[llvm-dev] LoopIdiomRegognize vs Preserved

[Mikael Holmén via llvm-dev]

Hi,

On 02/10/2016 01:23 AM, haicheng at codeaurora.org wrote:
> Thank you, Mikael.  I can reproduce what you saw and am looking into it.

Great!

> Just curious, why do you run loop-deletion before licm and loop-idiom?

As part of our internal testing we use Csmith to generate C-programs and 
then we run the compiler with random generated compiler flags on that 
input.

This bug was triggered in one of those runs, so the options to opt are 
not something we use in our normal pipeline, but used to find faults.

/Mikael

> The latter two can cause empty loops.
>
> Best,
>
> Haicheng
>
>> Hi Haicheng,
>>
>> Originally I ran into this on our out-of-tree target but I managed to
>> reproduce the crash on X86 as well now:
>>
>>    build-all/bin/opt -S -sroa -loop-rotate -loop-deletion -licm
>> -loop-idiom ../llvm/bugpoint-reduced-simplified.i8+.ll
>>
>> gives:
>>
>> While deleting: void %
>> An asserting value handle still pointed to this value!
>> UNREACHABLE executed at ../lib/IR/Value.cpp:696!
>> 0  opt             0x0000000001752bc8
>> llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
>> 1  opt             0x0000000001751376 llvm::sys::RunSignalHandlers() + 54
>> 2  opt             0x00000000017537ca
>> 3  libpthread.so.0 0x00007f1909a70340
>> 4  libc.so.6       0x00007f1908c98cc9 gsignal + 57
>> 5  libc.so.6       0x00007f1908c9c0d8 abort + 328
>> 6  opt             0x000000000170cddd
>> llvm::llvm_unreachable_internal(char const*, char const*, unsigned int)
>> + 461
>> 7  opt             0x000000000137bd3b
>> llvm::ValueHandleBase::ValueIsDeleted(llvm::Value*) + 1051
>> 8  opt             0x000000000137b5db llvm::Value::~Value() + 43
>> 9  opt             0x0000000001322319 llvm::CallInst::~CallInst() + 9
>> 10 opt             0x000000000131e676
>> llvm::Instruction::eraseFromParent() + 86
>> 11 opt             0x00000000015e8d14
>> 12 opt             0x00000000015e8630
>> 13 opt             0x00000000015e4e6a
>> 14 opt             0x0000000000f5404e
>> llvm::LPPassManager::runOnFunction(llvm::Function&) + 1086
>> 15 opt             0x000000000134a034
>> llvm::FPPassManager::runOnFunction(llvm::Function&) + 516
>> 16 opt             0x000000000134a27b
>> llvm::FPPassManager::runOnModule(llvm::Module&) + 43
>> 17 opt             0x000000000134a757
>> llvm::legacy::PassManagerImpl::run(llvm::Module&) + 903
>> 18 opt             0x000000000062aa9e main + 8782
>> 19 libc.so.6       0x00007f1908c83ec5 __libc_start_main + 245
>> 20 opt             0x0000000000618bcf
>> Stack dump:
>> 0.      Program arguments: build-all/bin/opt -S -sroa -loop-rotate
>> -loop-deletion -licm -loop-idiom
>> ../llvm/bugpoint-reduced-simplified.i8+.ll
>> 1.      Running pass 'Function Pass Manager' on module
>> '../llvm/bugpoint-reduced-simplified.i8+.ll'.
>> 2.      Running pass 'Loop Pass Manager' on function '@set_array'
>> 3.      Running pass 'Recognize loop idioms' on basic block '%bb4'
>> Abort
>>
>> /Mikael
>>
>> On 02/08/2016 05:50 PM, Haicheng Wu wrote:
>>> Hi Mikael,
>>>
>>> What is your compilation command to trig the assert?  I am trying to
>>> reproduce your problem.
>>>
>>> Thank you,
>>>
>>> Haicheng
>>>
>>> -----Original Message-----
>>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
>>> Mikael Holmén via llvm-dev
>>> Sent: Monday, February 08, 2016 4:33 AM
>>> To: llvm-dev at lists.llvm.org
>>> Subject: [llvm-dev] LoopIdiomRegognize vs Preserved
>>>
>>> Hi,
>>>
>>> I'm having problems with the LoopIdiomRegognizer crashing on me with
>>>
>>> An asserting value handle still pointed to this value!
>>> UNREACHABLE executed at ../lib/IR/Value.cpp:695!
>>>
>>> If I remove
>>>
>>>        AU.addPreserved<LoopInfoWrapperPass>();
>>>
>>> or
>>>
>>>        AU.addPreserved<AAResultsWrapperPass>();
>>>
>>> everything goes well.
>>>
>>> The C-code triggering this is
>>>
>>> void foo(int a[10][10])
>>> {
>>>        int	i, j, k;
>>>
>>>        for (i = 0; i < 1; i++) {
>>>            for (j = 0; j < 2; j++) {
>>>                for (k = 0; k < 10; k++) {
>>>                    a[j][k] = 42;
>>>                }
>>>            }
>>>        }
>>> }
>>>
>>> First LoopIdiomRecognize replaces the store in the inner loop with a
>>> memset in the outer loop, and later, when examining the outer loop it
>>> tries to replace that memset with an even bigger memset in the outermost
>>> loop. But then, when removing the "old" memset, the assert blows.
>>>
>>> I don't know LoopIdiomRecognize very well at all, is it obvious that
>>> AAResultsWrapperPass and/or LoopInfoWrapperPass should not be preserved
>>> here?
>>>
>>> Regards,
>>> Mikael
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
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