Michael Bartnett michaelbartnett
michaelbartnett
/ gist:29de734a08ea9f0d4be3327664efd806
Created
May 8, 2016 21:25
— forked from rygorous/gist:e0f055bfb74e3d5f0af20690759de5a7
A bit of background on compilers exploiting signed overflow
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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. |