[PATCH] D14694: [ARM] Prevent use of a value pointed by end() iterator when placing a jump table

[James Molloy via llvm-commits]

LGTM!
On Mon, 16 Nov 2015 at 08:37, Petr Pavlu via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> petpav01 created this revision.
> petpav01 added a subscriber: llvm-commits.
> Herald added subscribers: rengolin, aemerson.
>
> 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()`.
>
> The problem can be shown by compiling the following example with `clang
> --target=armv8a-none-none-eabi -O3`:
> ```
> void v0();
> void v1();
> void v2();
> void v3();
> void v4();
> int a;
>
> void test(void) {
>   switch (a) {
>     default:
>       v0();
>       break;
>     case 1:
>       v1();
>       break;
>     case 2:
>       v2();
>       break;
>     case 3:
>       v3();
>       break;
>     case 4:
>       v4();
>       break;
>   }
>   try {
>     throw 0;
>   } catch (int) {
>   }
> }
> ```
>
> http://reviews.llvm.org/D14694
>
> Files:
>   lib/Target/ARM/ARMConstantIslandPass.cpp
>
> Index: lib/Target/ARM/ARMConstantIslandPass.cpp
> ===================================================================
> --- lib/Target/ARM/ARMConstantIslandPass.cpp
> +++ lib/Target/ARM/ARMConstantIslandPass.cpp
> @@ -589,6 +589,8 @@
>    MachineBasicBlock *LastCorrectlyNumberedBB = nullptr;
>    for (MachineBasicBlock &MBB : *MF) {
>      auto MI = MBB.getLastNonDebugInstr();
> +    if (MI == MBB.end())
> +      continue;
>
>      unsigned JTOpcode;
>      switch (MI->getOpcode()) {
>
>
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