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8c8ddd6602
Summary: This switches the EH implementation to the new proposal: https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md (The previous proposal was https://github.com/WebAssembly/exception-handling/blob/master/proposals/old/Exceptions.md) - Instruction changes - Now we have one single `catch` instruction that returns a except_ref value - `throw` now can take variable number of operations - `rethrow` does not have 'depth' argument anymore - `br_on_exn` queries an except_ref to see if it matches the tag and branches to the given label if true. - `extract_exception` is a pseudo instruction that simulates popping values from wasm stack. This is to make `br_on_exn`, a very special instruction, work: `br_on_exn` puts values onto the stack only if it is taken, and the # of values can vay depending on the tag. - Now there's only one `catch` per `try`, this patch removes all special handling for terminate pad with a call to `__clang_call_terminate`. Before it was the only case there are two catch clauses (a normal `catch` and `catch_all` per `try`). - Make `rethrow` act as a terminator like `throw`. This splits BB after `rethrow` in WasmEHPrepare, and deletes an unnecessary `unreachable` after `rethrow` in LateEHPrepare. - Now we stop at all catchpads (because we add wasm `catch` instruction that catches all exceptions), this creates new `findWasmUnwindDestinations` function in SelectionDAGBuilder. - Now we use `br_on_exn` instrution to figure out if an except_ref matches the current tag or not, LateEHPrepare generates this sequence for catch pads: ``` catch block i32 br_on_exn $__cpp_exception end_block extract_exception ``` - Branch analysis for `br_on_exn` in WebAssemblyInstrInfo - Other various misc. changes to switch to the new proposal. Reviewers: dschuff Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits Differential Revision: https://reviews.llvm.org/D57134 llvm-svn: 352598
97 lines
4.1 KiB
C++
97 lines
4.1 KiB
C++
//===-- CodeGen/AsmPrinter/WasmException.cpp - Wasm Exception Impl --------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file contains support for writing WebAssembly exception info into asm
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// files.
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//
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//===----------------------------------------------------------------------===//
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#include "WasmException.h"
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#include "llvm/IR/Mangler.h"
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#include "llvm/MC/MCContext.h"
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#include "llvm/MC/MCStreamer.h"
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using namespace llvm;
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void WasmException::endModule() {
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// This is the symbol used in 'throw' and 'br_on_exn' instruction to denote
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// this is a C++ exception. This symbol has to be emitted somewhere once in
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// the module. Check if the symbol has already been created, i.e., we have at
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// least one 'throw' or 'br_on_exn' instruction in the module, and emit the
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// symbol only if so.
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SmallString<60> NameStr;
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Mangler::getNameWithPrefix(NameStr, "__cpp_exception", Asm->getDataLayout());
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if (Asm->OutContext.lookupSymbol(NameStr)) {
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MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol("__cpp_exception");
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Asm->OutStreamer->EmitLabel(ExceptionSym);
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}
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}
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void WasmException::markFunctionEnd() {
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// Get rid of any dead landing pads.
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if (!Asm->MF->getLandingPads().empty()) {
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auto *NonConstMF = const_cast<MachineFunction *>(Asm->MF);
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// Wasm does not set BeginLabel and EndLabel information for landing pads,
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// so we should set the second argument false.
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NonConstMF->tidyLandingPads(nullptr, /* TidyIfNoBeginLabels */ false);
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}
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}
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void WasmException::endFunction(const MachineFunction *MF) {
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bool ShouldEmitExceptionTable = false;
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for (const LandingPadInfo &Info : MF->getLandingPads()) {
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if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) {
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ShouldEmitExceptionTable = true;
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break;
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}
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}
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if (!ShouldEmitExceptionTable)
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return;
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MCSymbol *LSDALabel = emitExceptionTable();
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assert(LSDALabel && ".GCC_exception_table has not been emitted!");
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// Wasm requires every data section symbol to have a .size set. So we emit an
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// end marker and set the size as the difference between the start end the end
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// marker.
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MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end");
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Asm->OutStreamer->EmitLabel(LSDAEndLabel);
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MCContext &OutContext = Asm->OutStreamer->getContext();
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const MCExpr *SizeExp = MCBinaryExpr::createSub(
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MCSymbolRefExpr::create(LSDAEndLabel, OutContext),
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MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext);
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Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp);
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}
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// Compute the call-site table for wasm EH. Even though we use the same function
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// name to share the common routines, a call site entry in the table corresponds
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// to not a call site for possibly-throwing functions but a landing pad. In wasm
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// EH the VM is responsible for stack unwinding. After an exception occurs and
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// the stack is unwound, the control flow is transferred to wasm 'catch'
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// instruction by the VM, after which the personality function is called from
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// the compiler-generated code. Refer to WasmEHPrepare pass for more
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// information.
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void WasmException::computeCallSiteTable(
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SmallVectorImpl<CallSiteEntry> &CallSites,
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const SmallVectorImpl<const LandingPadInfo *> &LandingPads,
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const SmallVectorImpl<unsigned> &FirstActions) {
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MachineFunction &MF = *Asm->MF;
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for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) {
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const LandingPadInfo *Info = LandingPads[I];
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MachineBasicBlock *LPad = Info->LandingPadBlock;
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// We don't emit LSDA for single catch (...).
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if (!MF.hasWasmLandingPadIndex(LPad))
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continue;
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// Wasm EH must maintain the EH pads in the order assigned to them by the
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// WasmEHPrepare pass.
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unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad);
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CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]};
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if (CallSites.size() < LPadIndex + 1)
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CallSites.resize(LPadIndex + 1);
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CallSites[LPadIndex] = Site;
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}
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}
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