[llvm-dev] Dereferenceable load semantics & LICM (original) (raw)

Piotr Padlewski via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 6 09:28:37 PDT 2017

2017-04-06 17:57 GMT+02:00 Sanjoy Das <sanjoy at playingwithpointers.com>:

Hi Piotr,

On April 6, 2017 at 2:36:53 AM, Piotr Padlewski (piotr.padlewski at gmail.com) wrote: > I disagree, I find it different than the patch you mentioned. We don't have > any problems with code like this: > > ptr = load i8*, i8** %ptrptr, !dereferenceable !{i64 8} > if (false) { > > // ptr is not actually dereferenceable, even the load above has UB > // (since the metadata is "wrong"), but it is never executed so all is > well. > int val = *ptr; > } I was not talking about code like that. The problem is code like this: if (false) { ptr = load i8*, i8** %ptrptr, !dereferenceable !{i64 8, !"GlobalProperty} // ptr is not actually dereferenceable, even the load above has UB // (since the metadata is "wrong"), but it is never executed so all is well. int val = *ptr; } I did not mention this earlier, but I've assumed that %ptrptr itself is dereferenceable, which means you can hoist the load of ptr. Since because of !"GlobalProperty" you don't strip the !dereferenceable, you'll also be able to hoist the load of val, which would segfault because ptr was not dereferenceable. That is, with the !"GlobalProperty" bit in the picture, it is possible to make "if (false) { X }" introduce UB in a program for certain values of X. Hi Sanjoy, My point is that this it is exactly the same way as normal !dereferenceable introduces UB.

ptr = load i8*, i8** %ptrptr, !dereferenceable !{i64 8} if (false) { int val = *ptr; }

If frontend says that something is dereferenceable, which is not actually dereferenceable, then it is UB and everything can happen - like the execution of dead instruction. This is exactly the same with the global properties - we are giving a guarantee that pointer it is dereferenceable even if we would hoist or sink it, and if it is not true then it is UB.

I don't see why UB with normal dereferenceable is acceptable, but having this property globally is not. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170406/dad3e9a9/attachment.html>

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