RE: [RFC] Mitigating unexpected arithmetic overflow

[David Laight]

...
> Going the other way is similar:
> 
>         all_bits = low_bits + ((u64) high_bits << 16) << 16);
> 
> and again, the compiler will recognize this idiom and do the right
> thing (and if 'all_bits' is only 32-bit, the compiler will optimize
> the high bit noise away).

On a 32bit system the compiler might not do the expected thing.

I had terrible trouble with the 32bit div_u64_u32() code I was
playing with getting the compiler to do 'something sensible' for
that.
I just couldn't get it to stop generating two 64bit values
(in two register pairs) and then oring them together.
I didn't try using a union - that might work.
On x64 the asm "A" (edx:eax) and "a" and "d" constraints will DTRT
but force the values into edx:eax which is ok if you are doing divides.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)