I'm about to upgrade a tomcat installation from version 6.0 to version 8.5, and also upgrading the JVM from 6 to 8. Our java code is compiled with java 6 as for now.
I know that running java code on the latest JVM is always a good idea performance-wise (not to mention security-wise). In this way some code compiled with java 1.6 might run faster on JVM 8 compared to JVM 6.
But how about compilation? Does one gain anything by compiling code written in java 6 with a java 8 compiler (i.e. only java 6 features are used)? I.e. does code written with the intent of compiling with java 1.6 run faster on JVM 8 when the code is compiled with java 8 (targeted 8) and not java 6? Is the bytecode from java 8 optimized compared to java 6?
And regarding the target flag. Is anything gained by compiling java 6 code with a java 8 compiler targeted to java 6 (when the code is still to be run on JVM 6), compared to if the code was compiled with a java 6 compiler?
Is anything gained by compiling java 6 code with a java 8 compiler targeted to java 6 compared to if the code was compiled with a java 6 compiler?
No.
There is absolutely no performance gain, since the java compiler don't perform any kind of optimization, not even obvious ones.
Any optimization that could be performed on the bytecode produced by javac is done by the JIT engine once your application is running.
Edit: Look at pveentjer comment, a few simple optimizations (e.g. constant resolution) are performed by javac. But i'm still confident that the answer regarding the differences between 6 and 8 is still "no".