This adds a FunctionPointer class as well as a new FnPtr primtype.
It also adds a CallPtr opcode that pops a FunctionPointer from the stack and calls its function.
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[clang][Interp] Add function pointers ClosedPublic Authored by tbaeder on Jan 11 2023, 2:14 AM.
Details Summary This adds a FunctionPointer class as well as a new FnPtr primtype. It also adds a CallPtr opcode that pops a FunctionPointer from the stack and calls its function.
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tbaeder marked an inline comment as done. Comment Actions Design question -- do you anticipate this class handling *all* function pointers, including pointer to member functions that might be virtual: https://godbolt.org/z/hT8fMY37n Comment Actions
I had to make a few adjustments but I have that example working locally now, of course no idea if that would work generally. Do you see any problem with the proposed FunctionPointer class? According to https://clang.llvm.org/docs/ConstantInterpreter.html#pointers, there was a MemberPointer class that sounds related to this, but I don't know more about it. I guess, since you made func() virtual, that matters and becomes a bytecode-runtime decision instead of being known at compile-time, which seems to be the case in your example? Comment Actions
Member pointers (for functions or for data) are weird in that they're not the typical pointer width. They're actually a pointer and between one-to-three other fields in a trenchcoat, depending on the circumstances. You generally need the function pointer, but you also may need various offsets (to this, to the vtable, etc). There's some more information about how it's done in MSVC (which is different from Itanium ABI, but we can do what we want for the constant expression interpreter): https://rants.vastheman.com/2021/09/21/msvc/ I don't think there's a problem with FunctionPointer per se, I'm more wondering are you planning to also add a MemberPointer type or are you planning to reuse FunctionPointer to handle function members (and presumably something else for data members)? As for virtual functions in general, the standard has rules: http://eel.is/c++draft/expr.const#5.6 and http://eel.is/c++draft/expr.const#7 Comment Actions
I was thinking that the dynamicDispatch in https://godbolt.org/z/rf9Ks77Wo would be a good reproducer since the actual function to call is only known when calling dynamicDispatch(), but that example already works when doing a few changes to classify() the right types and adding a if (BO->isPtrMemOP()) { return visit(RHS); } to VisitBinaryOperator(). Comment Actions
That is a reasonable test, but probably not sufficient as nothing is really testing the layout of those objects (the calls return a constant). How about a test like: https://godbolt.org/z/rhhhvxYxf where there are offsets to member variables involved? Comment Actions
Ah I get what you mean now. The pointer we have on the stack is of type S but then we end up calling a function expecting a S2 pointer, and which one we're calling is not known at compile time. I'm not sure what to do about this right now. I was wondering about restructuring pointers so all the metadata is before all the actual data, but that would be a large refactoring. Comment Actions
For right now, we can move forward with basic function pointer support and we can revisit this when you get around to doing member function pointers.
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FWIW, I did that refactoring locally but it didn't really turn out to work like that. This example works now and there should be a test for it AFAIK.
Comment Actions Okay, fixed this by using visitZeroInitializer() so now it's not dead code anymore. TODO: Test zero-initialization of floats. This revision is now accepted and ready to land.Mar 24 2023, 9:23 AM This revision was landed with ongoing or failed builds.Mar 30 2023, 6:38 AM Closed by commit rG3ad167329aaf: [clang][Interp] Implement function pointers (authored by tbaeder). · Explain Why This revision was automatically updated to reflect the committed changes.
Revision Contents
Diff 507966 clang/lib/AST/Interp/ByteCodeExprGen.cpp
clang/lib/AST/Interp/Context.cpp
clang/lib/AST/Interp/Descriptor.cpp
clang/lib/AST/Interp/FunctionPointer.h
clang/lib/AST/Interp/Interp.h
clang/lib/AST/Interp/InterpStack.h
clang/lib/AST/Interp/Opcodes.td
clang/lib/AST/Interp/PrimType.h
clang/lib/AST/Interp/PrimType.cpp
clang/test/AST/Interp/functions.cpp
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Can we still reach this bit now?