[LLVMdev] MemoryDependenceAnalysis

[Anthony Danalis]

On Apr 25, 2009, at 5:05 PM, Chris Lattner wrote:

>
> On Apr 13, 2009, at 12:01 PM, Anthony Danalis wrote:
>
>> I'm attaching the .bc file.  Note that my analysis pass is invoked
>> after "-O1" and that's why the IR I included in the original email
>> is optimized.
>
> Hi Anthony,
>
> Sorry for the delay, things have been crazy lately.
>
> The MemDep API assumes that you will call getDependency() first, and
> then only call getNonLocalPointerDependency() if it returns non-
> local.  This will return the first instruction that the instruction is
> dependent on.
>
> As Eli mentioned, MemDep does have some more rich APIs that are
> private.  We could look the export those so that you can query each
> argument at a time.

Actually there is no need to export it.  I wrote a little  
ReachingDefinitions pass based on the API from MemDep that gives me  
the information I want.  I should probably send it in, but I'm not  
sure how to do so.  Should I submit it as a patch to the list, or try  
to commit it in the svn?

>  The problem is that (without more information
> about language semantics) there is no way to know how much data is
> accessed off each pointer by the function, nor whether it accesses
> them through other aliases.  For example, "bar" could capture the N
> pointer, etc.
>
> -Chris
>
>
>>
>> <simple_loops_F2_4list.bc>
>> On Apr 13, 2009, at 2:21 PM, Chris Lattner wrote:
>>
>>> On Apr 13, 2009, at 9:06 AM, Anthony Danalis wrote:
>>>> Hello,
>>>>
>>>> I have a code similar to the following:
>>>
>>> Hi Anthony,
>>>
>>> Can you please attach the .bc file for this?
>>>
>>> -Chris
>>>
>>>>
>>>>
>>>>      program test
>>>>          integer i, j, N
>>>>          real B(10)
>>>>
>>>>          call bar(N, 8)
>>>>          N = N+1
>>>>          do i = 1, N
>>>>              B(i) = (i+5)/(i+3)
>>>>          enddo
>>>>
>>>>          j = N/2
>>>>          N = N+7
>>>>
>>>>          call IMPORTANT_F(B, N, i, j)
>>>>
>>>>      end program
>>>>
>>>> and I am trying to use dependence analysis on the second and fourth
>>>> actual parameters of IMPORTANT_F().  Since it's Fortran, they are
>>>> really pointers, and the IR of the last block looks like this:
>>>>
>>>>      %11 = sdiv i32 %1, 2            ; <i32> [#uses=1]
>>>>      store i32 %11, i32* %j, align 4
>>>>      %12 = add i32 %0, 8             ; <i32> [#uses=1]
>>>>      store i32 %12, i32* %n, align 4
>>>>      call void (...)* @IMPORTANT_F_([10 x float]* %b, i32* %n,
>>>> i32* %i, i32* %j) nounwind
>>>>      ret void
>>>>
>>>> My problem is that if I use getNonLocalPointerDependency() I get  
>>>> the
>>>> instruction "N=N+1" as a Def for the second parameter, "N",  and no
>>>> Def for the fourth, "j", (only the call to "bar()" as clobber).
>>>> If I use getDependency() on the call, I get the instruction "N=N
>>>> +7" as
>>>> a clobber.
>>>>
>>>> How can I use MemoryDependenceAnalysis (or any other analysis for
>>>> that
>>>> matter) to gather that the instructions
>>>> j = N/2  (store i32 %11, i32* %j, align 4)
>>>> and
>>>> N = N+7  (store i32 %12, i32* %n, align 4)
>>>> are the ones that define the parameters "j" and "N" respectively?
>>>>
>>>> thanks,
>>>> Anthony
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
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