[LLVMdev] Question on instruction itineraries

Miguel G miguel at esenciatech.com
Mon Aug 15 18:03:29 CDT 2011


Hi everyone

I'm fairly new with LLVM and I've been searching around but couldn't find
info on this subject.
I started working on a target for a new cpu and I realizing my initial
simple understanding of instruction itineraries may be completely off.
I'm trying to model a CPU that has a latency of 2 cycles for multiplications
fully pipelined (so it can start a new one after one cycle)
First of all, is there a document that describes the instruction itinerary
model in some detail ?

For example looking at MBlaze target MBlaeSchedule.td I can see something
like

MblazeSchedule.td
...
def IIImul             : InstrItinClass;
...
  InstrItinData<IIImul             , [InstrStage<17, [IMULDIV]>]>,


Does that mean Mul's are expected to have a latency of 17 clks ? Mips target
has something similar.
In Mblaze case I can see the result being used the very next cycle

        mul       *r3*, r6, r5
        addik     r3,* r3*, 4

similarly for my target (instead of 17 I'm specifying 2 above) and for Mips
I get the same result. Same for loads where I'm also specifying a larger
latency

What would be the right way to specify a latency of 2 with 1 clk initiation
interval for instance

Thanks
Miguel
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