Figure A.6 demonstrates how the preceding snippet maps into the original source code. in .NET

Draw qr barcode in .NET Figure A.6 demonstrates how the preceding snippet maps into the original source code.
Figure A.6 demonstrates how the preceding snippet maps into the original source code.
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High-Level Code
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[Variable1], 100
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if (Variable1 == 100 || Variable2 == 50) SomeFunction(); ...
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je ConditionalBlock cmp [Variable2], 50
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je ConditionalBlock jmp AfterConditionalBlock
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ConditionalBlock: call SomeFunction AfterConditionalBlock: ...
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Figure A.6 High-level/low-level view of a compound conditional statement with two conditions combined using the OR operator.
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Again, the most noticeable element in this snippet is the sequence of conditional jumps all pointing to the same code. Keep in mind that with this approach the conditional jumps actually point to the conditional block (as opposed to the previous cases that have been discussed, where conditional jumps point to the code that follows the conditional blocks). This approach is employed by GCC and several other compilers and has the advantage (at least from a reversing perspective) of being fairly readable and intuitive. It does have a minor performance disadvantage because of that final JMP that s reached when neither condition is met. Other optimizing compilers such as the Microsoft compilers get around this problem of having an extra JMP by employing a slightly different approach for implementing the OR operator. The idea is that only the second condition is reversed and is pointed at the code after the conditional block, while the first condition still points to the conditional block itself. Figure A.7 illustrates what the same logic looks like when compiled using this approach. The first condition checks whether Variable1 equals 100, just as it s stated in the source code. The second condition has been reversed and is now checking whether Variable2 doesn t equal 50. This is so because you want the first condition to jump to the conditional code if the condition is met and the second condition to not jump if the (reversed) condition is met. The second condition skips the conditional block when it is not met.
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cmp je cmp jne
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[Variable1], 100 ConditionalBlock [Variable2], 50 AfterConditionalBlock
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if (Variable1 == 100 || Variable2 == 50) Result = 1; ...
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ConditionalBlock: mov [Result], 1
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AfterConditionalBlock: ...
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Figure A.7 High-level/low-level view of a conditional statement with two conditions combined using a more efficient version of the OR operator.
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Appendix A
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Simple Combinations
What happens when any of the logical operators are used to specify more than two conditions Usually it is just a straightforward extension of the strategy employed for two conditions. For GCC this simply means another condition before the unconditional jump. In the snippet shown in Figure A.8, Variable1 and Variable2 are compared against the same values as in the original sample, except that here we also have Variable3 which is compared against 0. As long as all conditions are connected using an OR operator, the compiler will simply add extra conditional jumps that go to the conditional block. Again, the compiler will always place an unconditional jump right after the final conditional branch instruction. This unconditional jump will skip the conditional block and go directly to the code that follows it if none of the conditions are satisfied. With the more optimized technique, the approach is the same, except that instead of using an unconditional jump, the last condition is reversed. The rest of the conditions are implemented as straight conditional jumps that point to the conditional code block. Figure A.9 shows what happens when the same code sample from Figure A.8 is compiled using the second technique.
Assembly Language Code
High-Level Code
cmp je cmp je cmp jne jmp
[Variable1], 100 ConditionalBlock [Variable2], 50 ConditionalBlock [Variable3], 0 ConditionalBlock AfterConditionalBlock
if (Variable1 == 100 || Variable2 == 50 || Variable3 != 0) SomeFunction(); ...
ConditionalBlock: call SomeFunction
AfterConditionalBlock:
Figure A.8 High-level/low-level view of a compound conditional statement with three conditions combined using the OR operator.
Deciphering Code Structures
Assembly Language Code
High-Level Code
cmp je cmp je cmp je
[Variable1], 100 ConditionalBlock [Variable2], 50 ConditionalBlock [Variable3], 0
Not Reversed
if (Variable1 == 100 || Variable2 == 50 || Variable3 != 0) SomeFunction(); ...
Not Reversed
Reversed
AfterConditionalBlock
ConditionalBlock:
call SomeFunction
AfterConditionalBlock:
Figure A.9 High-level/low-level view of a conditional statement with three conditions combined using a more efficient version of the OR operator.
The idea is simple. When multiple OR operators are used, the compiler will produce multiple consecutive conditional jumps that each go to the conditional block if they are satisfied. The last condition will be reversed and will jump to the code right after the conditional block so that if the condition is met the jump won t occur and execution will proceed to the conditional block that resides right after that last conditional jump. In the preceding sample, the final check checks that Variable3 doesn t equal zero, which is why it uses JE. Let s now take a look at what happens when more than two conditions are combined using the AND operator (see Figure A.10). In this case, the compiler simply adds more and more reversed conditions that skip the conditional block if satisfied (keep in mind that the conditions are reversed) and continue to the next condition (or to the conditional block itself) if not satisfied.