[llvm-dev] double to unsigned char cast

[Jay K via llvm-dev]

This seems disappointing.

Casting any value to an integral type should yield a value within the range of that integral type.

The value can be anything, but it should be within range.

i.e. it should be as if I anded it with (type)~(type)0.


i.e. int i = (signed char)x;

assert(i >= -128 && i <= 127);


int i = (unsigned char)x;

assert(i >= 0 && i <= 255);

seem very reasonable, assuming there is no "trap"  before the assert.


 - Jay


________________________________
From: Tim Northover <t.p.northover at gmail.com>
Sent: Thursday, April 5, 2018 4:41 PM
To: Jay K
Cc: llvm-dev at lists.llvm.org; lewurm at gmail.com
Subject: Re: [llvm-dev] double to unsigned char cast

Hi Jay,

On 5 April 2018 at 07:49, Jay K via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> wrt
> https://github.com/mono/mono/commit/fb91fce5d339bb9ffe507588f5bc1d8d6f244d9b

>
>
> This doesn't seem right to me.

Nevertheless, it's what the standard says. C99 6.3.1.4: When a finite
value of real floating type is converted to an integer type other than
_Bool, the fractional part is discarded (i.e. the value is truncated
toward zero). If the value of the integral part cannot be represented
by the integer type, the behavior is undefined.

By definition any double you're allowed to convert to an unsigned char
doesn't need the "and".

Cheers.

Tim.
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