[llvm-dev] A bug in DependenceAnalysis?

[Philip Pfaffe via llvm-dev]

Hi Stan,

in the first example, I get an "anti [*|<]" result. DA doesn't look through
zext expressions, so it needs to overapproximate.

In the second example I get a "consistent anti [0|<]" result, which is
wrong. The cause of this bug is that DA falsely ignores the base pointer,
and only looks at the indices.

Please file a bug report for this, including a reproducing example, and put
me on CC.

Cheers,
Philip

2017-06-22 10:59 GMT+02:00 Stanislav Manilov <stanislav.manilov at gmail.com>:

> 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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