[llvm-dev] Unsigned int displaying as negative

Manuel Jacob via llvm-dev
2017-02-15T18:44:38Z

Hi Ryan,

It is important to get clear about that LLVM IR integers (and operations
if they don't have to) have no sign.  But IR integers have to be printed
somehow and it was decided to print them as being signed.

I'm not a SelectionDAG and tablegen expert really, but I'm sure it is
the same in the code generator.  Sometimes the signedness is important
for an instruction because flags are affected.  But I'm ignoring that
for now, as they would be represented as llvm.*.with.overflow in the IR
with explicit signedness.

In cases where flags don't matter, just select the best instruction.
I'd advise against trying to reconstruct the signedness of an operation.
  That's impossible to do in general and there's no good reason to do
that.

-Manuel

On 2017-02-15 19:19, Ryan Taylor via llvm-dev wrote:
> I'm curious why 'unsigned short w = 0x8000' is displayed as
-32768 in
> the
> IR?
>
> This propagates to the DAG and hence tablegen, where I am missing
> matching
> on some immediates because the immediate is not being looked at as
> unsigned? For example, we have no issue if instead of 0x8000 we use
> 0x7FFF,
> then everything is fine, the match is fine.
>
> I can understand that it's just being printed as 'signed' even
when
> it's
> unsigned due to the bit pattern (2s complement)  but it seems to affect
> matching.
>
> We'd like;
>
> unsigned short x, y;
> int main() {
>    unsigned short w = 0x8000;
>    y = w - x;
>    return 0;
> }
>
> To match to something like 'sub16u $0x8000, x, y' (if I set w =
0x7FFF,
> then we get sub16u $0x7FFF, x, y' but not when using 0x8000).
>
> We have some code to determine if the operation is a signed or unsigned
> operation in tablegen. Can anyone suggest a good way to get around
> this?
>
> Thanks,
> Ryan
>
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