Il 12/06/2018 02:07, J. Gareth Moreton ha scritto:
Someone pointed out that if you perform "(x shl 8) shr 8", where x is of type Word, the result is always x, even though logic dictates that the upper 8 bits should be shifted out and hence the result actually be equal to "x and $FF".
IMHO logic dictates that (x shl 8) shr 8 should give x, because the shr cancels the effect of shl, and that a further compiler optimization might simply suppress the code at all. Register size when performing a calculation is not dictated by logic, but by physical constraints, which have nothing to do with logic.
If the programmer wishes to mask out some bits he should explicitly use an *and* instruction, making the code both more readable and more portable. Generating a less efficient code, with and instructions added with the sole purpose of supporting the improper usage of a shl instruction wouldn't be very smart.
I'm in favour of a compiler which generates the most efficient code, and leaves to the programmer the responsibility to mask unwanted bits.
Just my 2 cents. Giuliano _______________________________________________ fpc-devel maillist - [email protected] http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
