[llvm-dev] Unsigned int displaying as negative

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 15 10:24:27 PST 2017

LLVM IR integers have no sign. You can't reliably tell whether an add or
subtract was signed or unsigned when generating code.

On Wed, Feb 15, 2017 at 10:19 AM, Ryan Taylor via llvm-dev <
llvm-dev at lists.llvm.org> 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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