llvm dev - Nov 2002 - [LLVMdev] Instruction comparison

I am looking at making my recompilation analysis smarter by
automatically detecting which functions have changed from one source
code modification to the next.  So, I have one Module representing the
original code, and another module representing the new code.

I think it's fairly straight forward to match functions and global
variables between these modules.  However, mathching functions for
equality , or just trying to detect if they are still the same, seems to
be quite difficult.  It seems that I cannot use a simple heuristic of
testing opcode or number of params, etc. because I must also detect very
subtle changes, like the changing of a constant.  

Are there any supports in llvm for drawing conclusions about equivalence
of instructions or functions.

Or, is it possible to be certain of equality just by looking at
per-instruction properties and ignoring the problem of proving
equivalence of the instructions supplying the values to the current
instruction.


Thanks,
James

> Or, is it possible to be certain of equality just by looking at
> per-instruction properties and ignoring the problem of proving
> equivalence of the instructions supplying the values to the current
> instruction.
The answer to your overall question is: No.  There is no easy way to check
for equivalence.

What you really want to check for is whether the SSA graphs defined by a
function are isomorphic to each other.  In this case, I think the problem
is restricted enough to make it possible to do in linear time (you have
roots), but it's still non-trivial to code up.

Is analysis time really that much of a problem?

-Chris

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