[llvm-dev] GEP with a null pointer base

[Peter Lawrence via llvm-dev]

> On Jul 20, 2017, at 11:22 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Wed, Jul 19, 2017 at 10:17 AM Peter Lawrence via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Chandler,
>                The only thing David made clear that wasn’t already clear
> is that he believes UB to be “comparatively rare”, which is in agreement
> with what Hal already said which is that he does not expect deleting
> UB will be of benefit to for example SPEC benchmarks.
> 
> Given that it is “comparatively rare”, why all the effort to delete it ? 
> And why make deleting it the default rather than warning about it ? 
> 
> There seems to be some confusion/misunderstanding here. My best understanding is that when David said this:
> 
> "The cases where the compiler can statically prove that undefined behaviour is present are comparatively rare."
> 
> What he was referring to/describing was a contrast with the optimizations described prior to that.
> 
> It's something like this:
> 
> UB-based optimizations don't prove UB is present - they optimize on the assumption that it is not present due to some unproven (by the compiler, but assumed to be known by the developer) invariants in the program.
> 
> Think about a simple case like array bounds - the compiler emits an unconditional load to the memory because it assumes the developer correctly validated the bounds or otherwise constructed so that out of bounds indexes never reach that piece of code. This is quite common - that UB is assumed to not happen, and the compiler optimizes on this fact.
> 
> What is less common, is for the compiler to be able to (in reasonable time) prove that UB /does/ happen (in many cases this would require complex interprocedural analysis - the array is defined in one function, maybe with a complex dynamic bound, then passed to another function and indexed using a non-trivial dynamic expression... statically proving that to be true or false is complex/expensive and so basically not done by any compiler - so any cases that are caught by the compiler are relatively trivial ("oh, you declared a const null pointer value, then dereferenced it within the same function", etc) & so don't happen very often (because they're also fairly obvious to developers too))
> 
> Does that help explain the difference/distinction being drawn here?



Dave,
          perhaps you missed these parts of the discussion

Here is the definition, acknowledged by Hal, of what we’re doing

> 1. Sometimes there are abstraction penalties in C++ code
> 2. That can be optimized away after template instantiation, function inlining, etc
> 3. When they for example exhibit this pattern
> 	if (A) {
> 		stuff;
> 	} else {
> 		other stuff including “undefined behavior”;
> 	}
> 4. Where the compiler assumes “undefined behavior” doesn’t actually happen because
>   In the C language standard it is the users responsibility to avoid it
> 5. Therefore in this example the compiler can a) delete the else-clause
>    b) delete the if-cond, c) assume A is true and propagate that information


And, here’s the optimization that according to Sean we’re using to delete UB

> [ … ]
> 
> In other words, if we can prove  "when program statement A executes then
> program statement B is guaranteed to execute and have undefined behavior"
> then we can assume that program statement A is never executed.
> 
> In particular, deleting program statement A is a correct transformation.
> Deleting program statement B itself is a special case of this (when A = B).
> 
> And yes, this does include everything up to and including `main`,
> intraprocedurally and interprocedurally.
> 
> 
> [ … ]
> 
> -- Sean Silva


This is entirely a separate issue from what Dan Gohman did to optimize sign extension
of i32 induction variables out of loops for LP64 target machines, where the optimization
is justified based on “the assumption that UB does not happen”, and no actual UB
exists either statically or dynamically.

But when it comes to actual provable UB the plan is to delete it.
On that there is no confusion, and there is no mis-understanding.


Peter Lawrence.




> 
> - Dave
> 
> Peter
> 
> 
>> On Jul 13, 2017, at 2:15 PM, Chandler Carruth <chandlerc at gmail.com <mailto:chandlerc at gmail.com>> wrote:
>> 
>> On Thu, Jul 13, 2017 at 5:13 PM Peter Lawrence via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> David,
>>           Here is the definition accepted by Hal of what we’re doing
>> 
>> > 1. Sometimes there are abstraction penalties in C++ code
>> > 2. That can be optimized away after template instantiation, function inlining, etc
>> > 3. When they for example exhibit this pattern
>> >       if (A) {
>> >               stuff;
>> >       } else {
>> >               other stuff including “undefined behavior”;
>> >       }
>> > 4. Where the compiler assumes “undefined behavior” doesn’t actually happen because
>> >    In the C language standard it is the users responsibility to avoid it
>> > 5. Therefore in this example the compiler can a) delete the else-clause
>> >     b) delete the if-cond, c) assume A is true and propagate that information
>> 
>> 
>> 
>> We are actively deleting undefined behavior, and the question is why
>> given that doing so potentially masks a real source code bug.
>> At the very least deleting undefined behavior should not be the default.
>> 
>> You are asserting this (again), but others have clearly stated that they disagree. David gave detailed and clear reasons why. Continuing to re-state positions is not productive.
>> 
>> -Chandler
> 
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