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Add the blurb about the new exception handling.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143042 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -689,12 +689,117 @@ in this section.
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<p>LLVM IR has several new features for better support of new targets and that
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expose new optimization opportunities:</p>
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<p>One of the biggest changes is that 3.0 has a new exception handling
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system. The old system used LLVM intrinsics to convey the exception handling
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information to the code generator. It worked in most cases, but not
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all. Inlining was especially difficult to get right. Also, the intrinsics
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could be moved away from the <code>invoke</code> instruction, making it hard
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to recover that information.</p>
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<p>The new EH system makes exception handling a first-class member of the IR. It
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adds two new instructions:</p>
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<ul>
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<!--
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<li></li>
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-->
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<li><a href="LangRef.html#i_landingpad"><code>landingpad</code></a> —
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this instruction defines a landing pad basic block. It contains all of the
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information that's needed by the code generator. It's also required to be
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the first non-PHI instruction in the landing pad. In addition, a landing
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pad may be jumped to only by the unwind edge of an <code>invoke</code>
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instruction.</li>
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<li><a href="LangRef.html#i_resume"><code>resume</code></a> — this
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instruction causes the current exception to resume traveling up the
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stack. It replaces the <code>@llvm.eh.resume</code> intrinsic.</li>
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</ul>
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<p>Converting from the old EH API to the new EH API is rather simple, because a
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lot of complexity has been removed. The two intrinsics,
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<code>@llvm.eh.exception</code> and <code>@llvm.eh.selector</code> have been
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superceded by the <code>landingpad</code> instruction. Instead of generating
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a call to <code>@llvm.eh.exception</code> and <code>@llvm.eh.selector</code>:
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<div class="doc_code">
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<pre>
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Function *ExcIntr = Intrinsic::getDeclaration(TheModule,
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Intrinsic::eh_exception);
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Function *SlctrIntr = Intrinsic::getDeclaration(TheModule,
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Intrinsic::eh_selector);
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// The exception pointer.
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Value *ExnPtr = Builder.CreateCall(ExcIntr, "exc_ptr");
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std::vector<Value*> Args;
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Args.push_back(ExnPtr);
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Args.push_back(Builder.CreateBitCast(Personality,
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Type::getInt8PtrTy(Context)));
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<i>// Add selector clauses to Args.</i>
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// The selector call.
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Builder.CreateCall(SlctrIntr, Args, "exc_sel");
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</pre>
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</div>
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<p>You should instead generate a <code>landingpad</code> instruction, that
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returns an exception object and selector value:</p>
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<div class="doc_code">
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<pre>
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LandingPadInst *LPadInst =
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Builder.CreateLandingPad(StructType::get(Int8PtrTy, Int32Ty, NULL),
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Personality, 0);
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Value *LPadExn = Builder.CreateExtractValue(LPadInst, 0);
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Builder.CreateStore(LPadExn, getExceptionSlot());
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Value *LPadSel = Builder.CreateExtractValue(LPadInst, 1);
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Builder.CreateStore(LPadSel, getEHSelectorSlot());
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</pre>
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</div>
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<p>It's now trivial to add the individual clauses to the <code>landingpad</code>
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instruction.</p>
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<div class="doc_code">
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<pre>
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<i><b>// Adding a catch clause</b></i>
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Constant *TypeInfo = getTypeInfo();
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LPadInst->addClause(TypeInfo);
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<i><b>// Adding a C++ catch-all</b></i>
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LPadInst->addClause(Constant::getNullValue(Builder.getInt8PtrTy()));
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<i><b>// Adding a cleanup</b></i>
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LPadInst->setCleanup(true);
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<i><b>// Adding a filter clause</b></i>
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std::vector<Constant*> TypeInfos;
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Constant *TypeInfo = getFilterTypeInfo();
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TypeInfos.push_back(Builder.CreateBitCast(TypeInfo, Builder.getInt8PtrTy()));
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ArrayType *FilterTy = ArrayType::get(Int8PtrTy, TypeInfos.size());
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LPadInst->addClause(ConstantArray::get(FilterTy, TypeInfos));
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</pre>
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</div>
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<p>Converting from using the <code>@llvm.eh.resume</code> intrinsic to
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the <code>resume</code> instruction is trivial. It takes the exception
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pointer and exception selector values returned by
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the <code>landingpad</code> instruction:</p>
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<div class="doc_code">
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<pre>
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Type *UnwindDataTy = StructType::get(Builder.getInt8PtrTy(),
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Builder.getInt32Ty(), NULL);
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Value *UnwindData = UndefValue::get(UnwindDataTy);
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Value *ExcPtr = Builder.CreateLoad(getExceptionObjSlot());
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Value *ExcSel = Builder.CreateLoad(getExceptionSelSlot());
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UnwindData = Builder.CreateInsertValue(UnwindData, ExcPtr, 0, "exc_ptr");
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UnwindData = Builder.CreateInsertValue(UnwindData, ExcSel, 1, "exc_sel");
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Builder.CreateResume(UnwindData);
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</pre>
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</div>
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</div>
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<!--=========================================================================-->
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