llvm dev - Dec 2017 - Question about pickTracePred() in MachineTraceMetrics

Hi all,

I have a question about MachineTraceMetrics.

The definition of depth of block in MachineTraceMetrics is defined as
the accumulated number of instructions in the trace above the block.
Does not include instructions in this block.

In MachineTraceMetrics::Ensemble::computeDepthResources(), we could
see the definition when computing depth of a basic block as follow.

TBI->InstrDepth = PredTBI->InstrDepth + PredFBI->InstrCount;

TBI is current basic block's trace info.
PredTBI is predecessor's trace info.
PredFBI is predecessor's fixed trace info.
That is, current block depth is summation of predecessor's depth and
predecessor's instruction count.

However, in MinInstrCountEnsemble::pickTracePred(), it uses
predecessor's depth plus current basic block's instruction count as
depth to pick the smallest one. Why? The summation is not aligned with
the definition.
Is there any reason to do so or is it a bug in MinInstrCountEnsemble?

Thanks in advance.

Hi stoklund,

I found that MachineTraceMetrics is created by you. I think it is a
bug in MachineTraceMetrics. Could you help me to review it? I have
created a patch for it.

https://reviews.llvm.org/D41572

In addition, I created a slide to write down some notes about
MachineTraceMetrics. I think it is helpful for someone interested in
MachineTraceMetrics implementation. Welcome to give me comments about
the slide.

https://www.slideshare.net/chimerawang/machine-trace-metrics-84887214

Thanks in advance,
Hsiangkai

On Thu, Dec 21, 2017 at 11:30 AM, Hsiangkai Wang <hsiangkai at gmail.com>
wrote:
> Hi all,
>
> I have a question about MachineTraceMetrics.
>
> The definition of depth of block in MachineTraceMetrics is defined as
> the accumulated number of instructions in the trace above the block.
> Does not include instructions in this block.
>
> In MachineTraceMetrics::Ensemble::computeDepthResources(), we could
> see the definition when computing depth of a basic block as follow.
>
> TBI->InstrDepth = PredTBI->InstrDepth + PredFBI->InstrCount;
>
> TBI is current basic block's trace info.
> PredTBI is predecessor's trace info.
> PredFBI is predecessor's fixed trace info.
> That is, current block depth is summation of predecessor's depth and
> predecessor's instruction count.
>
> However, in MinInstrCountEnsemble::pickTracePred(), it uses
> predecessor's depth plus current basic block's instruction count as
> depth to pick the smallest one. Why? The summation is not aligned with
> the definition.
> Is there any reason to do so or is it a bug in MinInstrCountEnsemble?
>
> Thanks in advance.