[???] FBC 1.01.0 misevaluation (addition+wrap+compare)

General FreeBASIC programming questions.
DOS386
Posts: 798
Joined: Jul 02, 2005 20:55

[???] FBC 1.01.0 misevaluation (addition+wrap+compare)

Postby DOS386 » Jan 30, 2015 9:47

Code: Select all

#define incv(Q) Q=Q+1
type UINT8 as UBYTE
DIM KK8 AS UINT8
KK8=251 : ? "!!! BUG (addition+wrap+compare) !!!"
DO
  ? STR$(KK8) + " " ;
  IF ((KK8+1)>128) THEN '' Broken in FBC 1.01.0 !!!
    ? "Truue" ;
  ELSE
    ? "False" ;
  ENDIF
  ? " " ;
  IF (CAST(UINT8,(KK8+1))>128) THEN '' This evaluates OK !!!
    ? "Truue" ;
  ELSE
    ? "False" ;
  ENDIF
  ? : incv(KK8)
  IF (KK8=4) THEN EXIT DO '' Enough !!!
LOOP


seems that the compiler "optimizes" my ((KK8+1)>128) into (KK8>127) ... NOT the idea behind my code :-(

http://www.freebasic.net/wiki/wikka.php ... PgLiterals
http://www.freebasic.net/wiki/wikka.php ... blVarTypes
Last edited by DOS386 on Feb 09, 2015 8:33, edited 1 time in total.
counting_pine
Site Admin
Posts: 6245
Joined: Jul 05, 2005 17:32
Location: Manchester, Lancs

Re: [BUG] FBC 1.01.0 misevaluation (addition+wrap+compare)

Postby counting_pine » Jan 30, 2015 10:50

The compiler doesn't optimise things like that - it doesn't try to rebalance comparisons.
I would have expected any operands to have been cast up to Integer. Especially 'KK8+1'.
I can't test at the moment. My guess is either that both sides are treated as signed bytes, or that 128 is being treated as -128 somehow.

What architecture and compiler parameters are you using?
DOS386
Posts: 798
Joined: Jul 02, 2005 20:55

Re: [BUG] FBC 1.01.0 misevaluation (addition+wrap+compare)

Postby DOS386 » Jan 30, 2015 10:54

counting_pine wrote:compiler doesn't optimise things like that - it doesn't try to rebalance comparisons.


Nevertheless the result is wrong. Can anyone reproduce it ?

> I would have expected any operands to have been cast up to Integer

But this is AFAIK NOT supposed to happen.

> What architecture and compiler parameters are you using?

Very default simple compilation to target DOS or Win32.
dkl
Site Admin
Posts: 3221
Joined: Jul 28, 2005 14:45
Location: Germany

Re: [BUG] FBC 1.01.0 misevaluation (addition+wrap+compare)

Postby dkl » Jan 30, 2015 11:26

I think the result is ok - the comparisons and math operations are indeed done at Integer precision, not at UByte precision. That's also why the cast to UByte is needed to truncate the value to the range of a UByte, if that is wanted.
counting_pine
Site Admin
Posts: 6245
Joined: Jul 05, 2005 17:32
Location: Manchester, Lancs

Re: [BUG] FBC 1.01.0 misevaluation (addition+wrap+compare)

Postby counting_pine » Jan 31, 2015 11:21

I see - yes, the result is as expected.
Most operations will have a return type at least as big as the operands passed to it. I don't think FB makes any guarantees to limit the size of the return type.
Besides, the '1' in 'KK8+1' is an Integer. Perhaps upcasting on UByte+UByte isn't desirable, but surely it is on UByte+Integer.
DOS386
Posts: 798
Joined: Jul 02, 2005 20:55

Re: [BUG] FBC 1.01.0 misevaluation (addition+wrap+compare)

Postby DOS386 » Feb 09, 2015 8:41

counting_pine wrote:I see - yes, the result is as expected. Most operations will have a return type at least as big as the operands passed to it.


All operands in my code are UINT8, so I expected 8-bit processing.

counting_pine wrote:Besides, the '1' in 'KK8+1' is an Integer


Then the documentation http://www.freebasic.net/wiki/wikka.php ... PgLiterals is bad:

> Integer size suffixes
> If an integer literal suffix is not given, the number field size required to hold the
> literal is automatically calculated. Specifying a size suffix guarantees that the
> compiler will consider a number as a specific integer size.

I was under the assumption that "1" is UINT8 and "1UL" is UINT32. But if "1" is already UINT32, then the "UL" suffix seems to be pretty pointless. Is the precision and wrapping documented somewhere ? I can't find anything .... http://www.freebasic.net/wiki/wikka.php ... KeyPgOpAdd ...

Return to “General”

Who is online

Users browsing this forum: No registered users and 15 guests