Looking for a conditional macro processing algorithm

I am writing a cpp's macro processor for a special application. The
program needs to propagate C preprocessor's conditional macro to assembly
code and generate total size for defined data. For example, C definition:


                  #ifdef D
                        struct X a; /* X and Y are structs * /
                  #else
                        struct Y b;
                  # ifdef SHORT
                              short B;
                  # else
                              int B;
                  # endif
                  #endif
                  long C;


needs to be converted into:


                  IFDEF D
                      S equ sizeof(X)+sizeof(long)
                  ELSE
                  IFDEF SHORT
                      S equ sizeof(Y)+sizeof(short)+sizeof(long)
                  ELSE
                      S equ sizeof(Y)+sizeof(int)+sizeof(long)
                  ENDIF
                  ENDIF


where sizeof() represents the size of its parameter.


I wrote a decision-tree type of algorithm for it. But I feel there may
already exist algorithms and even code for this problem. Your help are
appreciated. Thanks.
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