On 2012-10-31 01:44, Peter Maydell wrote: > On 30 October 2012 15:34, Jia Liu <pro...@gmail.com> wrote: >> On Mon, Oct 29, 2012 at 9:40 PM, Jovanovic, Petar <pet...@mips.com> wrote: >>>> imm = (int16_t)(imm << 6) >> 6; >>> >>> result of a bitwise shift of a signed type and a negative vlaue is >>> implementation-defined, so you can not rely on that. >>> >> >> I think it will take a 10bits signed value sign extend into 16bits >> signed value, and I've tested it with negative values, it working >> well. > > You cannot rely on the behaviour of a specific compiler implementation > as evidence that a piece of code is correct. C has a standard which > defines what is and is not valid.
Indeed. The only portable way is val = ((val & (sign | (sign - 1))) ^ sign) - sign with all unsigned types, and "sign" set to the sign bit. > > Having said that, right shift of negative signed integers is one of > those bits of implementation defined behaviour which we allow ourselves > to rely on in QEMU because all the platforms we care about behave > that way. (That integers are 2s complement representation is another.) Also very true. I don't like seeing the code in question though. We've several implementations of sign-extend-to-N-bits functions throughout qemu; we ought to unify them. r~
