register width, but (for backwards compat) int is 32-bit on all
major 64-bit targets, and this causes quite hairy problems for code generation and optimization in some
fairly common cases. The signed overflow UB exploitation is an attempt to work around this.

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Registers
    Caller-saved                                Callee-saved
    RAX RCX RSP RDI RSI RDX R8 R9 R10 R11       RBP RBX R12 R13 R14 R15

    Args: RDI, RSI, RDX, RCX, R8, R9, XMM0–7
    Return: RAX

Simple Compile
    yasm -f macho64 foo.asm && gcc foo.c foo.o -Wall -Wextra -g -O1

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Using <details> in GitHub

Suppose you're opening an issue and there's a lot noisey logs that may be useful.

Rather than wrecking readability, wrap it in a <details> tag!

<details>
 <summary>Summary Goes Here</summary>

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Achieving warp speed with Rust

Contents:

• Number one optimization tip: don't
• Never optimize blindly
• Don't bother optimizing one-time costs
• Improve your algorithms
• CPU architecture primer
• Keep as much as possible in cache
• Keep as much as possible in registers