This patch is the Part-1 (FE Clang) implementation of HW Exception handling.
This new feature adds the support of Hardware Exception for Microsoft Windows SEH (Structured Exception Handling). This is the first step of this project; only X86_64 target is enabled in this patch. The plan will add AArch64 & ARM and X86 targets later.
For clang-cl.exe, the option is -EHa, the same as MSVC.
For clang.exe, the extra option is -feh-asynch, plus -triple x86_64-windows -fexceptions and -fcxx-exceptions as usual.
The rules for C code:
For C-code, one way (MSVC approach) to achieve SEH -EHa semantic is to follow three rules. First, no exception can move in or out of _try region., i.e., no "potential faulty instruction can be moved across _try boundary. Second, the order of exceptions for instructions 'directly' under a _try must be preserved (not applied to those in callees). Finally, global states (local/global/heap variables) that can be read outside of _try region must be updated in memory (not just in register) before the subsequent exception occurs.
The impact to C++ code:
Although SEH is a feature for C code, -EHa does have a profound effect on C++ side. When a C++ function (in the same compilation unit with option -EHa ) is called by a SEH C function, a hardware exception occurs in C++ code can also be handled properly by an upstream SEH _try-handler or a C++ catch(…). As such, when that happens in the middle of an object’s life scope, the dtor must be invoked the same way as C++ Synchronous Exception during unwinding process.
A natural way to achieve the rules above in LLVM today is to allow an EH edge added on memory/computation instruction (previous iload/istore idea) so that exception path is modeled in Flow graph preciously. However, tracking every single memory instruction and potential faulty instruction can create many Invokes, complicate flow graph and possibly result in negative performance impact for downstream optimization and code generation. Making all optimizations be aware of the new semantic is also substantial.
This design does not intend to model exception path at instruction level. Instead, the proposed design tracks and reports EH state at BLOCK-level to reduce the complexity of flow graph and minimize the performance-impact on CPP code under -EHa option.
One key element of this design is the ability to compute State number at block-level. Our algorithm is based on the following rationales:
A _try scope is always a SEME (Single Entry Multiple Exits) region as jumping into a _try is not allowed.
The single entry must start with a seh_try_begin() invoke with a correct State number that is the initial state of the SEME.
Through control-flow, state number is propagated into all blocks.
Side exits marked by seh_try_end() will unwind to parent state based on existing SEHUnwindMap.
Side exits can ONLY jump into parent scopes (lower state number).
Thus, when a block succeeds various states from its predecessors, the lowest State triumphs others.
If some exits flow to unreachable, propagation on those paths terminate, not affecting remaining blocks.
For CPP code, object lifetime region is usually a SEME as SEH _try. However there is one rare exception: jumping into a lifetime that has Dtor but has no Ctor is warned, but allowed:
note: jump bypasses variable with a non-trivial destructor
In this case, this region is actually a MEME (multiple entry multiple exits). Our solution is to inject a eha_scope_begin() invoke in the side entry block to ensure a correct State.
Part-1: Clang implementation described below.
- Two intrinsic are created to track CPP object scopes; eha_scope_begin() and eha_scope_end(). _scope_begin() is immediately added after ctor() is called and EHStack is pushed. So it must be an invoke, not a call. With that it's also guaranteed an EH-cleanup-pad is created regardless whether there exists a call in this scope. _scope_end is added before dtor(). These two intrinsics make the computation of Block-State possible in downstream code gen pass, even in the presence of ctor/dtor inlining.
- Two intrinsic, seh_try_begin() and seh_try_end(), are added for C-code to mark _try boundary and to prevent from exceptions being moved across _try boundary.
- All memory instructions inside a _try are considered as 'volatile' to assure 2nd and 3rd rules for C-code above. This is a little sub-optimized. But it's acceptable as the amount of code directly under _try is very small.
Part-2: LLVM implementation described below.
- For both C++ & C-code, the state of each block is computed at the same place in BE (WinEHPreparing pass) where all other EH tables/maps are calculated. In addition to _scope_begin & _scope_end, the computation of block state also rely on the existing State tracking code (UnwindMap and InvokeStateMap).
- For both C++ & C-code, the state of each block with potential trap instruction is marked and reported in DAG Instruction Selection pass, the same place where the state for -EHsc (synchronous exceptions) is done.
- If the first instruction in a reported block scope can trap, a Nop is injected before this instruction. This nop is needed to accommodate LLVM Windows EH implementation, in which the address in IPToState table is offset by +1. (note the purpose of that is to ensure the return address of a call is in the same scope as the call address.
- The handler for catch(...) for -EHa must handle HW exception. So it is 'adjective' flag is reset (it cannot be IsStdDotDot (0x40) that only catches C++ exceptions).
- Suppress push/popTerminate() scope (from noexcept/noTHrow) so that HW exceptions can be passed through.
Original llvm-dev [RFC] discussions can be found in these two threads below: