[llvm-dev] -speculative-execution moving load before store

[Mikael Holmén via llvm-dev]

Hi,

A few days ago I stumbled upon a problem where SpeculativeExecution
changed the order of a load and a store to the same address.

I wrote

  https://bugs.llvm.org//show_bug.cgi?id=32964

about it but no response there so far.

In the input we have

   store i8 0, i8* @i
   %.pre = load i8, i8* @i

and then in the output the load is moved so it's before the store which 
clearly makes it load the wrong value.

isSafeToSpeculativelyExecute happily says "true" for the load, and there 
is a comment on isSafeToSpeculativelyExecute saying

   /// This method can return true for instructions that read memory;
   /// for such instructions, moving them may change the resulting value.

And that is indeed the case in PR32964.

How is this supposed to work?

I started seeing this after the commit

  SpeculativeExecution: Stop using whitelist for costs

Before that, e.g. a load got the cost UINT_MAX which I suppose disabled 
hoisting, but since the commit TTI.getUserCost(I) is used for all 
instructions.

Regards,
Mikael