[fpc-devel] Unexpected behaviour with intermediate results

J. Gareth Moreton gareth at moreton-family.com
Tue Jun 12 02:07:37 CEST 2018

 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?

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