Am 31.10.2024 um 17:25 schrieb Takashi Yano:
On Thu, 31 Oct 2024 11:15:59 +0100
Corinna Vinschen wrote:
Yes, I will, but this is still puzzeling. While negative shift values
are undefined in C, there's this:
[...]
I guess the compiler ommitted the undefined calculation.
Precisely. Using negative arguments on the right hand side of shift
operators is undefined behaviour. The compiler can do whatever tickles
its fancy with that code.
For about a decade now, compilers like GCC have raised quite some ruckus
by what some people view as an abuse of this "poetic license": they
completely remove any provably undefined behaviour they discover, and
from that point on may assume whichever outcome makes the following code
the easiest to compile. Their reasoning goes something like this:
writing code with undefined behaviour means the user has relinquished
any rights they ever had to get reasonable output. So instead the
compiler writers decided to make life easy for the people who still
mattered at that point: themselves.
What that means for us compiler users, even as we're implementing the
runtime library, i.e. a substantial part of the C compiler toolchain
itself, here, is relatively clear: we must not write such code.
Oh, and BTW, __builtin_clzl() is the wrong function to use on signed
inputs, anyway :-)
