https://llvm.org/bugs/show_bug.cgi?id=31797
Bug ID: 31797
Summary: Poor code generation depending on constexprness of
function
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: ldionn...@gmail.com
CC: dgre...@apple.com, llvm-bugs@lists.llvm.org
Classification: Unclassified
Hi,
The following snippet results in much different codegen depending on whether
the `make_pair` function is marked as `constexpr` or not:
#include <utility>
template <typename F, typename S>
// constexpr // <--- TRY UNCOMMENTING THIS, THE DYNAMIC CALL DISAPPEARS
std::pair<F, S> make_pair(F f, S s) { return std::pair<F, S>{f, s}; }
template <typename Pair>
struct map {
explicit constexpr map(Pair p) : pair_(p) { }
Pair pair_;
};
template <typename Pair>
constexpr map<Pair> make_map(Pair pair)
{ return map<Pair>{pair}; }
struct increment_tag { };
using VTable = map<std::pair<increment_tag, void (*)(void*)>>;
template <typename T>
void increment(void* self) { ++*static_cast<T*>(self); }
template <typename T>
static VTable const vtable = make_map(make_pair(increment_tag{},
&increment<T>));
struct any_iterator {
template <typename It>
explicit any_iterator(It it) : vptr_{&vtable<It>}, self_{new It(it)} { }
VTable const* vptr_;
void* self_;
};
int main() {
int array[3] = {0};
any_iterator it{&array[0]};
it.vptr_->pair_.second(it.self_);
}
Without the `constexpr` qualifier, the generated code (-O3) is this:
main: # @main
sub rsp, 24
mov dword ptr [rsp + 16], 0
mov qword ptr [rsp + 8], 0
mov edi, 8
call operator new(unsigned long)
lea rcx, [rsp + 8]
mov qword ptr [rax], rcx
mov rdi, rax
call qword ptr [rip + vtable<int*>+8]
xor eax, eax
add rsp, 24
ret
__cxx_global_var_init: # @__cxx_global_var_init
mov qword ptr [rip + vtable<int*>+8], void increment<int*>(void*)
ret
void increment<int*>(void*): # @void
increment<int*>(void*)
add qword ptr [rdi], 4
ret
With the `constexpr` qualifier, the optimizer seems to see through the
initialization of the vtable, and it is able to collapse everything:
main: # @main
xor eax, eax
ret
Same example on Godbolt: https://godbolt.org/g/dSzAe2
More involved example on Wandbox, where the codegen difference makes a
significant performance difference:
http://melpon.org/wandbox/permlink/06SYXAs9q5D19sxx
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