[LLVMdev] Question regarding the x86 SBB instruction.

Duncan Sands
Fri, 28 Jun 2013 13:04:22 -0000

Hi James, you could use the sadd_with_overflow intrinsic.

Ciao, Duncan.

On 28/06/13 14:51, James Courtier-Dutton wrote:
> Hi,
>
> I have the x86 SBB instruction. how should I represent this in LLVM
> IR.  (as part of a decompiler from binary to LLVM IR)
>
> Pre-conditions:
> %eax = 0xffffffff
> %edx = 0xffffffff
> %carry = 1
>
> SBB %eax, %edx       // %edx is the destination doing %edx = %edx -
> (%eax + carry)
> JC  jump_destination1    // If the Carry flag is set, jump to
jump_destination1
>
> How do I represent this correctly in LLVM IR?
> In the above case, the carry flag should be set by the SBB because:
> %eax + carry == 0x100000000     (33 bits)  or 0x0 (32 bits)
> %edx - (%eax + carry) == %edx   with Carry set.
>
> If I use LLVM IR:
> %eax2 = ADD i32 %eax1, %carry    (%eax2 == 0)
> [%edx2,%carry] = llvm.ssub.with.overflow.i32 %edx1, %eax2    (carry =>
0, but I want it to be 1)
>
> So, the problem only occurs with the edge case of %eax == 0xffffffff
> and carry == 1
>
> Any ideas how I could make this work accurately in LLVM IR ?
> I could put an if round it:
> if ((%eax1 == 0xffffffff)) && (%carry1 == 1)) {
>      %carry2 = 1
> } else {
> %eax2 = ADD i32 %eax1, %carry
> [%edx2,%carry3] = llvm.ssub.with.overflow.i32 %edx1, %eax2
> }
> %carry4 = phi (%carry2, %carry3)      (true branch, else branch)
> %edx3 = phi (%edx1, %edx2)
> branch cond %carry4  jump_destination
>
> Any better ideas?
>
> James
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