https://bugs.freepascal.org/view.php?id=33851 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". After analysing the assembly language that's produced, it turns out that the optimiser works with the full 32-bits of the register. For example:
is_this_a_bug:=((counter shl 8) shr 8); ...becomes the following under -O3 (under -O1, %r13w and %si are replaced with references)... movzwl %r13w,%eax shl $0x8,%eax shr $0x8,%eax mox %al,%si A similar situation happens if the variables are of type Byte as well - the intermediate values use the full 32 bits of the register. I'm not certain if this is a bug or intended behaviour though. Can someone more senior make a decision on this? Gareth
_______________________________________________ fpc-devel maillist - [email protected] http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
