While playing with the new compiler memory statistic, I saw that +StressRecompile causes a ~x2 increase in memory used during compilations. That confused me since I would have thought a recompilation starts memory-wise with a clean slate.

----

Baseline: highest peak during compilation is ~60 MB, caused by compilation of java/lang/invoke/LambdaForm$Kind::<clinit> with ~18k nodes.

`./images/jdk/bin/java -XX:CompileCommand='collectmemstat,*.*' -XX:-StressRecompilation -Xcomp -Xbatch -cp $REPROS_JAR de.stuefe.repros.Simple`


NMT:

```
- Compiler (reserved=275KB, committed=275KB)
                            (malloc=79KB #30) (peak=93KB #35)
                            (arena=196KB #4) (peak=59292KB #11) << peak
```


Compiler.memory:

```
total NA RA #nodes time type #rc thread method
(3/1425)
59063K 13406K 40025K 18345 6,174 c2 2 0x00007fdc2c28e7b0 java/lang/invoke/LambdaForm$Kind::<clinit>(()V)
```

----

StressRecompile: highest peak during compilation is ~112 MB, same method, sameish node count.

`./images/jdk/bin/java -XX:CompileCommand='collectmemstat,*.*' -XX:+StressRecompilation -Xcomp -Xbatch -cp $REPROS_JAR de.stuefe.repros.Simple`


NMT:

```
- Compiler (reserved=289KB, committed=289KB)
                            (malloc=93KB #35) (peak=107KB #40)
                            (arena=196KB #4) (peak=115628KB #11) << peak
```

Compiler.memory:

```
total NA RA #nodes time type #rc thread method
(16/1434)
112M 13469K 94987K 18791 16,451 c2 2 0x00007f790428e7b0 java/lang/invoke/LambdaForm$Kind::<clinit>(()V)

```

----

60M -> 112M. In both cases, a large part of the memory was allocated in resource areas. Its relative size compared to the total spike size increased.

Therefore it looks to me like we accrue a lot of memory in ResourceArea over recompilations.

As a test, I artificially increased the number of recompilations even further. As expected, memory footprint went up further.