[llvm-dev] Instruction Itineraries: question about operand latencies

Ehsan Amiri via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 6 15:10:23 PDT 2016

Hi Phil

There are some comments in "include/llvm/Target/TargetItinerary.td" where
class InstrItinData is defined.

 B is the number of cycles after issue where the first operand of the
instruction is defined. A is the number of cycles that the instruction will
stay in that particular stage in the pipeline. So for simple cases, like
your example, one would expect that A and B should have the same value.But
there is different API for accessing to A and B.

An example of accessing to B in the source code can be found here:
PPCInstrInfo::getInstrLatency. You can also look at getStageLatency in
include/llvm/MC/MCInstrItineraries.h. From this two you can probably find
other relevant places.

Hope this helps

On Mon, Jun 6, 2016 at 2:37 PM, Phil Tomson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> In our architecture loads from certain memory locations take a long time
> to complete (on the order of 150 clock cycles). Since we don't have a way
> to tell at compile time if the address being loaded from lies in slow or
> fast memory, I've gone ahead and made all of the load numbers high, such as:
>   InstrItinData< II_LOAD1,     [InstrStage<150, [AGU]>]>,
> However, I see that there is another field which I haven't specified where
> operand latencies are specified.  Here's an example from
> ARMScheduleA8.td:
>   InstrItinData<IIC_iALUi ,[InstrStage<1, [A8_Pipe0, A8_Pipe1]>], [2, 2]>,
> Now I'm wondering if Instead of what I had above, I should instead have
> specified:
>   InstrItinData< II_LOAD1,     [InstrStage<150, [AGU]>],[150,1,1]>,
> ?
> but is that first '150' parameter there redundant? Since it's specified in
> the operand latency list ([150,1,1] - the first element of that array being
> the latency for the output)?
> To clarify, for values of  'A' and 'B' below:
>   InstrItinData< II_LOAD1,     [InstrStage<A, [AGU]>], [B,1,1]>,
> ...what is the difference in the meaning for 'A' and 'B'? Are they
> essentially the same value since only one functional unit is specified?
> ([AGU])
> Phil
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> llvm-dev at lists.llvm.org
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