[llvm-dev] The undef story

[Peter Lawrence via llvm-dev]

> On Jun 29, 2017, at 5:48 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> 
> On 06/29/2017 07:26 PM, Peter Lawrence wrote:
>> Hal,
>>       Mehdi points out I mis-quoted you, I apologize sincerely.
>> 
>> Mehdi,
>>            Thank you for forcing me to go back and re-read what Hal wrote,
>> I could have sworn Hal and I were in agreement at the time I wrote you,
>> Must have been asleep at the wheel, not enough sleep last night
>> However my request for a more concrete example stands
>> 
>> Here’s what I said
>>> 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”.
>> 
>> Here’s what Hal said in response
>> > 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.
>> 
>> 
>> The problem is I don’t know how to interpret what Hal said here,
>> I can’t construct the “real” example from the “representative” example
>> given his advise.
> 
> Hopefully, the description I later provided helps. I can't share the "real" code, but I reconstructed this example from my memory of the original code plus some of the intermediate IR I'd looked at.
> 

Hal,
      I’d like to see if I understand you, here’s what I think, let me know if this is correct

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


Peter Lawrence.








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