If you run a simple benchmark like this:

    private String s;
    private int idx;

    @Setup
    public void setup() {
        idx = 13;
        s = "Improve the scope of range check optimizations";
    }

    @Benchmark
    public void test_charAt() {
        trampoline_charAt();
    }

    @CompilerControl(CompilerControl.Mode.DONT_INLINE)
    public char trampoline_charAt() {
        return s.charAt(idx);
    }

...then you will see this generated code with the latest jdk9-dev:

 mov %eax,-0x14000(%rsp)
 push %rbp
 sub $0x20,%rsp
 mov 0xc(%rsi),%r11d ; get field $idx
 mov 0x10(%rsi),%r10d ; get field $s
 mov 0xc(%r12,%r10,8),%r9d ; get field $s.value
 test %r11d,%r11d
 jl 0x00007f6204b5cc19 ; range check 1, jump if ($idx < 0)
 mov 0xc(%r12,%r9,8),%ebp ; get $s.value.arraylength
 cmp %ebp,%r11d
 jge 0x00007f6204b5cc31 ; range check 2, jump if ($idx >= arraylength)
 cmp %ebp,%r11d ; <--- REDUNDANT COMPARE
 jae 0x00007f6204b5cc03 ; jump if overflow?
 lea (%r12,%r9,8),%r10 ; unpack $s.value array reference
 movzwl 0x10(%r10,%r11,2),%eax ; get $s.value[$idx]
 add $0x20,%rsp ; epilogue and return
 pop %rbp
 test %eax,0x15f1a3fe(%rip)
 retq

As you can see, we do double compare on the same arguments, even though jumps are not affecting the flags, and there seem to be no incoming branches to the second cmp.