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[llvm-mca][docs] Improve the "How LLVM-MCA works" section. 

llvm-svn: 338410
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Andrea Di Biagio 2018-07-31 18:19:15 +00:00
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28
llvm/docs/CommandGuide/llvm-mca.rst
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@ -287,10 +287,30 @@ for a total of 900 dynamically executed instructions.
The report is structured in three main sections. The first section collects a
few performance numbers; the goal of this section is to give a very quick
overview of the performance throughput. In this example, the two important
performance indicators are the predicted total number of cycles, and the IPC.
IPC is probably the most important throughput indicator. A big delta between
the Dispatch Width and the computed IPC is an indicator of potential
performance issues.
performance indicators are **IPC** and **Block RThroughput** (Block Reciprocal
Throughput).
IPC is computed dividing the total number of simulated instructions by the total
number of cycles. A delta between Dispatch Width and IPC is an indicator of a
performance issue. In the absence of loop-carried data dependencies, the
observed IPC tends to a theoretical maximum which can be computed by dividing
the number of instructions of a single iteration by the *Block RThroughput*.
IPC is bounded from above by the dispatch width. That is because the dispatch
width limits the maximum size of a dispatch group. IPC is also limited by the
amount of hardware parallelism. The availability of hardware resources affects
the resource pressure distribution, and it limits the number of instructions
that can be executed in parallel every cycle. A delta between Dispatch
Width and the theoretical maximum IPC is an indicator of a performance
bottleneck caused by the lack of hardware resources. In general, the lower the
Block RThroughput, the better.
In this example, ``Instructions per iteration/Block RThroughput`` is 1.50. Since
there are no loop-carried dependencies, the observed IPC is expected to approach
1.50 when the number of iterations tends to infinity. The delta between the
Dispatch Width (2.00), and the theoretical maximum IPC (1.50) is an indicator of
a performance bottleneck caused by the lack of hardware resources, and the
*Resource pressure view* can help to identify the problematic resource usage.
The second section of the report shows the latency and reciprocal
throughput of every instruction in the sequence. That section also reports