[LLVMdev] help decompiling x86 ASM to LLVM IR

[Duncan Sands]

Hi James,

On 12/03/13 17:20, James Courtier-Dutton wrote:
> Hi,
>
> I am looking to decompile x86 ASM to LLVM IR.
> The original C is this:
> int test61 ( unsigned value ) {
>          int ret;
>          if (value < 1)
>                  ret = 0x40;
>          else
>                  ret = 0x61;
>          return ret;
> }
>
> It compiles with GCC -O2 to (rather cleverly removing any branches):
> 0000000000000000 <test61>:
>     0:   83 ff 01                cmp    $0x1,%edi
>     3:   19 c0                   sbb    %eax,%eax
>     5:   83 e0 df                and    $0xffffffdf,%eax
>     8:   83 c0 61                add    $0x61,%eax
>     b:   c3                      retq
>
> How would I represent the SBB instruction in LLVM IR?

you could use an llvm.ssub.with.overflow.i32 intrinsic to get the
sub-with-carry, and then explicitly extend the carry flag to i32 and
subtract it off too.  See

http://llvm.org/docs/LangRef.html#llvm-ssub-with-overflow-intrinsics

Ciao, Duncan.

> Would I have to first convert the ASM to something like:
>     0000000000000000 <test61>:
>     0:                   cmp    $0x1,%edi        Block A
>     1:                   jb     4:               Block A
>     2:                   mov    0x61,%eax        Block B
>     3:                   jmp    5:               Block B
>     4:                   mov    0x40,%eax        Block C
>     5:                   retq                    Block D  (Due to join point)
>
> ...before I could convert it to LLVM IR ?
> I.e. Re-write it in such a way as to not need the SBB instruction.
>
> The aim is to be able to then recompile it to maybe a different target.
> The aim is to go from binary -> LLVM IR -> binary for cases where the
> C source code it not available or lost.
>
> I.e. binary available for x86 32 bit.  Re-target it to ARM or x86-64bit.
> The LLVM IR should be target agnostic, but would permit the
> re-targetting task without having to build AST and structure as a C or
> C++ source code program.
>
> Any comments?
>
> James
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