Re: [PATCH] Better __ashlDI3, __ashrDI3 and __lshrDI3 functions, plus fixed __bswapsi2 function

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Nov 11, 2020 at 09:33:00AM +0100, Stefan Kanthak wrote:
> Ouch: that's but not the point here; what matters is the undefined behaviour of
>       ((u) & 0x000000ff) << 24
> 
> 0x000000ff is a signed int, so (u) & 0x000000ff is signed too -- and producing
> a negative value (or overflow) from the left-shift of a signed int, i.e.
> shifting into (or beyond) the sign bit, is undefined behaviour!

Only in some language dialects.
It is caught by -fsanitize=shift.
In C++20, if the shift count is within bounds, all signed as well as
unsigned left shifts well defined.
In C99/C11 there is one extra rule:
For signed x << y, in C99/C11, the following:
     (unsigned) x >> (uprecm1 - y)
     if non-zero, is undefined.
and for C++11 to C++17 another one:
  /* For signed x << y, in C++11 and later, the following:
     x < 0 || ((unsigned) x >> (uprecm1 - y)) > 1
     is undefined.  */
So indeed, 0x80 << 24 is UB in C99/C11 and C++98, unclear in C89 and
well defined in C++11 and later.  I don't know if C2X is considering
mandating two's complement and making it well defined like C++20 did.

Guess we should fix that, though because different languages have different
rules, GCC itself except for sanitization doesn't consider it UB and only
treats shifts by negative value or shifts by bitsize or more UB.

	Jakub