[LLVMdev] LLVM IR, Instructions, Backend, AsmPrinter

[John Criswell]

On 6/19/14, 10:10 PM, pratik dand wrote:
> Dear John,
>
> Since the pass that I have can transform IR, I was thinking to put in 
> an inline assembly code using the module asm in llvm.

I believe what you want to do is to first create an InlineAsm object in 
your Module that represents the inline assembly code that you want to 
execute.  You then create a CallInst that "calls" the inline assembly 
code with the desired inputs.  See 
http://llvm.org/doxygen/classllvm_1_1InlineAsm.html.

That said, I think "Module asm" is for inline assembly code that does 
not belong in a function, so what you want to create is "regular" inline 
assembly code.

>
> Replacing the llvm IR instruction %tmp4 = add nsw i32 %tmp2, %tmp3 by 
> something like  module asm "ADDenc dst,src1,src2" (instruction in the 
> new X86 architecture).

Yes, this is (more or less) what you want to do.
>
> But here I am not sure about the register allocation to tmp4,tmp2 and 
> tmp3. Can you comment?

I believe you first create an InlineAsm object which acts like an LLVM 
function: you specify a function type (which describes its inputs and 
return values) and then an inline assembly constraint string that 
describes whether the operands need to be pointers to memory, register, 
etc.  You then create a CallInst in which the value called is the 
InlineAsm object and the arguments are the LLVM values that you pass 
into the inline assembly code (in this case, %tmp2 and %tmp3).  The 
CallInst object itself will be the result of the inline assembly code.

LLVM's inline assembly is just like GCC's and uses the exact same 
constraints.  First learn how to use GCC inline assembly (if you don't 
know already), and then understanding LLVM's will be straightforward.

Regards,

John Criswell

>
> Regards,
> Pratik
>
>
> On Wed, Jun 18, 2014 at 11:20 PM, John Criswell <criswell at illinois.edu 
> <mailto:criswell at illinois.edu>> wrote:
>
>     On 6/18/14, 3:01 AM, pratik dand wrote:
>>     Dear,
>>
>>     I am new to llvm and hence have very little idea about a problem
>>     that is my university project.
>>     I am supposed to assume a X86 CPU supporting an instruction
>>     ADDenc that adds two encrypted operands. The original ADD also
>>     exists and should operate on unencrypted operands.
>>
>>     My task is to transform C programs into the new X86 assembly that
>>     supports ADDenc. I have very little idea about this.
>>
>>     I have been suggested the following approaches
>>     1) Adding a new Instruction ADDenc in the current X86 LLVM
>>     backend and make the necessary changes.
>>     2) Adding a new LLVM IR instruction addenc that recognises the
>>     operands at this level instead of general LLVM add, then add an
>>     instruction in the target X86 to transform addenc of LLVM to
>>     ADDenc of X86.
>
>     There is a third option: if you have an assembler that understands
>     the new ADDenc instruction, you can probably add inline assembly
>     code that performs the AddEnc instruction.  This will only work if
>     you're doing ahead-of-time compilation and clang is configured to
>     use your new assembler, but if you're transforming C code, that is
>     most likely what you're doing.
>
>     That said, if you're sufficiently confident with working with the
>     LLVM code generator, I think you should add support for the AddEnc
>     instruction in the X86 Backend.  As to whether you should add an
>     intrinsic or modify the backend to just figure out where to use
>     AddEnc, I'm guessing that adding an intrinsic would be better, but
>     people more familiar with the code generator infrastructure should
>     comment.
>
>     Regards,
>
>     John Criswell
>
>>
>>     I have been given an LLVM pass that can run on LLVM IR and decide
>>     which are the encrypted operands.
>>
>>     Any help of any kind will be helpful as I know very little about
>>     LLVM.
>>
>>     -- 
>>     Pratik
>>
>>
>>
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>
>
>
>
> -- 
> Pratik
>

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