Re: Doubt : Help

[Ian Lance Taylor <ian at airs dot com>]

Sachin Vijay Sonawane <sachinvs@cse.iitb.ac.in> writes:

>   In .md file, while writing Function-unit specifications, we have to
> define an attribute called "simultaneity". I am still confused to get
> the correct meaning it. Can anyone plz elaborate on it? I have found
> the following definition from GCC manual.
> 
>   "simultaneity specifies the maximum number of insns that can be
> executing in each instance of the function unit simultaneously or zero
> if the unit is pipelined and has no limit."

First I'll note that all that define_function_unit is gone in the
current compiler, along with its documentation.  The new approved
approach is to use define_cpu_unit and define_insn_reservation.

That said, simultaneity is attempting to describe a feature of
processor functional units.  Consider a multiplier.  Let's say that
the multiplier takes several cycles to complete the operation.  What
happens if a second multiply instruction comes along while the
multiplier is working?  One option is that the second multiply
instruction stalls until the first one is complete (for example, the
XScale works this way).  In this case, the multiply unit has a
"simultaneity" of 1: one instruction may occupy the functional unit at
one time.  Another option is that the multiplier is fully pipelined,
and the second multiply instruction simply begins executing in the
pipeline (e.g., Pentium II, many other high end processors).  In this
case, the multiply unit has a "simultaneity" of 0, meaning that there
is no limit on the number of instructions which may occupy the
functional unit at one time.  It is of course possible to have two
non-pipelined multiplier units, in which case a second multiply
instruction would begin executing immediately, while a third one would
be stalled until one of them completed.  For example, I believe that
the Athlon has three floating point multiplication units, and
therefore has a "simultaneity" of 3.

Hope this helps.

Ian