Eliminate redundant memcpy operation in jni_GetStringUTFRegion

###@###.### wrote:
> I also found that a memory allocation and a memory copying can be
> removed from the following function defined in
> hotspot/src/share/vm/prims/jni.cpp. The function was a source of the
> MISALIGN.MEM.REF events and the events could not be decreased with the
> above macro:
>
> JNI_ENTRY(void, jni_GetStringUTFRegion(JNIEnv *env, jstring string,
> jsize start, jsize len, char *buf))
> JNIWrapper("GetStringUTFRegion");
> DTRACE_PROBE5(hotspot_jni, GetStringUTFRegion__entry, env, string,
> start, len, buf);
> DT_VOID_RETURN_MARK(GetStringUTFRegion);
> oop s = JNIHandles::resolve_non_null(string);
> int s_len = java_lang_String::length(s);
> if (start < 0 || len < 0 || start + len > s_len) {
> THROW(vmSymbols::java_lang_StringIndexOutOfBoundsException());
> } else {
> //%note jni_7
> if (len > 0) {
> ResourceMark rm(THREAD);
> char *utf_region = java_lang_String::as_utf8_string(s, start,
> len);
> int utf_len = (int)strlen(utf_region);
> memcpy(buf, utf_region, utf_len);
> buf[utf_len] = 0;
> } else {
> // JDK null-terminates the buffer even in len is zero
> if (buf != NULL) {
> buf[0] = 0;
> }
> }
> }
> JNI_END
>
> The java_lang_String::as_utf8_string function allocates a temporary
> buffer in which result of utf8 encoding is put. The function returns the
> allocated buffer. The data from the temporary buffer is copied into the
> specified buffer. After all the temporary buffer is freed. I wrote the
> java_lang_String::copy_as_utf8_string function which writes results of
> encoding directly into the specified buffer. This optimization reduces a
> number of CPU_CLK/JOP from 3883 to 1540, a path length/JOP - from 1580
> to 793.

Sample source is attached.