So I spent some more time working on the decompiler. The part I was working on was subgraph recognition. Each flow control structure has a unique sructure, and by recognizing these structures, we can identify code blocks and the control operations.

Here are several example graphs to help explain this better. Each letter represents a block of non-branching code. The arrows show the possible execution paths between these blocks.

Example 1: Do while loop

A->B->C

^--'

Execution starts at block A, proceeds to block B and then either loops back to A or proceeds to C. This graph is equivalent to the following pseudocode:

do {

A

} while(B);

C

Example 2: While loop

.-----v

A->B->C

^--'

This is similar to the do while loop, but the loop determination is in block A. Here's the equivalent pseudocode:

while(A) {

B

}

C

Example 3: If

A->B->C

'-----^

This is a subgraph of the while loop, so the detection logic needs to distinguish between them. Again the decision whether to execute block B is made in block A. In either case, execution proceeds to block C. Here's the equivalent pseudocode:

if(A) {

B

}

C

Example 4: If else

.-----v

A->B C->D

'-----^

I think you get the idea by now. Here's the equivalent pseudocode:

if(A) {

B

} else {

C

}

D

There's still tons to do for the decompiler, but getting this right is one of the central problems that needed to be solved. It's pretty neat to see this working.

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