Various methods in libjava use this macro in io_util_md.h:

#define GET_FD(this, fid) \
    (*env)->GetObjectField(env, (this), (fid)) == NULL ? \
        -1 : (*env)->GetIntField(env, (*env)->GetObjectField(env, (this), (fid)), IO_fd_fdID)

Problem: compilers can't fold the repeated GetObjectField into one call

Solution: Turn the macros into functions which call GetObjectField only once:

FD getFD(JNIEnv *env, jobject this, jfieldID fid) {
  jobject fd = (*env)->GetObjectField(env, this, fid);
  if (fd == NULL) {
    return -1;
  }
  return (*env)->GetIntField(env, fd, IO_fd_fdID);
}

On existing micro RandomAccessRead:

Before:
Benchmark (fileSize) Mode Cnt Score Error Units
RandomAccessRead.test 1000000 avgt 25 0.832 ± 0.001 us/op

After:
Benchmark (fileSize) Mode Cnt Score Error Units
RandomAccessRead.test 1000000 avgt 25 0.771 ± 0.005 us/op

~60 ns/op reduction, or an 8% speed-up. (Relative gain diminishes for larger operations, e.g., reading an 8kb buffer).

Startup label added since this has an effect on opening and reading small entries from jar/zip files.