another difference in gcc (2v) ?

[Gabriel Paubert]

On Wed, 11 Apr 2001, Kevin B. Hendricks wrote:

>
> Hi,
>
> Lost in the huge diff was one other change, this time in the actual code
> itself.
>
> Can anyone tell me if this code resequence is legal?  The diff (-) lines
> show the nonworking disassembled code (generated by -O2) while the
> diff's (+) lines show the working code (generated by -O2
> -fno-schedule-insns -fno-schedule-insns2.
>
> Notice that when correct, the "cmpwi r3,0" code comes right before the
> branch ("beq") while in the non-working code it has been resequenced to
> come before lots of other instructions that may be setting a condition
> register value (subf and srawi).
>
> Is this a correct resequence?  I seem to remember something about a "."
> being added to some assmebler instructions to indicate that it impacts a
> condition register yet I see no indication of it in this sequence.

The reordering looks perfectly valid. You are right, the only instructions
that affect condition codes and do not end in a dot are the compare
instructions.

For most other arithmetic/logic instructions you have the choice of
updating cr0 or not. For a very small subset of instructions, cr0 is
always updated (andi., andis., and stwcx./stdcx.), note that all these
mnemonics end in a dot, and they have no dotless form.

Unless you've found a processor bug, I doubt this is the problem.

>
> -     1c4:      80 61 00 38     lwz     r3,56(r1)
> -     1c8:      80 01 00 44     lwz     r0,68(r1)
> -     1cc:      2c 03 00 00     cmpwi   r3,0
> -     1d0:      7c 03 00 50     subf    r0,r3,r0
> -     1d4:      7c 00 16 70     srawi   r0,r0,2
> +     1c4:      80 01 00 44     lwz     r0,68(r1)
> +     1c8:      80 61 00 38     lwz     r3,56(r1)
> +     1cc:      7c 03 00 50     subf    r0,r3,r0
> +     1d0:      7c 00 16 70     srawi   r0,r0,2
> +     1d4:      2c 03 00 00     cmpwi   r3,0
>        1d8:      41 82 00 14     beq     1ec
> <configmgr::configapi::implGetHiera\
> rchicalName(configmgr::configapi::NodeAccess &)+0x1e8>

	Regards,
	Gabriel.


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