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This classof() is effectively saying that a MachineCodeEmitter "is-a" JITEmitter, but JITEmitter is in fact a descendant of MachineCodeEmitter, so this is not semantically correct. Consequently, none of the assertions that rely on these classof() actualy check anything. Remove the RTTI (which didn't actually check anything) and use static_cast<> instead. Post-Mortem Bug Analysis ======================== Cause of the bug ---------------- r55022 appears to be the source of the classof() and assertions removed by this commit. It aimed at removing some dynamic_cast<> that were solely in the assertions. A typical diff hunk from that commit looked like: - assert(dynamic_cast<JITEmitter*>(MCE) && "Unexpected MCE?"); - JITEmitter *JE = static_cast<JITEmitter*>(getCodeEmitter()); + assert(isa<JITEmitter>(MCE) && "Unexpected MCE?"); + JITEmitter *JE = cast<JITEmitter>(getCodeEmitter()); Hence, the source of the bug then seems to be an attempt to replace dynamic_cast<> with LLVM-style RTTI without properly setting up the class hierarchy for LLVM-style RTTI. The bug therefore appears to be simply a "thinko". What initially indicated the presence of the bug ------------------------------------------------ After implementing automatic upcasting for isa<>, classof() functions of the form static bool classof(const Foo *) { return true; } were removed, since they only serve the purpose of optimizing statically-OK upcasts. A subsequent recompilation triggered a build failure on the isa<> tests within the removed asserts, since the automatic upcasting (correctly) failed to substitute this classof(). Key to pinning down the root cause of the bug --------------------------------------------- After being alerted to the presence of the bug, some thought about the semantics which were being asserted by the buggy classof() revealed that it was incorrect. How the bug could have been prevented ------------------------------------- This bug could have been prevented by better documentation for how to set up LLVM-style RTTI. This should be solved by the recently added documentation HowToSetUpLLVMStyleRTTI. However, this bug suggests that the documentation should clearly explain the contract that classof() must fulfill. The HowToSetUpLLVMStyleRTTI already explains this contract, but it is a little tucked away. A future patch will expand that explanation and make it more prominent. There does not appear to be a simple way to have the compiler prevent this bug, since fundamentally it boiled down to a spurious classof() where the programmer made an erroneous statement about the conversion. This suggests that perhaps the interface to LLVM-style RTTI of classof() is not the best. There is already some evidence for this, since in a number of places Clang has classof() forward to classofKind(Kind K) which evaluates the cast in terms of just the Kind. This could probably be generalized to simply a `static const Kind MyKind;` field in leaf classes and `static const Kind firstMyKind, lastMyKind;` for non-leaf classes, and have the rest of the work be done inside Casting.h, assuming that the Kind enum is laid out in a preorder traversal of the inheritance tree. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165764 91177308-0d34-0410-b5e6-96231b3b80d8 |
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.. | ||
CMakeLists.txt | ||
JIT.cpp | ||
JIT.h | ||
JITDwarfEmitter.cpp | ||
JITDwarfEmitter.h | ||
JITEmitter.cpp | ||
JITMemoryManager.cpp | ||
LLVMBuild.txt | ||
Makefile |