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

Juneyoung Lee via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 15 08:26:58 PST 2019

As with many things in the C and C++ specifications, this has very little relationship to real-world code. The biggest example I've seen that violates this rule is the way that the FreeBSD kernel implements per-CPU storage, but in a fairly ad-hoc analysis of existing C/C++ code we found quite a few cases where subtraction occurred between objects.

Hello David, If C programmer wants to get distance between two different objects, s/he can use (intptr_t)p - (intptr_t)q instead of p - q. I believe this situation is similar to one adopting optimizations/analyses that exploit signed overflow / TBAA / etc.

Juneyoung Lee

On Tue, Jan 15, 2019 at 11:59 AM David Chisnall via llvm-dev < llvm-dev at lists.llvm.org> wrote:

On 14/01/2019 20:55, Juneyoung Lee via llvm-dev wrote: > Correctness of psub is guaranteed by the specification of pointer > subtraction of C/C++. > When two pointers are subtracted, both shall point to elements of the > same array object, or one past the last element of the array object > (6.5.6.9).

As with many things in the C and C++ specifications, this has very little relationship to real-world code. The biggest example I've seen that violates this rule is the way that the FreeBSD kernel implements per-CPU storage, but in a fairly ad-hoc analysis of existing C/C++ code we found quite a few cases where subtraction occurred between objects. David

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