[llvm-dev] The undef story

Peter Lawrence via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 29 08:41:59 PDT 2017

> On Jun 29, 2017, at 4:39 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> On 06/28/2017 05:33 PM, Peter Lawrence wrote:
>> Chandler,
>>                where we disagree is in whether the current project is moving the issue
>> forward.  It is not.  It is making the compiler more complex for no additional value.
>> The current project is not based in evidence, I have asked for any SPEC benchmark
>> that shows performance gain by the compiler taking advantage of “undefined behavior”
>> and no one can show that.
> I can't comment on SPEC, but this does remind me of code I was working on recently. To abstract the relevant parts, it looked something like this:
> template <typename T>
> int do_something(T mask, bool cond) {
>   if (mask & 2)
>     return 1;
>   if (cond) {
>     T high_mask = mask >> 48;
>     if (high_mask > 5)
>       do_something_1(high_mask);
>     else if (high_mask > 3)
>       do_something_2();
>   }
>   return 0;
> }
> This function ended up being instantiated on different types T (e.g. unsigned char, unsigned int, unsigned long, etc.) and, dynamically, cond was always false when T was char. The question is: Can the compiler eliminate all of the code predicated on cond for the smaller types? In this case, this code was hot, and moreover, performance depended on the fact that, for T = unsigned char, the function was inlined and the branch on cond was eliminated. In the relevant translation unit, however, the compiler would never see how cond was set.
> Luckily, we do the right thing here currently. In the case where T = unsigned char, we end up folding both of the high_mask tests as though they were false. That entire part of the code is eliminated, the function is inlined, and everyone is happy.
> Why was I looking at this? As it turns out, if the 'else if' in this example is just 'else', we don't actually eliminate both sides of the branch. The same is true for many other variants of the conditionals (i.e. we don't recognize all of the code as dead).

I apologize in advance if I have missed something here and am misreading your example...

This doesn’t make sense to me, a shift amount of 48 is “undefined” for unsigned char,
How do we know this isn’t a source code bug,
What makes us think the the user intended the result to be “0”.

This strikes me as odd, we are mis-interpreting the user’s code 
In such a way so as to improve performance, but that isn’t necessarily what the user intended.

Here’s one way to look at this issue, if something is “C undefined behavior” then
The standard says, among other things, that we could trap here
Why aren’t we doing that rather than optimizing it ?

Here’s another way to look at it, no one has ever filed a bug that reads
“I used undefined behavior in my program, but the optimizer isn’t taking advantage of it”
But if they do I think the response should be 
“you should not expect that, standard says nothing positive about what undefined behavior does"

> Once we have a self-consistent model for undef, we should be able to fix that. The user was confused, however, why seemingly innocuous changes to the code changed the performance characteristics of their application. The proposed semantics by John, et al. should fix this uniformly.
> In any case, to your point about:
>>   if (a == a)
>>     S;
> I have the same thought. If a == undef here, the code should be dead. Dead code must be aggressively dropped to enable inlining and further optimization. This is an important way we eliminate abstraction penalties. Dead code also has costs in terms of register allocation, speculative execution, inlining, etc.

And yet  IIRC Sanjoy in his last email was arguing for consistent behavior in cases like
If (x != 0) {
	/* we can optimize in the then-clause assuming x != 0 */
And in the case above when it is a function that gets inlined

Here’s what Sanjoy said about the function-inline case

> This too is fixed in the semantics mentioned in the paper.  This also
> isn't new to us, it is covered in section 3.1 "Duplicate SSA Uses".

So this issue seems to be up in the air

> I've also seen cases where templated types are used with fixed-sized arrays where the compiler to leveraged knowledge of UB on uninitialized values and out-of-bounds accesses to eliminate unnecessary part of the code. In short, "optimizing on undefined behavior" can end up being an important tool.

As you can tell from my first comments, I am not yet convinced, and would still like to see real evidence

Peter Lawrence.

>  -Hal
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory

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