On 24 June 2014 15:24, Richard Henderson <r...@twiddle.net> wrote: > On 06/24/2014 03:00 AM, Peter Maydell wrote: >> This is undefined behaviour in C because of the shift into >> the sign bit. Better to shift first and then signextend: >> >> offset = sextract32(ctx->opcode << 3, 0, 21); > > Not true. Because we know from the extract that the value has > 13 copies of the sign bit. Shifting by 3 isn't going to cause > problems. It's shifting a *different* bit into the sign position > that's (one's compliment) undefined.
C99 6.5.7 says that for E1 << E2, "If E1 has a signed type and nonnegative value, and E1 * 2^E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined." As I read that, shifting any negative value is undefined, as well as shifting a 1 into the sign bit. > PS: Honestly, all these compilers/sanitizers should grow a "No One's > Compliment" switch to disable all the stupid stuff. Heartily agreed. Unfortunately until they do, I don't trust the compiler not to decide it can be fantastically clever and speed up specmark by 0.00003% if it breaks the 2s complement behaviour for signed shifts. thanks -- PMM
