llvm dev - Jan 2011 - [LLVMdev] Adding a new instruction to Alpha ISA

Hi,

I want to add an extra instruction to the Alpha ISA.  This is not a new
operation; it is just an addition with a lower latency, let's call it
fast-add.  Could you give me advice on what's an easy and clean approach to
this?  I am thinking of adding an extra instruction to the LLVM IR to
represent this fast-add operation (I want to make the decision of using
either the normal addition or fast addition during loop unroll pass, which
happens before code generation; so I think I need an extra IR instruction).
 The LLVM documentation warns against adding new IR instruction unless it is
absolutely necessary, so I would like to ask to make sure.

Thank you very much.

Giang
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20110112/6d491223/attachment.html>

On Jan 12, 2011, at 7:49 AM, Giang Hoang wrote:
> Hi,
> 
> I want to add an extra instruction to the Alpha ISA.  This is not a new
operation; it is just an addition with a lower latency, let's call it
fast-add.  Could you give me advice on what's an easy and clean approach to
this?  I am thinking of adding an extra instruction to the LLVM IR to represent
this fast-add operation (I want to make the decision of using either the normal
addition or fast addition during loop unroll pass, which happens before code
generation; so I think I need an extra IR instruction).  The LLVM documentation
warns against adding new IR instruction unless it is absolutely necessary, so I
would like to ask to make sure.
> 
Hi Giang,

If you need to have special instructions like this, the best way is to create an
intrinsic that represents the operation. For instance, LLVM has the
"llvm.smul.with.overflow" intrinsic, which represents a normal
(signed) multiply instruction, but is specially lowered to check for overflow.

-bw


-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20110112/a47a08f2/attachment.html>