[llvm] r253222 - [ARM] Prevent use of a value pointed by end() iterator when placing a jump table

[Petr Pavlu via llvm-commits]

When I add the suggested test assertion, the following tests fail because they hit it:
Failing Tests (4):
    LLVM :: CodeGen/ARM/2008-04-11-PHIofImpDef.ll
    LLVM :: CodeGen/ARM/2011-12-19-sjlj-clobber.ll
    LLVM :: CodeGen/ARM/eh-dispcont.ll
    LLVM :: CodeGen/ARM/sjljehprepare-lower-empty-struct.ll

> From: David Blaikie [mailto:dblaikie at gmail.com] 
> Sent: 16 November 2015 16:58
> To: Petr Pavlu
> Cc: llvm-commits
> Subject: Re: [llvm] r253222 - [ARM] Prevent use of a value pointed by end() iterator when placing a jump table
>
> Is this covered by existing tests? (if I replaced the if/continue with assert (!= end) would it fire on the existing regression suite?)
>
> On Mon, Nov 16, 2015 at 8:41 AM, Petr Pavlu via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> Author: petr.pavlu
> Date: Mon Nov 16 10:41:13 2015
> New Revision: 253222
>
> URL: http://llvm.org/viewvc/llvm-project?rev=253222&view=rev
> Log:
> [ARM] Prevent use of a value pointed by end() iterator when placing a jump table
>
> Function ARMConstantIslands::doInitialJumpTablePlacement() iterates over all
> basic blocks in a machine function. It calls `MI = MBB.getLastNonDebugInstr()`
> to get the last instruction in each block and then uses MI->getOpcode() to
> decide what to do. If getLastNonDebugInstr() returns MBB.end() (for example,
> when the block does not contain any instructions) then calling getOpcode() on
> this value is incorrect. Avoid this problem by checking the result of
> getLastNonDebugInstr().
>
> Differential Revision: http://reviews.llvm.org/D14694
>
> Modified:
>    llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp
>
> Modified: llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp?rev=253222&r1=253221&r2=253222&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp (original)
> +++ llvm/trunk/lib/Target/ARM/ARMConstantIslandPass.cpp Mon Nov 16 10:41:13 2015
> @@ -589,6 +589,8 @@ void ARMConstantIslands::doInitialJumpTa
>   MachineBasicBlock *LastCorrectlyNumberedBB = nullptr;
>   for (MachineBasicBlock &MBB : *MF) {
>     auto MI = MBB.getLastNonDebugInstr();
> +    if (MI == MBB.end())
> +      continue;
>
>     unsigned JTOpcode;
>     switch (MI->getOpcode()) {
>