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

Sanjoy Das via llvm-dev llvm-dev at lists.llvm.org
Fri Apr 7 10:25:22 PDT 2017

Hi Piotr,

On April 7, 2017 at 8:02:01 AM, Piotr Padlewski (piotr.padlewski at gmail.com) wrote:

But we already have that behavior with dereferenceable variables.

void foo(i8* dereferenceable(8) %ptr) { if (false) { ; what if %x is not actually dereferenceable? %x = load i8, i8* %ptr } }

For that example ^, we'd say that calling foo with a non-dereferenceable pointer will immediately cause UB.

-- Sanjoy

Now we can hoist the load, even that it would be never executed. This is the same with global properties - it gives us guarantee that property holds non locally and if it doesn't then it is UB and things like this might happen.

I understand your concerns about the speculate function attribute, the examples that you showed in the review are pretty disturbing, but loads and stores seems to be much safere in that maner. Maybe you know some examples like this, but with load and stores? It just seems weird to me that we already depend on very similar mechanics, but this one seems like a bad idea. Best Piotr

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