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Last active April 13, 2016 22:56
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implementation sketch for faster integer overflow checking in clang/llvm
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overflow.md

I want to speed up integer overflow checking for hardening purposes by keeping a sticky overflow flag and only trapping when necessary. I want to keep it super simple while hopefully giving the optimizers room to do their thing.

In the codegen part of clang:

  • each function gets an i1 for storing overflow information, initialized to 0
  • each integer overflow check ORs its result into the overflow flag
  • before each function call, return instruction, or other side-effecting operation, execude ud2 if overflow is set

Reasonable?

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regehr commented Mar 25, 2016

Ok, I'm convinced, thanks guys, I'll start reading the passes relevant for check removal.

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