[llvm-dev] Reducing the number of ptrtoint/inttoptrs that are generated by LLVM (original) (raw)

David Chisnall via llvm-dev llvm-dev at lists.llvm.org
Wed Jan 16 01:40:50 PST 2019

On 15/01/2019 16:26, Juneyoung Lee wrote:

If C programmer wants to get distance between two different objects, s/he can use (intptrt)p - (intptrt)q instead of p - q.

That's fine for new code, but unless you're volunteering to audit the (roughly) 8 billion lines of open source C code and a corresponding (probably larger) amount of proprietary code, then making this assumption is going to break things.

From the relatively small survey that we did of C idioms as part of the CHERI project, I can tell you that this is very widespread. This was something that we had to support (we considered only allowing pointer subtraction to yield a valid result when the operands referred to the same object and it broke pretty much everything we looked at).

David

P.S. If you want to talk about the standard with respect to intptr_t, you might want to reread what the standard actually says about it, specifically that a) it's optional, and b) if it does exist the only operation that is guaranteed to work is a cast from a pointer to an intptr_t and back again - all arithmetic on intptr_t is implementation defined.

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