[llvm-dev] undef * 0

[Manuel Jacob via llvm-dev]

I think you're looking at "undef" the wrong way.  "Undef" is not a 
value, "undef" is a placeholder where the compiler can choose an 
arbitrary value at every occurence of "undef".

You could choose different values for "undef" in your example, so the 
law of distribution holds:

undef * (1 - 1) ~> undef * 1 + undef * (-1) ~> 0 * 1 + 0 * (-1) ~> 0 + 0 
= 0

or even

undef * (1 - 1) ~> undef * 1 + undef * (-1) ~> undef + undef ~> undef 
(same as yours so far) ~> 0

Here's a blog post which describes "undef" in more detail, also 
describing a case where it inhibits optimization: 
http://www.playingwithpointers.com/problem-with-undef.html

On 2016-09-12 09:36, Soham Chakraborty via llvm-dev wrote:
> Hi,
> 
> Back to the original question - undef * 0 = 0 may have some issues.
> 
> Consider the evaluation of the expression undef * (1 - 1)
> 
> (1) undef * (1 - 1) ~> undef * 0 ~> 0 (as suggested)
> 
> (2) undef * (1 - 1) ~> undef * 1 + undef * (-1) ~> undef + undef ~> 
> undef
> and now the evaluation of undef can be some other value than 0.
> 
> Thus it does not follow the law of distribution of multiplication over
> addition property and the algebraic transformations are ambiguous.
> 
> Similarly associativity of boolean algebra is also violated as follows:
> 
> undef & a & !a
> (1) ~> (undef & a) & !a ~> undef & !a ~> undef
> (2) ~> undef (a & !a) ~> undef & 0 ~> 0
> 
> Is there any clear set of rules how LLVM handles such cases?
> 
> Best Regards,
> soham
> 
> 
>> I don't know of a way to do it from the command-line, but if you're
>> willing
>> to change the IR, you can add the optsize (for -Os) or minsize (for 
>> -Oz)
>> IR
>> attribute to the function you're compiling.
>> 
>> On Fri, Sep 2, 2016 at 5:59 AM, Bruce Hoult via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>> 
>>> Idle question, if anyone is reading still ... how do you get llc to 
>>> do
>>> -Os
>>> or -Oz? The docs say the argument must be an integer, and anything 
>>> other
>>> than 0..3 is rejected. (in fact .. bug report ... 10 thru 39 are also
>>> silently accepted as are 100 thru 399 etc)
>>> 
>>> 
>> 
> 
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