[LLVMdev] Strategy for leveraging llvm optimizations in vm

John Criswell criswell at illinois.edu
Tue Sep 13 07:38:50 PDT 2011

On 9/13/11 9:22 AM, Michael Clagett wrote:
> Oooohhh!  That's interesting.   Does the optimizer just work around 
> these inline fragments?  Is there documentation for this facility?

CC'ing the mailing list.

The inline assembly feature is described in the LLVM Language Reference 
Manual (http://llvm.org/docs/LangRef.html).  I'm not really familiar 
with what the LLVM optimizers will do with inline assembly code; all I 
really know is that there's a well-defined interface between LLVM code 
and inline assembly code that allows the optimizers to "do the right 
thing" (e.g., an inline assembly fragment can specify that it needs one 
register for input and two for outputs, and the LLVM code generator will 
assign physical registers for you, or you can tell it that you're using 
%eax, and the LLVM code generator won't generate code that clobbers %eax).

It's basically the same as GCC inline assembly.

-- John T.

> ------------------------------------------------------------------------
> Date: Tue, 13 Sep 2011 09:20:03 -0500
> From: criswell at illinois.edu
> To: mclagett at hotmail.com
> CC: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Strategy for leveraging llvm optimizations in vm
> On 9/13/11 8:14 AM, Michael Clagett wrote:
>     Hi --
>     I'm still very much a newbie with llvm, but am looking (hopefully)
>     to use it to compile into native intel code a set of source that
>     is a combination of byte codes for my own custom vm and intel code
>     that has been coded in assembly language directly.
>     In an earlier exchange, I already discovered that llvm does not do
>     any optimizations on intel assembly language code.  This would be
>     an interesting thing to add, I believe, and I may make some
>     progress in that direction as I proceed.  But in the meantime I'm
>     wondering if the following makes sense:  To write two separate
>     translation mechanisms; one that translates the assembler code
>     associated with each of my byte codes into a sequence of llvm
>     intermediate code instructions, and the other to do the same with
>     instructions coded directly in intel assembly (i.e there would be
>     a one-to-one correspondence between intel assembly language
>     statements and llvm intermediate code statements, substituting
>     real register assignments with llvm virtual register assignments.
> Another option for handling the assembly language fragments is to 
> convert them into inline assembly code within the LLVM IR that you 
> generate.  This might be worth exploring if converting the assembly 
> code into LLVM IR is either difficult or causes a performance hit.
> -- John T.
>     It strikes me that if I did this, I would then have instruction
>     sequences (whether originating in my byte code or in intel
>     assembly language) that could be submitted to the llvm
>     optimization engine.  Does this make sense to any of you old hands?
>     Thanks.
>     Mike
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