unreachable branch in codegen for switch statement

[Lukas Stadler]

The problem in this particular case is that a switch with a default target is not recognized as a non-fall-through instruction in LIRGenerator.endsWithJump().
I’ve just pushed some changes that get rid of this unnecessary jump, by taking into account the (somewhat ill-named) FallThroughOps.

There’s still a problem left - occasionally, the a SwitchOp emits useless jumps to the next instruction, because the control flow optimization logic doesn’t know how to optimize blocks ending in switches.
Getting rid of this will require a bit more refactoring.

- Lukas

On 06 Nov 2013, at 3:22, Venkatachalam, Vasanth <Vasanth.Venkatachalam at amd.com> wrote:

> Doug or others-
> 
> When I ran the test case below through the AMD64 backend, I noticed an unreachable branch in the code generated below.  Namely, the jmp instruction at 0x00007facc8145a60.  It appears this is coming from the call to emitJump( ) in LIRGenerator.doBlock( ), which appends a jump instruction to the end of each basic block, which may later get optimized away by the reg. allocator.  Is there a reason why this jmp isn't being optimized away? Is there any way around this?
> I tried -G:+RemoveNeverExecutedCode but this isn't making a difference.
> 
> I'm hitting the same issue when I generate HSAIL code for this switch statement, but I get an assembler error due to this unreachable branch.
> 
> public int switchTest(int n) {
>        switch (n) {
>            case 0:
>                return 0;
>            case 1:
>                return 1;
>            case 2:
>                return 2;
>            default:
>                return 3;
>        }
> }
> 
> 0x00007facc8145a40: cmp    edx,0x0
>  0x00007facc8145a43: je     0x00007facc8145a6b
>  0x00007facc8145a49: cmp    edx,0x1
>  0x00007facc8145a4c: je     0x00007facc8145a72
>  0x00007facc8145a52: cmp    edx,0x2
>  0x00007facc8145a55: je     0x00007facc8145a79
>  0x00007facc8145a5b: jmp    0x00007facc8145a65
>  0x00007facc8145a60: jmp    0x00007facc8145a6b //unreachable code
>  0x00007facc8145a65: mov    eax,0x3
>  0x00007facc8145a6a: ret
>  0x00007facc8145a6b: mov    eax,0x0            //case 0
>  0x00007facc8145a70: jmp    0x00007facc8145a6a
>  0x00007facc8145a72: mov    eax,0x1            //case 1
>  0x00007facc8145a77: jmp    0x00007facc8145a6a
>  0x00007facc8145a79: mov    eax,0x2            //case 2
>  0x00007facc8145a7e: jmp    0x00007facc8145a6a