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by moloch-hai 1302 days ago
To omit the check, the compiler would need to know that the loop range matches or subtends the array bound. That is commonly easy for built-in arrays, uncommonly for user-defined types. Most types are user-defined.

We trust the library author to get it right, despite (in Rust) wrapping accesses in "unsafe" or (in C++) not. Compilers are not particularly better at everything than library authors.
1 comments
titzer 1301 days ago
> would need to know that the loop range matches or subtends the array bound

Some compilers have pretty sophisticated analyses aimed at just that: determining affine relations to statically bound indexed accesses. Failing that, some compilers will resort to loop versioning, generating two versions of the loop and then partitioning the iteration space into the definitely-in-bounds range from possibly-out-of-bounds range, then selecting which portions of which loop to execute by prefixing both with dynamic checks. Couple all of that with peeling and unrolling, and bounds checks start disappearing and getting amortized away.

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moloch-hai 1301 days ago
Libraries can do this too, in many cases more reliably.
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titzer 1301 days ago
Unless libraries are receiving a copy of the meta representation of the program and running integer equality relations over the dataflow chains, then no, not really.
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moloch-hai 1301 days ago
Yet, really.

The library author has certain knowledge of what the library is meant to achieve, where the compiler is obliged to guess according to whatever tea leaves it can find to descry.

In particular, the library author knows that the container won't be changing size over the duration of the loop, something the compiler would have difficulty proving.

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MaxBarraclough 1301 days ago
> The library author has certain knowledge of what the library is meant to achieve

What's special about libraries? Every programmer has such knowledge, and every programmer writes buggy code.

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moloch-hai 1301 days ago
What's special about compilers, then? Compilers are code, and therefore, as you say, buggy.

Library authors know things about what their code is meant to be doing that compilers cannot deduce, so cannot act on. But the library author can. A library, according to how heavily it is used, benefits from more thorough testing than generic application code gets.

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