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[MS] Apply adjustments after storing 'this'
Summary: The MS ABI convention is that the 'this' pointer on entry is the address of the vfptr that was used to make the virtual method call. In other words, the pointer on entry always points to the base subobject that introduced the virtual method. Consider this hierarchy: struct A { virtual void f() = 0; }; struct B { virtual void g() = 0; }; struct C : A, B { void f() override; void g() override; }; On entry to C::g, [ER]CX will contain the address of C's B subobject, and C::g will have to subtract sizeof(A) to recover a pointer to C. Before this change, we applied this adjustment in the prologue and stored the new value into the "this" local variable alloca used for debug info. However, MSVC does not do this, presumably because it is often profitable to fold the adjustment into later field accesses. This creates a problem, because the debugger expects the variable to be unadjusted. Unfortunately, CodeView doesn't have anything like DWARF expressions for computing variables that aren't in the program anymore, so we have to declare 'this' to be the unadjusted value if we want the debugger to see the right value. This has the side benefit that, in optimized builds, the 'this' pointer will usually be available on function entry because it doesn't require any adjustment. Reviewers: hans Subscribers: aprantl, cfe-commits Differential Revision: https://reviews.llvm.org/D40109 llvm-svn: 318440
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