[LLVMdev] C as used/implemented in practice: analysis of responses (original) (raw)

Renato Golin renato.golin at linaro.org
Wed Jul 1 05:29:29 PDT 2015

On 1 July 2015 at 13:10, Peter Sewell <Peter.Sewell at cl.cam.ac.uk> wrote:

while attractive from the compiler-writer point of view, is just not realistic, given the enormous body of C code out there which does depend on some particular properties which are not guaranteed by the ISO standard.

There is also an enormous body of code that is just wrong. Do we have to worry about getting that right, too? Trying to "understand" the authors' intentions and do that instead of what they asked?

Where do we draw the line? What do we consider "a reasonable deviation" from just "plain wrong"?

There is a large portion of non-standard documented behaviours in all compilers, and GCC and Clang are particularly important here. Most builtin functions, attributes, and extensions are supported by both compilers in a similar way, and people can somewhat rely on it. But the only true reliable sources are the standards.

However, the very definition of undefined behaviour is "here be dragons", and that's something that was purposely done to aid compilers at optimising code. You may try to unite the open source compilers in many ways (as I tried last year), but trying to regulate undefined behaviour is not one of them.

That code is not necessarily all gospel, of course, far from it - but its existence does have to be taken seriously.

And we do! Though, in a completely different direction than you would expect. :)

You advocate for better consistent support, which is ok and I, for one, have gone down that path multiple times. But in this specific case, the way we take it seriously is by warning the users of the potential peril AND abuse of it for performance reasons. This is a sweet spot because novice users will learn the language and advanced users will like the performance.

cheers, --renato

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