[llvm-mca][docs] Add documentation for the statistic outputs from mca. NFC

Summary: The original text was lifted from the MCA README.  I re-ran the dot-product example and updated the output seen in the docs.  I also added a few paragraphs discussing the instruction issued and retired histograms, as well as discussing the register file stats.

Reviewers: andreadb, RKSimon, courbet, gbedwell, filcab

Reviewed By: andreadb

Subscribers: tschuett

Differential Revision: https://reviews.llvm.org/D49614

llvm-svn: 337648

llvm/docs/CommandGuide/llvm-mca.rst

@@ -305,9 +305,9 @@ spent on average every iteration. The second table correlates the resource
 cycles to the machine instruction in the sequence. For example, every iteration
 of the instruction vmulps always executes on resource unit [6]
 (JFPU1 - floating point pipeline #1), consuming an average of 1 resource cycle
-per iteration.  Note that on Jaguar, vector floating-point multiply can only be
-issued to pipeline JFPU1, while horizontal floating-point additions can only be
-issued to pipeline JFPU0.
+per iteration.  Note that on AMD Jaguar, vector floating-point multiply can
+only be issued to pipeline JFPU1, while horizontal floating-point additions can
+only be issued to pipeline JFPU0.
 
 The resource pressure view helps with identifying bottlenecks caused by high
 usage of specific hardware resources.  Situations with resource pressure mainly
@@ -427,3 +427,125 @@ instructions.  When performance is mostly limited by the lack of hardware
 resources, the delta between the two counters is small.  However, the number of
 cycles spent in the queue tends to be larger (i.e., more than 1-3cy),
 especially when compared to other low latency instructions.
+
+Extra Statistics to Further Diagnose Performance Issues
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+The ``-all-stats`` command line option enables extra statistics and performance
+counters for the dispatch logic, the reorder buffer, the retire control unit,
+and the register file.
+
+Below is an example of ``-all-stats`` output generated by MCA for the
+dot-product example discussed in the previous sections.
+
+.. code-block:: none
+
+  Dynamic Dispatch Stall Cycles:
+  RAT     - Register unavailable:                      0
+  RCU     - Retire tokens unavailable:                 0
+  SCHEDQ  - Scheduler full:                            272
+  LQ      - Load queue full:                           0
+  SQ      - Store queue full:                          0
+  GROUP   - Static restrictions on the dispatch group: 0
+
+
+  Dispatch Logic - number of cycles where we saw N instructions dispatched:
+  [# dispatched], [# cycles]
+   0,              24  (3.9%)
+   1,              272  (44.6%)
+   2,              314  (51.5%)
+
+
+  Schedulers - number of cycles where we saw N instructions issued:
+  [# issued], [# cycles]
+   0,          7  (1.1%)
+   1,          306  (50.2%)
+   2,          297  (48.7%)
+
+
+  Scheduler's queue usage:
+  JALU01,  0/20
+  JFPU01,  18/18
+  JLSAGU,  0/12
+
+
+  Retire Control Unit - number of cycles where we saw N instructions retired:
+  [# retired], [# cycles]
+   0,           109  (17.9%)
+   1,           102  (16.7%)
+   2,           399  (65.4%)
+
+
+  Register File statistics:
+  Total number of mappings created:    900
+  Max number of mappings used:         35
+
+  *  Register File #1 -- JFpuPRF:
+     Number of physical registers:     72
+     Total number of mappings created: 900
+     Max number of mappings used:      35
+
+  *  Register File #2 -- JIntegerPRF:
+     Number of physical registers:     64
+     Total number of mappings created: 0
+     Max number of mappings used:      0
+
+If we look at the *Dynamic Dispatch Stall Cycles* table, we see the counter for
+SCHEDQ reports 272 cycles.  This counter is incremented every time the dispatch
+logic is unable to dispatch a group of two instructions because the scheduler's
+queue is full.
+
+Looking at the *Dispatch Logic* table, we see that the pipeline was only able
+to dispatch two instructions 51.5% of the time.  The dispatch group was limited
+to one instruction 44.6% of the cycles, which corresponds to 272 cycles.  The
+dispatch statistics are displayed by either using the command option
+``-all-stats`` or ``-dispatch-stats``.
+
+The next table, *Schedulers*, presents a histogram displaying a count,
+representing the number of instructions issued on some number of cycles.  In
+this case, of the 610 simulated cycles, single
+instructions were issued 306 times (50.2%) and there were 7 cycles where
+no instructions were issued.
+
+The *Scheduler's queue usage* table shows that the maximum number of buffer
+entries (i.e., scheduler queue entries) used at runtime.  Resource JFPU01
+reached its maximum (18 of 18 queue entries). Note that AMD Jaguar implements
+three schedulers:
+
+* JALU01 - A scheduler for ALU instructions.
+* JFPU01 - A scheduler floating point operations.
+* JLSAGU - A scheduler for address generation.
+
+The dot-product is a kernel of three floating point instructions (a vector
+multiply followed by two horizontal adds).  That explains why only the floating
+point scheduler appears to be used.
+
+A full scheduler queue is either caused by data dependency chains or by a
+sub-optimal usage of hardware resources.  Sometimes, resource pressure can be
+mitigated by rewriting the kernel using different instructions that consume
+different scheduler resources.  Schedulers with a small queue are less resilient
+to bottlenecks caused by the presence of long data dependencies.
+The scheduler statistics are displayed by
+using the command option ``-all-stats`` or ``-scheduler-stats``.
+
+The next table, *Retire Control Unit*, presents a histogram displaying a count,
+representing the number of instructions retired on some number of cycles.  In
+this case, of the 610 simulated cycles, two instructions were retired during
+the same cycle 399 times (65.4%) and there were 109 cycles where no
+instructions were retired.  The retire statistics are displayed by using the
+command option ``-all-stats`` or ``-retire-stats``.
+
+The last table presented is *Register File statistics*.  Each physical register
+file (PRF) used by the pipeline is presented in this table.  In the case of AMD
+Jaguar, there are two register files, one for floating-point registers
+(JFpuPRF) and one for integer registers (JIntegerPRF).  The table shows that of
+the 900 instructions processed, there were 900 mappings created.  Since this
+dot-product example utilized only floating point registers, the JFPuPRF was
+responsible for creating the 900 mappings.  However, we see that the pipeline
+only used a maximum of 35 of 72 available register slots at any given time. We
+can conclude that the floating point PRF was the only register file used for
+the example, and that it was never resource constrained.  The register file
+statistics are displayed by using the command option ``-all-stats`` or
+``-register-file-stats``.
+
+In this example, we can conclude that the IPC is mostly limited by data
+dependencies, and not by resource pressure.