Mac OS X should use -O3 optimization level to be consistent with other platforms and benefit Mac OS X java with the same performance levels as other platforms.

Mac OS X has used -Os level optimization since management of the platform JDK was handed over from Apple. This level of optimization was Apple's policy, not OpenJDK's. The policy was established in a time when memory was tight (I was there at the time.) Since then other platforms have moved on, making the decision to target speed over disk space. Specifically, using -O3 level optimization.

I've personally seen 20-30% improvement in performance in C/C++ applications using -O3 over -Os. But, of course, the performance team should verify the affect on the JDK.

A 11% real time improvement can be seen with a rough HelloWord compile/run.

>> time $JDK15Os/bin/java HelloWorld.java
Hello World!

real 0m0.385s
user 0m0.842s
sys 0m0.049s
>> time $JDK15Os/bin/java HelloWorld.java
Hello World!

real 0m0.368s
user 0m0.810s
sys 0m0.049s
>> time $JDK15Os/bin/java HelloWorld.java
Hello World!

real 0m0.382s
user 0m0.834s
sys 0m0.050s

>> time $JDK15O3/bin/java HelloWorld.java
Hello World!

real 0m0.365s
user 0m0.734s
sys 0m0.051s
>> time $JDK15O3/bin/java HelloWorld.java
Hello World!

real 0m0.363s
user 0m0.749s
sys 0m0.048s
>> time $JDK15O3/bin/java HelloWorld.java
Hello World!

real 0m0.367s
user 0m0.755s
sys 0m0.052s

The down side is about a 7% increasingly in the disk size of the JDK (520,941,898 bytes vs 486,650,895 bytes), but this is consistent with the difference on other platforms.

diff -r de8d5e5dd4e0 make/autoconf/flags-cflags.m4
--- a/make/autoconf/flags-cflags.m4 Sat Jun 06 10:05:20 2020 -0300
+++ b/make/autoconf/flags-cflags.m4 Mon Jun 08 09:52:41 2020 -0300
@@ -239,21 +239,11 @@
       C_O_FLAG_NONE="${C_O_FLAG_NONE} ${DISABLE_FORTIFY_CFLAGS}"
     fi
   elif test "x$TOOLCHAIN_TYPE" = xclang; then
- if test "x$OPENJDK_TARGET_OS" = xmacosx; then
- # On MacOSX we optimize for size, something
- # we should do for all platforms?
- C_O_FLAG_HIGHEST_JVM="-Os"
- C_O_FLAG_HIGHEST="-Os"
- C_O_FLAG_HI="-Os"
- C_O_FLAG_NORM="-Os"
- C_O_FLAG_DEBUG_JVM=""
- else
- C_O_FLAG_HIGHEST_JVM="-O3"
- C_O_FLAG_HIGHEST="-O3"
- C_O_FLAG_HI="-O3"
- C_O_FLAG_NORM="-O2"
- C_O_FLAG_DEBUG_JVM="-O0"
- fi
+ C_O_FLAG_HIGHEST_JVM="-O3"
+ C_O_FLAG_HIGHEST="-O3"
+ C_O_FLAG_HI="-O3"
+ C_O_FLAG_NORM="-O2"
+ C_O_FLAG_DEBUG_JVM="-O0"
     C_O_FLAG_SIZE="-Os"
     C_O_FLAG_DEBUG="-O0"
     C_O_FLAG_NONE="-O0"