[LLVMdev] Checked arithmetic

Jonathan S. Shapiro shap at eros-os.com
Wed Mar 26 06:19:21 PDT 2008

On Tue, 2008-03-25 at 21:18 -0700, Chris Lattner wrote:
> On Mar 25, 2008, at 8:25 PM, Jonathan S. Shapiro wrote:
> > In looking at the LLVM reference manual, it is conspicuous that (a)  
> > the
> > IR does not define condition codes, and (b) the IR does not define
> > opcodes that return condition results in addition to their  
> > computational
> > results.
> We currently don't have this because noone has implemented it yet.  It  
> would be great to have this.

I want to background process this for a bit, but it would be helpful to
discuss some approaches first.

There would appear to be three approaches:

  1. Introduce a CC register class into the IR. This seems to be a
     fairly major overhaul.

  2. Introduce a set of scalar and fp computation quasi-instructions
     that accept the same arguments as their computational counterparts,
     but produce *only* the condition code. For example:

        add i32 ...    produces result
        add_cc i32 ... produces condition codes

     Once this exists, re-combine the instructions in the back end with

  3. Handle CC as a black magic special case, which at least has the
     merit of tradition. :-)

Given the number of different ways that different bits of hardware
handle CCs, I am inclined to think that the right approach, in abstract,
is to introduce a CC register class and deal with CCs in a fully general
way. If so, this is not a small change.


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