https://gcc.gnu.org/bugzilla/show_bug.cgi?id=35560
--- Comment #16 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Witold Baryluk from comment #15) > I know this is a pretty old bug, but I was exploring some assembly of gcc > and clang on godbolt, and also stumbled into same issue. > > https://godbolt.org/z/qPzMhWse1 > > class A { > public: > virtual int f7(int x) const; > }; > > int g(const A * const a, int x) { > int r = 0; > for (int i = 0; i < 10000; i++) > r += a->f7(x); > return r; > } > > (same happens without loop, when just calling a->f7 multiple times) > > > > g(A const*, int): > push r13 > mov r13d, esi > push r12 > xor r12d, r12d > push rbp > mov rbp, rdi > push rbx > mov ebx, 10000 > sub rsp, 8 > .L2: > mov rax, QWORD PTR [rbp+0] # a vtable deref > mov esi, r13d > mov rdi, rbp > call [QWORD PTR [rax]] # f7 indirect call > add r12d, eax > dec ebx > jne .L2 > > add rsp, 8 > pop rbx > pop rbp > mov eax, r12d > pop r12 > pop r13 > ret > > > I was expecting mov rax, QWORD PTR [rbp+0] and call [QWORD PTR > [rax]], to be hoisted out of the loop (call converted to lea, and call > register). > > > A bit sad. > > Is there some recent work done on this optimization? > > Are there at least some cases where it is valid to do CSE, or change code so > it is moved out of the loop? GCC sees a->f() as possibly altering the virtual table [pointer] since the function gets passed 'a' and thus a pointer to it (and *a is global memory anyway, so GCC has to assume f() has access to it). In C++ probably there's probably no virtual function that could do this. A virtual DTOR would leave an uninitialized object. Not sure if class A { virtual void f() { } } class B : A { virtual void f() { new A (this); } } would be valid (maybe with first calling the DTOR on the existing object).
