What kind of C++ code would generate this x86 assembly instruction?

Question

I have been "reverse engineering" some of my own libraries to learn more about compiler optimizations. I have seen one of my simplest class constructors (a 4-D vector) get compiled to the following:

fldz                                                        ; push +0.0 to FPU stack
mov     eax, ecx                                            ; set eax to this (ecx)
mov     dword ptr [eax], offset data_??_7vector_t@data@@6B@ ; what is this doing?
fst     dword ptr [eax+4]                                   ; assign this->first
fst     dword ptr [eax+8]                                   ; assign this->second
fst     dword ptr [eax+0Ch]                                 ; assign this->third
fstp    dword ptr [eax+10h]                                 ; assign this->fourth, pop FPU stack
retn                                                        ; return this (eax)

On the third line, I have no clue what this is doing. I originally thought it might be some kind of optimization that is referencing some hard coded block of constant data.

In order to determine what it might be, I loaded the DLL into a container process and then attached a debugger and viewed the data at the location of data??_7vector_t@data@@6B@ , but it was just db offset unk??_7vector_t@data2@@6B@ . I followed that second label and there was a region of data which didn't correspond to anything I recognized in my project, even after converting the first 8 bytes into a double.

The compiler I'm using is MSVC++ with Visual Studio 2013 with full optimization without any advanced instruction sets (SSE, etc is turned off).

What C++ code would generate the instruction in question?

Answer 1

有问题的行是为正在构造的对象分配一个vtable指针。

Answer 2

Looks to me like it is applying a constructor to the uninitialized storage at ECX.

The steps of that would insert a vtable pointer in its first slot, and zero the 4d vector you say your application uses.

Answer 3

Not sure where the data_ prefix comes from, but the ? is the standard marker of decorated (mangled) names and you can use the undname tool to decipher them:

>undname ??_7vector_t@data@@6B@
Microsoft (R) C++ Name Undecorator
Copyright (C) Microsoft Corporation. All rights reserved.

Undecoration of :- "??_7vector_t@data@@6B@"
is :- "const data::vector_t::`vftable'"

So yes, it's just initializing the vtable pointer.