[llvm-dev] Dependence Analysis bug or undefined behavior?

[Juneyoung Lee via llvm-dev]

> Does this kind of IR have “undefined behavior”  under LLVM semantics or
is it acceptable?

I think this should be well defined. C11 says

6.5.2.3 Structure and union members
95) If the member used to read the contents of a union object is not the
same as the member last used to
store a value in the object, the appropriate part of the object
representation of the value is reinterpreted
as an object representation in the new type as described in 6.2.6 (a
process sometimes called ‘‘type
punning’’). This might be a trap representation.

(I'm using this pdf file for the text -
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf )

I guess peephole optimizations also can lead to the load/store with
different types (e.g. memcpy to/from an unsigned char array can be removed
if redundant), so the code seems to make sense.

Best Regards,
Juneyoung Lee

On Sun, Nov 18, 2018 at 11:03 PM De Azevedo Piovezan, Felipe via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
>
>
> Does this kind of IR have “undefined behavior”  under LLVM semantics or is
> it acceptable?
>
>
>
> (TLDR: a store of i64 at offset n, followed by a load of i32 at offset
> n+1.)
>
>
>
> define void @foo(i32* %A, i64 %n) {
>
> entry:
>
>   %arrayidx = getelementptr inbounds i32, i32* %A, i64 %n
>
>   %arrayidx_cast = bitcast i32* %arrayidx to i64*
>
>   store i64 0, i64* %arrayidx_cast, align 4
>
>   %add1 = add i64 %n, 1
>
>   %arrayidx2 = getelementptr inbounds i32, i32* %A, i64 %add1
>
>   %0 = load i32, i32* %arrayidx2, align 4
>
>   ret void
>
> }
>
>
>
> The result of Dependence Analysis is:
>
>
>
> opt -analyze -da BitCasts.ll
>
> Printing analysis 'Dependence Analysis' for function 'z0':
>
> da analyze - none!          <<< this is between the store and itself, ok
> to be “none”.
>
> da analyze - none!          <<< this is between the store and the load!!!
>
> da analyze - none!          <<< this is between the load and itself, ok to
> be “none”.
>
>
>
> Is dependence analysis right or wrong? This IR likely comes from some C++
> code doing funny things with unions…
>
>
>
> Thanks!
>
>
>
> --
>
> Felipe de Azevedo Piovezan
>
>
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>


-- 

Juneyoung Lee
Software Foundation Lab, Seoul National University
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