[llvm-dev] A bug in DependenceAnalysis?

[Stanislav Manilov via llvm-dev]

Hi Philip,

I forgot to mention that I was ignoring loop-independent dependences. If I
don't I get an inconsistent, ordered, anti, loop-independent dependence and
an inconsistent, ordered, flow, loop-carried dependence for example A. At
the same time I get just a consistent, ordered, anti, loop-independent
dependence for example B.

Here's the .ll code for example A:

*; Function Attrs: nounwind uwtable*
*define void @_Z8move_onePij(i32*, i32) #3 {*
*  br label %3*

*; <label>:3:                                      ; preds = %13, %2*
*  %.0 = phi i32 [ 0, %2 ], [ %14, %13 ]*
*  %4 = sub i32 %1, 1*
*  %5 = icmp ult i32 %.0, %4*
*  br i1 %5, label %6, label %15*

*; <label>:6:                                      ; preds = %3*
*  %7 = add i32 %.0, 1*
*  %8 = zext i32 %7 to i64*
*  %9 = getelementptr inbounds i32, i32* %0, i64 %8*
*  %10 = load i32, i32* %9, align 4*
*  %11 = zext i32 %.0 to i64*
*  %12 = getelementptr inbounds i32, i32* %0, i64 %11*
*  store i32 %10, i32* %12, align 4*
*  br label %13*

*; <label>:13:                                     ; preds = %6*
*  %14 = add i32 %.0, 1*
*  br label %3*

*; <label>:15:                                     ; preds = %3*
*  ret void*
*}*

Here's the .ll code for example B:

*; Function Attrs: nounwind uwtable*
*define void @_Z12move_one_altPij(i32*, i32) #3 {*
*  %3 = getelementptr inbounds i32, i32* %0, i64 1*
*  br label %4*

*; <label>:4:                                      ; preds = %13, %2*
*  %.0 = phi i32 [ 0, %2 ], [ %14, %13 ]*
*  %5 = sub i32 %1, 1*
*  %6 = icmp ult i32 %.0, %5*
*  br i1 %6, label %7, label %15*

*; <label>:7:                                      ; preds = %4*
*  %8 = zext i32 %.0 to i64*
*  %9 = getelementptr inbounds i32, i32* %3, i64 %8*
*  %10 = load i32, i32* %9, align 4*
*  %11 = zext i32 %.0 to i64*
*  %12 = getelementptr inbounds i32, i32* %0, i64 %11*
*  store i32 %10, i32* %12, align 4*
*  br label %13*

*; <label>:13:                                     ; preds = %7*
*  %14 = add i32 %.0, 1*
*  br label %4*

*; <label>:15:                                     ; preds = %4*
*  ret void*
*}*

Can you please check whether the anti dependeces that you get are
loop-carried or loop-independent?

Thanks,
 - Stan

On Wed, Jun 21, 2017 at 7:21 PM, Philip Pfaffe <philip.pfaffe at gmail.com>
wrote:

> Hi Stan,
>
> can you share your example.bc? Can you reproduce your issue with llvm 4.0
> or, better even, trunk?
>
> Cheers,
> Philip
>
> 2017-06-21 18:58 GMT+02:00 Stanislav Manilov <stanislav.manilov at gmail.com>
> :
>
>> Hi Philip,
>>
>> Thanks for checking!
>>
>> I'm running my own Foo pass that registers DependenceAnalysisWrapperPass
>> as a prerequisite and then I run it like so:
>>
>> opt -load libfoo.so -foo example.bc
>>
>> This is LLVM 3.9.
>>
>> Cheers,
>>  - Stan
>>
>> On Wed, Jun 21, 2017 at 5:40 PM, Philip Pfaffe <philip.pfaffe at gmail.com>
>> wrote:
>>
>>> Hi Stan,
>>>
>>> in both cases I get a consistent anti result. Can you show us the
>>> command lines you're using? Which version of llvm is this?
>>>
>>> Best,
>>> Philip
>>>
>>> 2017-06-21 17:56 GMT+02:00 Stanislav Manilov via llvm-dev <
>>> llvm-dev at lists.llvm.org>:
>>>
>>>> Hello llvm-dev,
>>>>
>>>> I'm running a pass that uses the result of
>>>> llvm::DependenceAnalysisWrapperPass to compute the dependencies
>>>> between all instructions of a loop. I have the following two examples of
>>>> code I wish to analyse:
>>>>
>>>> example A:
>>>>
>>>> ```
>>>> void move_one(int *A, unsigned n) {
>>>>   for (unsigned i = 0; i < n-1; ++i) {
>>>>     A[i] = A[i + 1];
>>>>   }
>>>> }
>>>> ```
>>>> and example B:
>>>> ```
>>>> void move_one_alt(int *A, unsigned n) {
>>>>   int *B = A + 1;
>>>>   for (unsigned i = 0; i < n-1; ++i) {
>>>>     A[i] = B[i];
>>>>   }
>>>> }
>>>> ```
>>>>
>>>> I would expect that I get the same result for both A and B, namely a
>>>> loop carried anti (WAR) dependence from the generated load instruction to
>>>> the generated store instruction. This should be the case, because on
>>>> iteration i+1 the loop is writing to the element that has been read in the
>>>> previous iteration - iteration i.
>>>>
>>>> However, in example A I get a loop carried flow (RAW) dependence from
>>>> the store instruction to the load instruction, while in example B I don't
>>>> get any dependence at all.
>>>>
>>>> Am I missing something, or is the result wrong?
>>>>
>>>> Thanks,
>>>>  - Stan
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>>
>
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