================ @@ -0,0 +1,266 @@ +======================================== +LLVM IR Generation for EH and Cleanups +======================================== + +.. contents:: + :local: + +Overview +======== + +This document describes how Clang's LLVM IR generation represents exception +handling (EH) and C++ cleanups. It focuses on the data structures and control +flow patterns used to model normal and exceptional exits, and it outlines how +the generated IR differs across common ABI models. + +Core Model +========== + +EH and cleanup handling is centered around an ``EHScopeStack`` that records +nested scopes for: + +- **Cleanups**, which run on normal control flow, exceptional control flow, or + both. These are used for destructors, full-expression cleanups, and other + scope-exit actions. +- **Catch scopes**, which represent ``try``/``catch`` handlers. +- **Filter scopes**, used to model dynamic exception specifications and some + platform-specific filters. +- **Terminate scopes**, used for ``noexcept`` and similar termination paths. + +Each cleanup is a small object with an ``Emit`` method. When a cleanup scope is +popped, the IR generator decides whether it must materialize a normal cleanup +block (for fallthrough, branch-through, or unresolved ``goto`` fixups) and/or an +EH cleanup entry (when exceptional control flow can reach the cleanup). This +results in a flattened CFG where cleanup lifetime is represented by the blocks +and edges that flow into those blocks. + +Key Components +============== + +The LLVM IR generation for EH and cleanups is spread across several core +components: + +- ``CodeGenModule`` owns module-wide state such as the LLVM module, target + information, and the selected EH personality function. It provides access to + ABI helpers via ``CGCXXABI`` and target-specific hooks. +- ``CodeGenFunction`` manages per-function state and IR building. It owns the + ``EHScopeStack``, tracks the current insertion point, and emits blocks, calls, + and branches. Most cleanup and EH control flow is built here. +- ``EHScopeStack`` is the central stack of scopes used to model EH and cleanup + semantics. It stores ``EHCleanupScope`` entries for cleanups, along with + ``EHCatchScope``, ``EHFilterScope``, and ``EHTerminateScope`` for handlers and + termination logic. +- ``EHCleanupScope`` stores the cleanup object plus state data (active flags, + fixup depth, and enclosing scope links). When a cleanup scope is popped, + ``CodeGenFunction`` decides whether to emit a normal cleanup block, an EH + cleanup entry, or both. +- Cleanup emission helpers implement the mechanics of branching through + cleanups, threading fixups, and emitting cleanup blocks. +- Exception emission helpers implement landing pads, dispatch blocks, + personality selection, and helper routines for try/catch, filters, and + terminate handling. +- ``CGCXXABI`` (and its ABI-specific implementations such as + ``ItaniumCXXABI`` and ``MicrosoftCXXABI``) provide ABI-specific lowering for + throws, catch handling, and destructor emission details. +- C++ expression, class, and statement emission logic drives construction and + destruction, and is responsible for pushing/popping cleanups in response to + AST constructs. + +These components interact along a consistent pattern: AST traversal in +``CodeGenFunction`` emits code and pushes cleanups or EH scopes; ``EHScopeStack`` +records scope nesting; cleanup and exception helpers materialize the CFG as +scopes are popped; and ``CGCXXABI`` supplies ABI-specific details for landing +pads or funclets. + +Normal Cleanups and Branch Fixups +================================= + +Normal control flow exits (``return``, ``break``, ``goto``, fallthrough, etc.) +are threaded through cleanups by creating explicit cleanup blocks. The +implementation supports unresolved branches to labels by emitting an optimistic +branch and recording a fixup. When a cleanup is popped, fixups are threaded ---------------- efriedma-quic wrote:
Talking about the details of how we implement "goto" is going a bit into the weeds... https://github.com/llvm/llvm-project/pull/176236 _______________________________________________ cfe-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
