[llvm-dev] GEP with a null pointer base
Sean Silva via llvm-dev
llvm-dev at lists.llvm.org
Tue Jul 25 00:12:35 PDT 2017
On Fri, Jul 21, 2017 at 6:29 PM, Peter Lawrence <peterl95124 at sbcglobal.net>
wrote:
> Sean,
>
> Dan Gohman’s “transform” changes a loop induction variable, but does not
> change the CFG,
>
> Hal’s “transform” deletes blocks out of the CFG, fundamentally altering it.
>
> These are two totally different transforms.
>
> And even the analysis is different,
>
> The first is based on an *assumption* of non-UB (actually there is no
> analysis to perform)
>
> the second Is based on a *proof* of existence of UB (here typically some
> non-trivial analysis is required)
>
> These have, practically speaking, nothing in common.
>
Ah, I think I see your point of confusion now. They are actually the same
thing, but one has a hidden step. You can consider the induction variable
transformation to be something like this:
```
a = add nsw i32 b, c
```
1. ==> widen `a` and expand the add into:
```
if ("add i32 b, c" does not overflow) {
a = add nsw i64 b, c
} else {
a = (cast to i64) add nsw i32 b, c
}
```
2. ==> if the `else` branch executes, then we provably have UB.
```
a = add nsw i64 b, c
```
The last step is the same as in Hal's example where we delete a part of the
CFG by proving that if we reach it there is guaranteed to be UB. In Hal's
example, the frontend marked the shift as having UB if the shift amount
exceeds the bitwidth. In this case, the frontend marked it as
non-overflowing. In this case, the fact that it is guaranteed to overflow
in the `else` branch has to be proven by looking at a dominating
conditional check instead of being manifest from a peephole observation
(after inlining/constant folding) as it is in Hal's example.
Hopefully that makes sense. If not, could you indicate specifically which
of transformations 1. and/or 2. you think is illegal and why?
-- Sean Silva
>
>
> Peter Lawrence.
>
>
>
> On Jul 21, 2017, at 5:00 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>
>
> On Jul 21, 2017 3:24 PM, "Peter Lawrence" <peterl95124 at sbcglobal.net>
> wrote:
>
>
> 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> 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.
>
>
> Sorry, the way I phrased this in terms of program statements may have made
> this unclear, but this is precisely the particular case A=B that I
> mentioned. In this case, A=B="the induction variable increment" and we use
> that to deduce that the statement will not execute and overflow, which is
> what justifies the widening.
>
> Notice that I mentioned that deleting code is only a particular case. In
> the general case we deduce that dynamically something simply does not
> happen, which is what we do in order to prove that induction variable
> widening is safe (overflow cannot happen).
>
> There is nothing special about induction variable widening with respect to
> UB. It is justified by applying the same principles as all other UB-related
> transformations.
>
>
> Briefly, there is only one axiom in the compiler writer's toolbox w.r.t.
> UB and that is "the input program does not execute UB". Everything else is
> derived from that by pure logical reasoning. Does that make sense? Can you
> see how all the descriptions I gave above are derivable from that axiom?
>
> The common application of this is that we can assume any program property
> P whatsoever (not just liveness, but variable ranges, etc.) if we can prove
> that the program would execute UB should that property P fail to hold.
>
>
> -- Sean Silva
>
>
> 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>
>> wrote:
>>
>> On Thu, Jul 13, 2017 at 5:13 PM Peter Lawrence via llvm-dev <
>> 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
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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