[llvm] r240521 - Hexagon: Avoid left shifting negative values (it's UB)

[David Majnemer]

On Tue, Jun 23, 2015 at 11:00 PM, Justin Bogner <mail at justinbogner.com>
wrote:

> Author: bogner
> Date: Wed Jun 24 01:00:53 2015
> New Revision: 240521
>
> URL: http://llvm.org/viewvc/llvm-project?rev=240521&view=rev
> Log:
> Hexagon: Avoid left shifting negative values (it's UB)
>
> Found by ubsan.
>
> Modified:
>     llvm/trunk/lib/Target/Hexagon/MCTargetDesc/HexagonShuffler.h
>
> Modified: llvm/trunk/lib/Target/Hexagon/MCTargetDesc/HexagonShuffler.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/Hexagon/MCTargetDesc/HexagonShuffler.h?rev=240521&r1=240520&r2=240521&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Target/Hexagon/MCTargetDesc/HexagonShuffler.h (original)
> +++ llvm/trunk/lib/Target/Hexagon/MCTargetDesc/HexagonShuffler.h Wed Jun
> 24 01:00:53 2015
> @@ -34,7 +34,7 @@ public:
>    HexagonResource(unsigned s) { setUnits(s); };
>
>    void setUnits(unsigned s) {
> -    Slots = s & ~(-1 << HEXAGON_PACKET_SIZE);
> +    Slots = s & ~(~0U << HEXAGON_PACKET_SIZE);
>

ISTM that this is equivalent to `s & ((1 << HEXAGON_PACKET_SIZE) - 1)`.
Personally, I find that this version is more obvious while still avoiding
UB.


>      setWeight(s);
>    };
>    unsigned setWeight(unsigned s);
>
>
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