[llvm-dev] Problem with the LLVM v8.0 IR 'select' instruction

[Mikael Egevig via llvm-dev]

Ouch!

Thanks for your help :-)


Cheers,
Mikael Egevig

Den man. 8. jul. 2019 kl. 05.53 skrev Craig Topper <craig.topper at gmail.com>:

> I believe your issue is that @Start has the readnone attribute. This is
> telling the optimizers that @Start doesn't read or write any memory.
>
> ~Craig
>
>
> On Sun, Jul 7, 2019 at 7:54 PM Mikael Egevig via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi again,
>>
>> It is probably me doing something wrong, but I have attached a trimmed
>> down sample that shows the issue and the output that Clang 8.0.0 makes
>> (using the binary release from llvm.org).
>>
>> I invoke clang as follows: clang -S -O3 -o Evergrowing.s -Wall
>> Evergrowing.ll
>>
>> I tried using -O1, -O2, and -O3, but all where the same.  Please notice
>> that I renamed 'main()' to 'mainfunc()' to be sure Clang isn't picking up
>> 'main()' as a reserved, special function.
>>
>> This on a Windows 10 x64 PC, but I got exactly the same behavior on
>> arm7hf and aarch64.
>>
>>
>> Cheers,
>> Mikael Egevig
>>
>>
>> Den man. 8. jul. 2019 kl. 04.30 skrev Mehdi AMINI <joker.eph at gmail.com>:
>>
>>> Hi,
>>>
>>>
>>>
>>> On Sun, Jul 7, 2019 at 4:26 PM Mikael Egevig via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi guys,
>>>>
>>>> I am doing a hobby compiler using LLVM.  I have a rather peculiar
>>>> problem with the 'select' instruction, as far as I can see.  Given this
>>>> code:
>>>>
>>>> ************************************************
>>>> ; Run-time Library Definitions
>>>> %Exception = type { i8 }
>>>>
>>>> define private %Exception* @Start(i1* %$result) #0
>>>> {
>>>> prologue:
>>>>     store i1 true, i1* %$result
>>>>     br label %epilogue
>>>> epilogue:
>>>>     ret %Exception* null
>>>> }
>>>>
>>>> ; Generated main() function.
>>>> define i32 @main(i32 %argc, i8** %argv)
>>>> {
>>>> prologue:
>>>>     %0 = alloca i1
>>>>     %1 = call %Exception *@Start(i1* %0)
>>>>     %2 = load i1, i1* %0
>>>>     %3 = select i1 %2, i32 0, i32 1
>>>>     ret i32 %3
>>>> }
>>>> ************************************************
>>>>
>>>> I'd expect the 'select' instruction in main() to yield zero, not one.
>>>>
>>>
>>> Seems correct to me.
>>>
>>>
>>>>   But when I compile the sample on x86_64, arm7, and aarch64, I always
>>>> get code that yields one.
>>>>
>>>
>>> Can you provide a reproducer?
>>> As far as I can tell the optimizer folds the code you provided above to
>>> yield 0: https://godbolt.org/z/T5wkSJ
>>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>>
>>>> The idea is that if the program, @Start, returns false, one should be
>>>> returned to the OS and vice versa.  I guess I could use 'not' instead of
>>>> 'select', but I don't understand the behavior of 'select'.
>>>>
>>>> The above code is automatically generated, except I have removed all
>>>> irrelevant code generated by the compiler.
>>>>
>>>> I'm not sure if LLVM gets confused by me returning the result in an
>>>> alloca'ed area, but I use the return value, of all functions, for the
>>>> exception pointer (null = no exception).
>>>>
>>>> Thanks in advance!
>>>>
>>>>
>>>> Cheers,
>>>> Mikael Egevig
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190708/f418d265/attachment.html>