[llvm-dev] The undef story

Peter Lawrence via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 29 10:43:01 PDT 2017

> On Jun 29, 2017, at 9:32 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> On 06/29/2017 10:41 AM, Peter Lawrence wrote:
>>> On Jun 29, 2017, at 4:39 AM, Hal Finkel <hfinkel at anl.gov <mailto: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”.
> As I said, this is representation of what the real code did, and looked like, after other inlining had taken place, etc. In the original form, the user's intent was clear. That code is never executed when T is a small integer type.

I will still have a hard time believing this until I see a real example, can you fill in the details ?

Peter Lawrence.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170629/018df4c5/attachment.html>

More information about the llvm-dev mailing list