Re: Fix for GCC Bugzilla Bug 36133

["Gunnar Von Boehn" <gunnar@genesi-usa.com>]

Hi Andreas,

Please have a look at the MOTOROLA 68k Programming model (M68000PRM)
for a definition of how the flags are set by the shift instructions.

ie. Page 218 for the LSL, LSR instructions:

Bit
X — Set according to the last bit shifted out of the operand;
unaffected for a shift
    count of zero.
N — Set if the result is negative; cleared otherwise.
Z — Set if the result is zero; cleared otherwise.
V — Always cleared.
C — Set according to the last bit shifted out of the operand; cleared
for a shift count
    of zero.


Lets compare how the flags are set to how the flags would be set by
the otherwise used TST instruction:

p 296 TST

X — Not affected.
N — Set if the operand is negative; cleared otherwise.
Z — Set if the operand is zero; cleared otherwise.
V — Always cleared.
C — Always cleared.

As you can see the flags N Z V are set 100% the same way by LSX and TST.


Do you agree with me that the flags N,Z,V are those that will be used by BCC?
Isn't it true that GCC does not allow the user to generate BCC
instructions which use the carry flag?


Kind regards
Gunnar von Boehn


On Tue, Jun 17, 2008 at 1:53 PM, Andreas Schwab <schwab@suse.de> wrote:
> "Gunnar Von Boehn" <gunnar@genesi-usa.com> writes:
>
>> Please apply the patch and recompile.
>> GCC is now aware that the LSR instruction does set the CC already and
>> will not create the unneeded TST.
>
> That does not look correct, see the comment in notice_update_cc:
>
>      case ASHIFT: case ASHIFTRT: case LSHIFTRT:
>      case ROTATE: case ROTATERT:
>        /* These instructions always clear the overflow bit, and set
>           the carry to the bit shifted out.  */
>        /* ??? We don't currently have a way to signal carry not valid,
>           nor do we check for it in the branch insns.  */
>        CC_STATUS_INIT;
>        break;
>
> Andreas.
>
> --
> Andreas Schwab, SuSE Labs, schwab@suse.de
> SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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> "And now for something completely different."
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