src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java

*** 955,996 ****
                      Exactness checks compare the final StringBuilder.capacity() with a resulting
                      String.length(). If these values disagree, that means StringBuilder had to perform
                      storage trimming, which defeats the purpose of exact strategies.
                   */
  
                  mv.visitInsn(DUP);
  
                  mv.visitMethodInsn(
                          INVOKEVIRTUAL,
                          "java/lang/StringBuilder",
                          "capacity",
                          "()I",
                          false
                  );
  
!                 mv.visitIntInsn(ISTORE, 0);
  
                  mv.visitMethodInsn(
                          INVOKEVIRTUAL,
                          "java/lang/StringBuilder",
                          "toString",
                          "()Ljava/lang/String;",
                          false
                  );
  
-                 mv.visitInsn(DUP);
- 
                  mv.visitMethodInsn(
                          INVOKEVIRTUAL,
                          "java/lang/String",
                          "length",
                          "()I",
                          false
                  );
  
-                 mv.visitIntInsn(ILOAD, 0);
- 
                  Label l0 = new Label();
                  mv.visitJumpInsn(IF_ICMPEQ, l0);
  
                  mv.visitTypeInsn(NEW, "java/lang/AssertionError");
                  mv.visitInsn(DUP);
--- 955,1008 ----
                      Exactness checks compare the final StringBuilder.capacity() with a resulting
                      String.length(). If these values disagree, that means StringBuilder had to perform
                      storage trimming, which defeats the purpose of exact strategies.
                   */
  
+                 /*
+                    The logic for this check is as follows:
+ 
+                      Stack before:     Op:
+                       (SB)              dup, dup
+                       (SB, SB, SB)      capacity()
+                       (int, SB, SB)     swap
+                       (SB, int, SB)     toString()
+                       (S, int, SB)      length()
+                       (int, int, SB)    if_icmpeq
+                       (SB)              <end>
+ 
+                    Note that it leaves the same StringBuilder on exit, like the one on enter.
+                  */
+ 
+                 mv.visitInsn(DUP);
                  mv.visitInsn(DUP);
  
                  mv.visitMethodInsn(
                          INVOKEVIRTUAL,
                          "java/lang/StringBuilder",
                          "capacity",
                          "()I",
                          false
                  );
  
!                 mv.visitInsn(SWAP);
  
                  mv.visitMethodInsn(
                          INVOKEVIRTUAL,
                          "java/lang/StringBuilder",
                          "toString",
                          "()Ljava/lang/String;",
                          false
                  );
  
                  mv.visitMethodInsn(
                          INVOKEVIRTUAL,
                          "java/lang/String",
                          "length",
                          "()I",
                          false
                  );
  
                  Label l0 = new Label();
                  mv.visitJumpInsn(IF_ICMPEQ, l0);
  
                  mv.visitTypeInsn(NEW, "java/lang/AssertionError");
                  mv.visitInsn(DUP);
*** 1001,1019 ****
                          "(Ljava/lang/Object;)V",
                          false);
                  mv.visitInsn(ATHROW);
  
                  mv.visitLabel(l0);
!             } else {
                  mv.visitMethodInsn(
                          INVOKEVIRTUAL,
                          "java/lang/StringBuilder",
                          "toString",
                          "()Ljava/lang/String;",
                          false
                  );
-             }
  
              mv.visitInsn(ARETURN);
  
              mv.visitMaxs(-1, -1);
              mv.visitEnd();
--- 1013,1031 ----
                          "(Ljava/lang/Object;)V",
                          false);
                  mv.visitInsn(ATHROW);
  
                  mv.visitLabel(l0);
!             }
! 
              mv.visitMethodInsn(
                      INVOKEVIRTUAL,
                      "java/lang/StringBuilder",
                      "toString",
                      "()Ljava/lang/String;",
                      false
              );
  
              mv.visitInsn(ARETURN);
  
              mv.visitMaxs(-1, -1);
              mv.visitEnd();