[cfe-users] Weird optimization

[Csaba Raduly]

This is, in fact, a great illustration of an optimization enabled by
undefined behavior. The compiler can say: "I don't have to make it
work correctly for 0x8000000000000000ul, and for every other case it's
faster this way".

A very good article about undefined behavior:
http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html

On Mon, Feb 23, 2015 at 12:22 PM, Artem Alimarin
<artem.alimarin at gmail.com> wrote:
> Hi Csaba,
>
> Yes, thanks. nasal demons, indeed. I forgot that the signed integer overflow
> causes undefined behavior unlike unsigned integer overflow, which causes
> wrap around.
>
> The code
>     return (long)(0ul - (unsigned long)x) > 0;
> does what I want.
>
> I am impressed by clang's ability to use this undefinedness to enable
> optimizations like (-x) > 0 to x < 0, which seems to be valid for all valid
> arguments.
>
> Best regards,
> Artem
>
>
> On 23-2-2015 9:44, Csaba Raduly wrote:
>>
>> cast to an unsigned type to negate
>> this value to itself
>>
>>
>



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