IA-64 "instruction group" query...

IA-64 "instruction group" query...

[Kiran VMS]

Greetings,

I am just curious to know if the following instruction bundles are
valid ones. In particular, I would like to know if the instruction
group comprised of the slots # 1 ( 3621 ),  # 2 ( 3622 ) of the first
instruction bundle and slot # 0 ( 3630 ) is a VALID group. If you
notice, the register R4 is being read as well as modified by these
instructions belonging to the same group. Is it a potential RAW --
Read After Write -- violation?

(Assume that the traget processor is Itanium with its feature that it
can execute 2 bundles at a time)

                          { .mmi
010800458100     3620                 adds    r4 = 44, r4 ;;
008080400200     3621                 ld4     r8 = [r4]
010800400480     3622                 adds    r18 = 0, r4
                           }
                           { .mmi
0119F84A8100     3630                 adds    r4 = -44, r4 ;;
000008000000     3631                 nop.m   0              
0000B0800200     3632                 sxt4    r8 = r8 ;;
			   }


Instruction group of my concern 
;;
008080400200     3621                 ld4     r8 = [r4]
010800400480     3622                 adds    r18 = 0, r4
                           }
                           { .mmi
0119F84A8100     3630                 adds    r4 = -44, r4 
;;

Thanks for your time and attention

Re: IA-64 "instruction group" query...

[Christian Hildner]

Hi Kiran,

your code sequence contains a WAR dependency (add r4=-44,r4). But this 
in general is allowed inside the IA64 architecture and the pipeline 
takes care that the writeback is the last step and all reads occur 
before. So the instruction group (3621-3630) is ok and does not need an 
additional stop. You'll find dependency rules in detail in the "Intel 
IA-64 architecture software developer's manual" volume 1.

Christian

Kiran VMS schrieb:

>Greetings,
>
>I am just curious to know if the following instruction bundles are
>valid ones. In particular, I would like to know if the instruction
>group comprised of the slots # 1 ( 3621 ),  # 2 ( 3622 ) of the first
>instruction bundle and slot # 0 ( 3630 ) is a VALID group. If you
>notice, the register R4 is being read as well as modified by these
>instructions belonging to the same group. Is it a potential RAW --
>Read After Write -- violation?
>
>(Assume that the traget processor is Itanium with its feature that it
>can execute 2 bundles at a time)
>
>                          { .mmi
>010800458100     3620                 adds    r4 = 44, r4 ;;
>008080400200     3621                 ld4     r8 = [r4]
>010800400480     3622                 adds    r18 = 0, r4
>                           }
>                           { .mmi
>0119F84A8100     3630                 adds    r4 = -44, r4 ;;
>000008000000     3631                 nop.m   0              
>0000B0800200     3632                 sxt4    r8 = r8 ;;
>			   }
>
>
>Instruction group of my concern 
>;;
>008080400200     3621                 ld4     r8 = [r4]
>010800400480     3622                 adds    r18 = 0, r4
>                           }
>                           { .mmi
>0119F84A8100     3630                 adds    r4 = -44, r4 
>;;
>
>Thanks for your time and attention
>-
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