llvm dev - Jul 2019 - Problem with the LLVM v8.0 IR 'select' instruction

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.  But
when I compile the sample on x86_64, arm7, and aarch64, I always get code
that yields one.

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
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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
>
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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
>>
>
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