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

[Mehdi AMINI via llvm-dev]

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
>
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