[llvm-dev] Understanding optimizations below LLVM IR.

[Krzysztof Parzyszek via llvm-dev]

On 9/2/2018 10:42 AM, James Courtier-Dutton via llvm-dev wrote:
> 
> I am writing a de-compiler "Binary -> LLVM IR". So, I obviously need to 
> treat SBB as a special case and transform it into something that can be 
> represented in LLVM IR.

Not a "special case", it's just an instruction whose function needs to 
be represented in the LLVM IR somehow.


> I wish to obtain a list of all the optimizations done by LLVM that 
> result in assembly that cannot immediately be represented in LLVM IR.

That won't take you anywhere.

Think of this as a compiler that takes source programs in ELF format 
(for example) and produces output in .ll format. The resulting .ll will 
never look exactly like the original bitcode, the best you can get is 
that it will have the same semantics. The SBB instruction uses the carry 
  bit and modifies the carry bit, so you need to represent the carry in 
your bitcode model somehow, and then do just that: write bitcode that 
produces the result of the subtraction and the value of the simulated 
carry bit. For things like EFLAGS are difficult to model because they 
are like a global variable, but if you assume some default value of it 
at function entries, you can still "decompile" functions that use it.

-Krzysztof