llvm-project

5a32fd3e - [lldb][NFCI] Rewrite UnwindAssemblyInstEmulation in terms of a CFG visit (#169630)

[lldb][NFCI] Rewrite UnwindAssemblyInstEmulation in terms of a CFG visit (#169630) Currently, UnwindAssemblyInstEmulation visits instructions in the order in which they appear in a function. This commit makes an NFCI change to UnwindAssemblyInstEmulation so that it follows the function's CFG: 1. The first instruction is enqueued. 2. While the queue is not empty: 2.1 Visit the instruction in the *back* queue to compute the new unwind state. 2.2 Push(+) the next instruction to the *back* of the queue. 2.3 If the instruction is a forward branch with a known branch target, push(+) the destination instruction to the *front* of the queue. (+) Only push if this instruction hasn't been enqueued before. (+) When pushing an instruction, the current unwind state is attached to it. Note that: * the "next instruction" is pushed to the *back* of the queue, * a branch target is pushed to the *front* of the queue, and * we always dequeue from the *back* of the queue. This means that consecutive instructions are visited one after the other; this is important to support "conditional blocks" [1] of instructions (see the line with "if last_condition != new_condition"). This is arguably a very Thumb specific thing, so maybe it shouldn't be in the generic algorithm; that said, it is already in the code, so we have to support it. The main reason this patch is NFCI and not NFC is that, now, the destination of a forward branch is visited in a slightly different moment than before. This should not cause any changes in output, as if a branch destination is reachable through two different paths, any well behaved compiler will generate the same unwind state in both paths. The motivation for this patch is to change step 2.2 so that it _only_ pushes the next instruction if the current instruction is not an unconditional branch / return, and to change step 2.3 so that backwards branches are also allowed, fixing the bug described by [2]. [1]: https://developer.arm.com/documentation/dui0473/m/arm-and-thumb-instructions/it [2]: https://github.com/llvm/llvm-project/pull/168398 Part of a sequence of PRs: [lldb][NFCI] Rewrite UnwindAssemblyInstEmulation in terms of a CFG visit #169630 [lldb][NFC] Rename forward_branch_offset to branch_offset in UnwindAssemblyInstEmulation #169631 [lldb] Add DisassemblerLLVMC::IsBarrier API #169632 [lldb] Handle backwards branches in UnwindAssemblyInstEmulation #169633 commit-id:dce6b515