On Sat, Jan 20, 2018 at 07:57:55PM +1100, Bruce Evans wrote: > The not-unused x86 arch is one that does this. IIRC, some history of this > is: > > - on the 8086, the shift count was taken mod 32. 16 bits was enough for > anyone, and shifting left or right by 16 through 31 (but not by 32) > shifted out all of the bits (in the unsigned case) to give 0. > > - for the 80386, someone forgot why the 8086 took the count mod 32 instead > of just 16, and kept using 32. 16 bits was not enough for anyone, and > shifting left or right by 32 had no effect (even in the signed case?). >
SDM rev 065 states: IA-32 Architecture Compatibility The 8086 does not mask the shift count. However, all other IA-32 processors (starting with the Intel 286 processor) do mask the shift count to 5 bits, resulting in a maximum count of 31. This masking is done in all operating modes (including the virtual-8086 mode) to reduce the maximum execution time of the instructions. Then later, the same section states that <=32bit mode uses mask 0x1f, and 64bit mode uses mask 0x3f. Of course this does not make the compiler a bit more useful or provide a reasoning for its useless behaviour. _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
