Messages in this thread | | | Subject | Re: [PATCH 4/9] drivers/iio: Sign extend without triggering implementation-defined behavior | From | Douglas Gilbert <> | Date | Wed, 30 Oct 2019 18:13:11 -0400 |
| |
On 2019-10-30 4:02 p.m., Peter Zijlstra wrote: > On Mon, Oct 28, 2019 at 01:06:55PM -0700, Bart Van Assche wrote: >> From the C standard: "The result of E1 >> E2 is E1 right-shifted E2 bit >> positions. If E1 has an unsigned type or if E1 has a signed type and a >> nonnegative value, the value of the result is the integral part of the >> quotient of E1 / 2E2 . If E1 has a signed type and a negative value, the >> resulting value is implementation-defined." > > FWIW, we actually hard rely on this implementation defined behaviour all > over the kernel. See for example the generic sign_extend{32,64}() > functions. > > AFAIR the only reason the C standard says this is implementation defined > is because it wants to support daft things like 1s complement and > saturating integers.
See: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2218.htm
That is in C++20 and on the agenda for C2x: https://gustedt.wordpress.com/2018/11/12/c2x/
Doug Gilbert
> Luckily, Linux doesn't run on any such hardware and we hard rely on > signed being 2s complement and tell the compiler that by using > -fno-strict-overflow (which implies -fwrapv). > > And the only sane choice for 2s complement signed shift right is > arithmetic shift right. > > (this recently came up in another thread, which I can't remember enough > of to find just now, and I'm not sure we got a GCC person to confirm if > -fwrapv does indeed guarantee arithmetic shift, the GCC documentation > does not mention this) >
| |