[llvm-dev] Hoisting instructions in presence of Undefined Behaviour

[Philip Reames via llvm-dev]

Nuno,

Correct me if I'm wrong, but I think there's an important subtlety 
here.  Don't we have to commit to a single interpretation of the 
freeze(undef) for the transformation to be legal?  Anna's framing of the 
question was in terms of hoisting, your proven examples seem to be in 
terms of dead code elimination.  I think that's an important distinction 
here isn't it?

To highlight the difference, considering the following:

c = freeze(undef)
if (!c)
   return
if (c)
   return
UB

In this example, the UB is dead because 'c' must be either true or 
false.  It would not be legal to hoist UB over the preceding "if (c)" 
branch *unless* we commit to having the "if (!c)" branch be taken.

Philip

On 11/30/20 3:34 PM, Nuno Lopes via llvm-dev wrote:
> Both transformations are correct, yes. See here:
> https://alive2.llvm.org/ce/z/EpqCUT
> https://alive2.llvm.org/ce/z/yyj9TQ
>
> For a fixed input, if the source triggers UB for *at least one* set of 
> chosen non-deterministic values (e.g., undef, freeze), then the source 
> is declared UB for that input. So you can optimize it away to UB.
>
> Nuno
>
> -----Original Message----- From: Anna Thomas
> Sent: Monday, November 30, 2020 10:45 PM
> Subject: [llvm-dev] Hoisting instructions in presence of Undefined 
> Behaviour
>
> We’d like to clarify whether the following transform is valid. Given 
> the code:
> ```
> if (freeze(undef))
>   return
> UB
> ```
>
> Can we hoist the UB above the `if` block:
> ```
> UB
> if (freeze(undef))
>  return
> ```
>
> The reasoning is that:
> 1. We were already having undefined behaviour in the code initially. 
> `if freeze(undef)` evaluates to true or false. So, a valid execution 
> of the program will fall through the `if` block and execute the UB.
> 2. Given #1, hoisting a UB to above the `if` block is valid.
>
>
> Taking this one step further, if the program was:
> ```
> if (freeze(undef))
>  return
> load
> ```
> Can we hoist the load over the if-block? I think we can.
>
> The `if freeze(undef)` being taken or not is independent of any other 
> program variables and the compiler is free to refine the code into one 
> where the if block is not taken.
> So, although the load is not guaranteedToExecute, we know that the 
> execution of the load is not actually control dependent on the branch.
>
> Anything incorrect with the above transforms?
>
>
> Thanks,
> Anna
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