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Specialization Histogram

This guide explains how to use the --engine.SpecializationStatistics option.

The specialization histogram requires Truffle DSL nodes to be generated in a special way. So if you use the plain specialization histogram option it will just print the following:

js --engine.SpecializationStatistics test.js

[engine] Specialization histogram:
No specialization statistics data was collected. Either no node with @Specialization annotations was executed or the interpreter was not compiled with -Atruffle.dsl.GenerateSpecializationStatistics=true e.g as parameter to the javac tool.

Follow the advice of the error and recompile our interpreter. For mx users this is as simple as:

mx build -c -A-Atruffle.dsl.GenerateSpecializationStatistics=true

After the rebuild, the specialization statistics are ready to be used. Make sure that your IDE does not recompile the sources automatically in the meantime. In this tutorial, a simple test.js script will be used:

function test() {
  var array = [42, "", {}, []]

  var globalVar = true;
  for (element of array) {
    globalVar = element;
  }
}
test();

Now the specialization statistics need to be enabled, in this example using the JavaScript launcher of GraalVM:

js --experimental-options --engine.SpecializationStatistics test.js

After the script is executed a histogram for each class will be printed. The histograms will be ordered by the sum of executions of each node, whereas the most frequently used node class will be printed last.

These are some of the histograms printed when executing test.js: (Note: The output is likely already outdated.)

 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Name                                                                         Instances          Executions     Executions per instance
 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| JSWriteCurrentFrameSlotNodeGen                                               8 (17%)            18 (12%)        Min=         1 Avg=        2.25 Max=          5  MaxNode= test.js~5-7:76-128
|   doBoolean <boolean>                                                          1 (13%)             1 (6%)         Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~4:52-71
|   doInt <int>                                                                  1 (13%)             1 (6%)         Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~5-7:76-128
|   doSafeIntegerInt                                                             0 (0%)              0 (0%)         Min=         0 Avg=        0.00 Max=          0  MaxNode=  -
|   doSafeInteger                                                                0 (0%)              0 (0%)         Min=         0 Avg=        0.00 Max=          0  MaxNode=  -
|   doLong                                                                       0 (0%)              0 (0%)         Min=         0 Avg=        0.00 Max=          0  MaxNode=  -
|   doDouble                                                                     0 (0%)              0 (0%)         Min=         0 Avg=        0.00 Max=          0  MaxNode=  -
|   doObject                                                                     7 (88%)            16 (89%)        Min=         1 Avg=        2.29 Max=          5  MaxNode= test.js~5-7:76-128
|     <DynamicObjectBasic>                                                         6 (86%)            12 (75%)        Min=         1 Avg=        2.00 Max=          5  MaxNode= test.js~5-7:76-128
|     <IteratorRecord>                                                             1 (14%)             1 (6%)         Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~1-8:16-130
|     <String>                                                                     2 (29%)             2 (13%)        Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~5-7:76-128
|     <Integer>                                                                    1 (14%)             1 (6%)         Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~6:105-123
|   --------------------------------------------------------------------------------------------------------------------------------------------------------------------
|   [doBoolean]                                                                  1 (13%)             1 (6%)         Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~4:52-71
|   [doInt, doObject]                                                            1 (13%)             4 (22%)        Min=         4 Avg=        4.00 Max=          4  MaxNode= test.js~5-7:76-128
|     doInt                                                                        1 (100%)            1 (25%)        Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~5-7:76-128
|     doObject                                                                     1 (100%)            3 (75%)        Min=         3 Avg=        3.00 Max=          3  MaxNode= test.js~5-7:76-128
|   [doObject]                                                                   6 (75%)            13 (72%)        Min=         1 Avg=        2.17 Max=          5  MaxNode= test.js~5-7:76-128
 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Name                                                                         Instances          Executions     Executions per instance
 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| JSReadCurrentFrameSlotNodeGen                                                8 (17%)            25 (17%)        Min=         1 Avg=        3.13 Max=          5  MaxNode= test.js~5-7:76-128
|   doBoolean                                                                    0 (0%)              0 (0%)         Min=         0 Avg=        0.00 Max=          0  MaxNode=  -
|   doInt <no-args>                                                              1 (13%)             1 (4%)         Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~5:81-87
|   doDouble                                                                     0 (0%)              0 (0%)         Min=         0 Avg=        0.00 Max=          0  MaxNode=  -
|   doObject <no-args>                                                           8 (100%)           24 (96%)        Min=         1 Avg=        3.00 Max=          5  MaxNode= test.js~5-7:76-128
|   doSafeInteger                                                                0 (0%)              0 (0%)         Min=         0 Avg=        0.00 Max=          0  MaxNode=  -
|   --------------------------------------------------------------------------------------------------------------------------------------------------------------------
|   [doInt, doObject]                                                            1 (13%)             4 (16%)        Min=         4 Avg=        4.00 Max=          4  MaxNode= test.js~5:81-87
|     doInt                                                                        1 (100%)            1 (25%)        Min=         1 Avg=        1.00 Max=          1  MaxNode= test.js~5:81-87
|     doObject                                                                     1 (100%)            3 (75%)        Min=         3 Avg=        3.00 Max=          3  MaxNode= test.js~5:81-87
|   [doObject]                                                                   7 (88%)            21 (84%)        Min=         1 Avg=        3.00 Max=          5  MaxNode= test.js~5-7:76-128
 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------

The histogram prints two inner tables for every node class.

The first table groups specialization and dynamic type combination. For example, in this histogram the node class JSWriteCurrentFrameSlotNodeGen was instantiated 8 and executed 18 times. This is 20% of the total instances and 11% of all node executions of the run.

Three specializations were instantiated in this script, namely doBoolean, doObject, and doInt. The doBoolean specialization was instantiated and executed only once which accounts for 13% of all instances and 6% of all executions of this node class. The doObject specializations was invoked using three different input value combinations: DynamicObjectBasic, IteratorRecord, and String. Similar to specializations, we can see the numbers of times per node they were used and how many times they were executed. For each line you can see minimum, average, and maximum execution numbers per instance. The last column prints the source section of the instance with the maximum executions.

The second table groups for each combination of specializations that were used by node class.

Here are some questions you would want to ask these specialization statistics:

1. Is a certain specialization combination used only rarely and can it be removed/consolidated into a single specialization?
2. Is there a specialization with a very common type combination that could benefit from further specialization?
3. Which specialization combination is common and could deserve its own specialization? This could indicate common polymorphism in the code that could be investigated.
4. What are common specializations, and does the order match the number of executions? Specializations that are most commonly used should be ordered first in the node class. This may lead to improvements in interpreter performance.
5. Are there unexpected specializations instantiated? If yes, investigate further using the printed source section.
6. Which specializations are instantiated often, and should therefore be optimized for memory footprint?
7. Were there nodes with the name Uncached in the profile? The use of uncached nodes should be rare. If they were used often, it can be worthwhile to dig deeper to see why.