Re: another difference in gcc (2v) ?

[Jerry Van Baren <vanbaren_gerald@si.com>]

I don't see any problem with the scheduled version.  The cmpwi mnemonic
doesn't use the "." because it always updates the condition register
(yes, they are inconsistent mnemonics).  The subf and srawi don't
affect the CR (no "." in them), so the CR is still valid when the
branch is evaluated.

gvb


At 10:15 AM 4/11/01 -0400, 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.
>
>-     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>
>
>
>Thanks,
>
>Kevin
>
>


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