[LLVMdev] [Propose] Add address-taken bit to GlobalVariable for disambiguation purpose

[Shuxin Yang]

Nadav:

    I don't think this is right approach for engineering.
The time-complexity of re-analyzing addr_taken for each single alias 
query depends on
    1. how many global variable
    2. how many occurrence of these global variables.
    3. how many queries the compiler have.

   3) depends on compiler.  You never know what we will have in the 
following few years.
1 and 2 depends on the program.  You never know what kind of program you 
will run into.
How can we use what we have today the extrapolate the future ignoring 
the highly
unpredictable complexity.

    It's interesting that recently, many EE magazine (circuit cellar, 
Elector, EE times) are
discussing buggy SW kill people.  I remember some posts complaining that 
some buggy program
have amazingly large # of global variables. I can find one post in 
Chinese website:

    http://forum.xitek.com/thread-1226816-5-1-1.html

    The 1st post says,  "a program has 11000 global variables"!

    As to "Can you provide this data"?  My answer is no, and I will not 
to implement the analysis
which perform on-the-fly analysis unless I'm convinced that saving 
addr_taken bit to llvm::GlobalVariable
is fundamentally flawed.

Shuxin





On 10/30/13 9:20 AM, Nadav Rotem wrote:
>
> On Oct 29, 2013, at 4:39 PM, Shuxin Yang <shuxin.llvm at gmail.com 
> <mailto:shuxin.llvm at gmail.com>> wrote:
>
>>   That said, I don't think it is still inexpensive if we re-analyze the
>>    addr-taken again and again *ON THE FLY".  It is really difficult to
>>    predict compile-time impact. You never know how many global variables
>>    in a program, and you never know how extensive they are used.
>>    Poorly-written program tends to have lots of global-vars.
>
> Actually, it is not that difficult to predict the effect on compile 
> time.  All you need to do is implement the analysis “on the fly” and 
> measure the compile time.  Can you provide this data ?  Compile time 
> measurements can help us make a decision.

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