8234841: Enhance buffering of byte buffers

Reviewed-by: alanb, ahgross, rhalade, psandoz

Author: Brian Burkhalter

src/java.base/share/classes/java/nio/ByteBufferAs-X-Buffer.java.template

@@ -80,7 +80,6 @@ class ByteBufferAs$Type$Buffer$RW$$BO$                  // package-private
     public $Type$Buffer slice() {
         int pos = this.position();
         int lim = this.limit();
-        assert (pos <= lim);
         int rem = (pos <= lim ? lim - pos : 0);
         long addr = byteOffset(pos);
         return new ByteBufferAs$Type$Buffer$RW$$BO$(bb, -1, 0, rem, rem, addr, segment);

src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template

@@ -213,7 +213,6 @@ class Direct$Type$Buffer$RW$$BO$
     public $Type$Buffer slice() {
         int pos = this.position();
         int lim = this.limit();
-        assert (pos <= lim);
         int rem = (pos <= lim ? lim - pos : 0);
         int off = (pos << $LG_BYTES_PER_VALUE$);
         assert (off >= 0);

src/java.base/share/classes/java/nio/Heap-X-Buffer.java.template

@@ -105,13 +105,15 @@ class Heap$Type$Buffer$RW$
     }
 
     public $Type$Buffer slice() {
-        int rem = this.remaining();
+        int pos = this.position();
+        int lim = this.limit();
+        int rem = (pos <= lim ? lim - pos : 0);
         return new Heap$Type$Buffer$RW$(hb,
                                         -1,
                                         0,
                                         rem,
                                         rem,
-                                        this.position() + offset, segment);
+                                        pos + offset, segment);
     }
 
     @Override

src/java.base/share/classes/java/nio/StringCharBuffer.java

@@ -43,12 +43,15 @@ class StringCharBuffer                                  // package-private
     }
 
     public CharBuffer slice() {
+        int pos = this.position();
+        int lim = this.limit();
+        int rem = (pos <= lim ? lim - pos : 0);
         return new StringCharBuffer(str,
                                     -1,
                                     0,
-                                    this.remaining(),
-                                    this.remaining(),
-                                    offset + this.position());
+                                    rem,
+                                    rem,
+                                    offset + pos);
     }
 
     @Override