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@shafik
shafik / WhatIsStrictAliasingAndWhyDoWeCare.md
Last active June 17, 2024 09:54
What is Strict Aliasing and Why do we Care?

What is the Strict Aliasing Rule and Why do we care?

(OR Type Punning, Undefined Behavior and Alignment, Oh My!)

What is strict aliasing? First we will describe what is aliasing and then we can learn what being strict about it means.

In C and C++ aliasing has to do with what expression types we are allowed to access stored values through. In both C and C++ the standard specifies which expression types are allowed to alias which types. The compiler and optimizer are allowed to assume we follow the aliasing rules strictly, hence the term strict aliasing rule. If we attempt to access a value using a type not allowed it is classified as undefined behavior(UB). Once we have undefined behavior all bets are off, the results of our program are no longer reliable.

Unfortunately with strict aliasing violations, we will often obtain the results we expect, leaving the possibility the a future version of a compiler with a new optimization will break code we th

@djg
djg / reading-list.md
Last active February 19, 2024 18:09
Fabian's Recommened Reading List
@rygorous
rygorous / gist:e0f055bfb74e3d5f0af20690759de5a7
Created May 8, 2016 06:54
A bit of background on compilers exploiting signed overflow
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.