------- Comment #8 from gunnar at greyhound-data dot com 2008-11-10 12:54
-------
(In reply to comment #7)
> (In reply to comment #4)
> > There are two causes where GCC generates unneeded TST instructions.
> > A) General arithmetic
> > lsr.l #1,D0
> > tst.l d0
> > jbne ...
> >
> > This tst instruction is unneeded as the LSR is setting the flags correctly
> > already.
>
> This is NOT correct. LSL does write to the condition codes, but not all of it.
> In particular, the bit involved in the not-equal test is not set.
>
> This TST *is* required.
What you is say is not correct.
The bit involved in the not-eval test is the "Z-Bit"
Both the LSR and TST do set the Z-Bit, 100% equally.
The TST instruction in the example is 100% unneeded and NOT required.
Please check the official 68K documentation for the which flags the conditinal
branch instruction BCC tests (in this case BNE), and verify the behavior of TST
and LSR in regards of setting these bits.
Best Regards
Gunnar von Boehn
--
gunnar at greyhound-data dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|INVALID |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36133
