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

[Shuxin Yang]

Hi, Hal:

  Thank you for your feedback, see following inline comment

Thanks

On 10/29/13 5:02 PM, Hal Finkel wrote:
> ----- Original Message -----
>> Hi, There:
>>
>>     I'd like to add bit, called "addr_not_taken", to GlobalVariable in
>> order to
>> indicate if a GlobalVariable doesn't has its address taken or not.
>>
>> 1.The motivation
>> ===============
>>      The motivation can be explained by the following example. In this
>> example,
>> variable x does not have its address taken, therefore, it cannot be
>> indirectly
>> access. So, we can prove "x" and "*p" don't overlap.
>>     -----------------
>>     static int x;
>>     foo(int *p) {
>>       x = 1;
>>       *p = 2;   // do not modify x
>>       x = x + 1;
>>     }
>>     ---------------
>>
>>     The semantic of llvm::GlobalVariable::addr_not_taken is:
>>      1). Of course, the global variable in question doesn't have its
>> address taken, and
>>      2). the global variable is not volatile, and
> Why are you also imposing this restriction? I'm also not quite sure what it means, in the context of LLVM, where only loads and stores can be volatile.
2) is just for being pedantic :-)

One might otherwise argue in this snippet, x apparently does not have 
its addr taken,
however, it's illegal to say  that "x" and "*p" don't alias.


  volatile static int x;
    foo(int *p) {
      x = 1;
      *p = 2;   // do not modify x
      x = x + 1;
    }


  >  Q4: Can we save the not-addr-taken to an analysis pass
  >  -----------------------------------------------------
  >  A4: If we implement this way:
  >      o. we have add a statement to most, if not all, passes to
  >      claim it
  >         preserve this addr-taken analysis result, which is bit
  >         anonying.
  >
  >      o. The address-taken information collected before LTO (aka
  >      pre-IPO),
  >         cannot be fed to LTO, unless we re-analyze them from ground
  >         up.

> So is the idea that lib/Analysis/IPA/GlobalsModRef.cpp will set this bit when it runs?
>
>
I'm not quire sure your question. Let me try to answer.

In my implementation, I set address_no_taken in 
lib/Transformation/IPO/GlobalOpt.cpp.
In this pass, it tries to see if a global var has its address taken 
before it tries to optimize
the global variable.

ModRef is immaterial in this regard.

What I'm try to say in "A4"  is that it is really *troublesome* if we 
save the
addr-taken-analysis result to a analysis-pass (rather than cache the 
result to
GlobalVariable::addr_not_taken).  If we really have to implement this way,
we need to at very least implement following two steps:

    --------------------------

        o. we have add a statement to most, if not all, passes to
        claim it preserve this addr-taken analysis result, which is bit
           anonying.

        o. In LTO mode, re-run the address-taken-analysis right after all input IR are linked
           together. I need to to do so because the addr-taken information we collected
           before LTO is missing.

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