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Thread: Part 3: Optimizing and Compiling

  1. #1

    Part 3: Optimizing and Compiling

    The reader should inspect this unoptimized IR listing before continuing. In an attempt to keep this entry from becoming unnecessarily long, the example snippets will be small, but for completeness a more thorough running example is linked throughout the text.

    We begin by removing the stack machine features of the IR. Since VMProtect operates on disassembled x86 code, and x86 itself is not a stack machine, this aspect of the protection is unnatural and easily removed. Here is a 15-line fragment of VMProtect IR.

    push Dword(-88)
    push esp
    push Dword(4)
    pop t3
    pop t4
    t5 = t3 + t4
    push t5
    push flags t5
    pop DWORD Scratch:[Dword(52)]
    pop t6
    pop t7
    t8 = t6 + t7
    push t8
    push flags t8
    pop DWORD Scratch:[Dword(12)]
    pop esp
    All but two instructions are pushes or pops, and the pushes can be easily matched up with the pops. Tracking the stack pointer, we see that, for example, t3 = Dword(4). A simple analysis allows us to "optimize away" the push/pop pairs into assignment statements. Simply iterate through each instruction in a basic block and keep a stack describing the source of each push. For every pop, ensure that the sizes match and record the location of the corresponding push. We wish to replace the pop with an assignment to the popped expression from the pushed expression, as in

    t3 = Dword(4)
    t4 = esp 
    t7 = Dword(-88)
    With the stack aspects removed, we are left with a more conventional listing containing many assignment statements. This optimization substantially reduces the number of instructions in a given basic block (~40% for the linked example) and opens the door for other optimizations. The newly optimized code is eight lines, roughly half of the original:

    t3 = Dword(4)
    t4 = esp
    t5 = t3 + t4
    DWORD Scratch:[Dword(52)] = flags t5
    t6 = t5
    t7 = Dword(-88)
    t8 = t6 + t7
    DWORD Scratch:[Dword(12)] = flags t8
    esp = t8
    A complete listing of the unoptimized IR versus the one with the stack machine features removed is here, which should be perused before proceeding.

    Now we turn our attention to the temporary variables and the scratch area. Recall that the former were not part of the pre-protected x86 code, nor the VMProtect bytecode -- they were introduced in order to ease the IR translation. The latter is part of the VMProtect bytecode, but was not part of the original pre-protected x86 code. Since these are not part of the languages we are modelling, we shall eliminate them wholesale. On a high level, we treat each temporary variable, each byte of the scratch space, and each register as being a variable defined within a basic block, and then eliminate the former two via the compiler optimizations previously discussed.

    Looking again at the last snippet of IR, we can see several areas for improvement. First, consider the variable t6. It is clearly just a copy of t5, neither of which are redefined before the next use in the assignment to t8. Copy propagation will replace variable t6 with t5 and eliminate the former. More generally, t3, t4, and t7 contain either constants or values that are not modified between their uses. Constant and copy propagation will substitute the assignments to these variables in for their uses and eliminate them.

    The newly optimized code is a slender three lines compared to the original 15; we have removed 80% of the IR for the running example.

    DWORD Scratch:[Dword(52)] = flags Dword(4) + esp
    esp = Dword(4) + esp + Dword(-88)
    DWORD Scratch:[Dword(12)] = flags Dword(4) + esp + Dword(-88)
    The side-by-side comparison can be found here.

    The IR now looks closer to x86, with the exception that the results of computations are being stored in the scratch area, not into registers. As before, we apply dead-store elimination, copy and constant propagation to the scratch area, removing dependence upon it entirely in the process. See here for a comparison with the last phase.

    Here is a comparison of the final, optimized code against the original x86:

    push ebp                               push    ebp                    
    ebp = esp                              mov     ebp, esp               
    push Dword(-1)                         push    0FFFFFFFFh             
    push Dword(4525664)                    push    450E60h                
    push Dword(4362952)                    push    offset sub_4292C8      
    eax = DWORD FS:[Dword(0)]              mov     eax, large fs:0
    push eax                               push    eax                    
    DWORD FS:[Dword(0)] = esp              mov     large fs:0, esp        
    eflags = flags esp + Dword(-88)                            
    esp = esp + Dword(-88)                 add     esp, 0FFFFFFA8h        
    push ebx                               push    ebx                    
    push esi                               push    esi                    
    push edi                               push    edi                    
    DWORD SS:[Dword(-24) + ebp] = esp      mov     [ebp-18h], esp         
    call DWORD [Dword(4590300)]            call    dword ptr ds:unk_460ADC
    vmreturn Dword(0) + Dword(4638392)
    Code generation is an afterthought.

  2. #2
    I was enjoying reading all 4 articles, great job

  3. #3
    yes great job :P keep working

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