[core] "Instruction removed from duplicated block ... please report this as an issue" on correct output

Issue details

A short-circuit if (a || b) { ... } whose body is a shared block with two predecessors gets that block marked DUPLICATED (RegionMaker.java#L62-L64). When that block also holds a StringBuilder chain in the dex2jar/R8 idiom (the builder kept in a local and reloaded for each append, with the append return popped), the later string-concat simplification removes the now-redundant astore/aload/pop, and InsnRemover.remove emits Instruction removed from duplicated block: ..., please report this as an issue (InsnRemover.java#L224).

The decompiled output is correct; the warning is a false positive that tells users to report a bug. It shows only on .class input from dex2jar (R8 -> dex -> dex2jar), not on the original dex or on javac output, since only that path both shares the block (the ||) and reloads the builder through a local.

Sample

Java equivalent:

static String f(boolean a, boolean b, String s) {
    if (a || b) {
        s = s + ".mp4";
    }
    return s;
}

Minimal input, no Android tooling (Jasmin; .class/.jar build with dex2jar's bundled d2j-jasmin2jar.sh):

.method public static f(ZZLjava/lang/String;)Ljava/lang/String;
    iload 0
    ifne L0            ; a -> L0
    aload 2
    astore 3
    iload 1
    ifeq L1            ; !b -> skip
  L0:                  ; shared block, two predecessors
    new java/lang/StringBuilder
    dup
    invokespecial java/lang/StringBuilder/<init>()V
    astore 3           ; builder in a local (dex2jar idiom)
    aload 3
    aload 2
    invokevirtual java/lang/StringBuilder/append(Ljava/lang/String;)Ljava/lang/StringBuilder;
    pop
    aload 3
    ldc ".mp4"
    invokevirtual java/lang/StringBuilder/append(Ljava/lang/String;)Ljava/lang/StringBuilder;
    pop
    aload 3
    invokevirtual java/lang/StringBuilder/toString()Ljava/lang/String;
    astore 3
  L1:
    aload 3
    areturn
  .limit locals 4
  .limit stack 2
.end method

Output on master (8c28a85), correct code with two spurious warnings:

/* JADX WARN: Code duplicated, block: B:6:0x000a  */
/* JADX WARN: Instruction removed from duplicated block: B:6:0x000a, please report this as an issue */
public static String f(boolean z, boolean z2, String str) {
    String str2;
    if (z) {
        str2 = str + ".mp4";
    } else {
        str2 = str;
        if (z2) {
            str2 = str + ".mp4";
        }
    }
    return str2;
}

Found in a public app: com.rubenmayayo.reddit (Boost for Reddit) 1.12.12, method rb.FileCacheManager.getFileNameFromUrl (after running the APK through dex2jar).

Jadx version

master 8c28a85 (current HEAD)