Eliot Miranda wrote:
Hi All,
anyone know the x86/IA32 really well? If so, read on. Otherwisesave yourself the yawn.
I just tried to save an instruction in Cog;s generated bitShift: primitive. It seems to me that SARL (shift arithmetic right long) should set the sign flag based on the result, in fact it says as much in the manual; I quote from IA-32 Intel® Architecture Software Developer's Manual Volume 2B: Instruction Set Reference, N-Z p 4-192
Flags Affected
The CF flag contains the value of the last bit shifted out of the destination operand; it is unde-
fined for SHL and SHR instructions where the count is greater than or equal to the size (in bits)
of the destination operand. The OF flag is affected only for 1-bit shifts (see "Description"
above); otherwise, it is undefined. The SF, ZF, and PF flags are set according to the result. If the
count is 0, the flags are not affected. For a non-zero count, the AF flag is undefined.
(my emphasis added). But neither the Bochs simulator nor my Intel Core Duo set the flags when doing sarl $1, %eax when %eax contains -1. Have I misread, or is the manual wrong?
Interesting. FWIW, the AMD64 arch manual (vol 3, p.220) also says that an SAR will affect the SF flag.
Regards,
-Martin