[llvm-dev] A bug in DependenceAnalysis?
Stanislav Manilov via llvm-dev
llvm-dev at lists.llvm.org
Fri Jun 23 07:07:04 PDT 2017
Hi Philip,
Done. See https://bugs.llvm.org/show_bug.cgi?id=33567.
Cheers,
- Stan
On Fri, Jun 23, 2017 at 11:52 AM, Philip Pfaffe <philip.pfaffe at gmail.com>
wrote:
> 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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