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GCC Clobbering Branch Predict Bit in CF3 Cores

Question asked by TomE on May 18, 2014
Latest reply on Apr 24, 2017 by TomE

I've searched for this apparent bug on gcc.gnu.org/bugzilla, but didn't find anything.

 

Many of those reading this will be using Code Warrior, and I assume this doesn't apply to you. I'd be interested in seeing the compiler's generated code for bit testing to see if it is every doing this though.

 

I'm using:

 

$ m68k-elf-gcc --version

m68k-elf-gcc.exe (Sourcery G++ Lite 4.3-208) 4.3.3

Copyright (C) 2008 Free Software Foundation, Inc.

 

The command line is:

 

m68k-elf-gcc -MD -MF adl3.d0 -gdwarf-2 -mcpu=5329 -Wall -std=c99 -g  -Os ...

 

Note the "-mcpu=5329" line.

 

A small snippet of source code:

 

#define cd_BUS_OFF 0x2000

#define cd_BUS_OFF 0x1000

#define cd_OVERRUN 0x0008

 

    else if (cc->status & cd_BUS_OFF) {

40109020:       202a 003c       movel %a2@(60),%d0

40109024:       0800 000d       btst #13,%d0

40109028:       6710            beqs 4010903a <comm_check_status+0x58>

(other code)

    } else if (cc->status & cd_BUS_RWARN) {

4010903a:       0800 000c       btst #12,%d0

4010903e:       6604            bnes 40109044 <comm_check_status+0x62>

    } else if (cc->status & cd_OVERRUN) {

40109040:       44c0            movew %d0,%ccr

40109042:       6a02            bpls 40109046 <comm_check_status+0x64>

 

Note the weird word-saving trick in the last compare? It copies the data to the CCR and then tests the "N" bit.

 

It only seems to do this trick for the "N" and "Z" bits (bits 2 and 3). it should be able to do it for "C" and "V" (0 and 1), but I haven't seen any code doing this. I've also seen code like:

 

4012193e:       44c2            movew %d2,%ccr

40121940:       57c0            seq %d0

40121942:       44c2            movew %d2,%ccr

40121944:       5bc1            smi %d1

 

What's the problem? Bit 7 of the CCR is documented in the CFPRM and MCF5329 Reference Manual as:

 

    Branch prediction (Version 3 only). Alters the static

    prediction algorithm used by the branch acceleration

    logic in the instruction fetch pipeline on forward

    conditional branches. Refer to a V3 core or device

    user’s manual for further information on this bit.

 

So the use of the "movew %d0,%ccr" instruction is changing the CPU's branch prediction randomly. It shouldn't be doing that.

 

Does anyone know if this was ever fixed?

 

Tom

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