On 30/12/2021 21:28, Martin Frb via fpc-devel wrote:
Well the issue is, that at some time it needs to be extended, as the
full expression is
Result := Result + (pn8^ shr 7) and ((not pn8^) shr 6);
Result is either 64 or 32 depending on architecture.
pn8 is pint8
On intel
- only the byte is loaded. "Not" applies to the byte only.
- Then on intel the value is sign extended, even though an (logical)
"shr 6" could be done on the byte.
- For the addition, sign extension is needed.
That means that due to the early sign extension, the bytes 8..31 (or
8..64) before the shift (or 2..25 after) might be set to 1.
Since shr is not arithmetic, and does not keep/fill the "sign bit", I do
not see why a sign extension is needed at that point. (But that is
another issue)
It's because, as mentioned earlier, in Pascal virtually all operations
are performed using the native integer type. Converting a signed byte to
the native integer type requires a sign extension to keep the value equal.
Jonas
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