[LLVMdev] reg_iterator Caveats

Evan Cheng evan.cheng at apple.com
Mon Mar 31 16:32:37 PDT 2008

On Mar 31, 2008, at 2:53 PM, David Greene wrote:

> On Monday 31 March 2008 00:57, Chris Lattner wrote:
>> On Mar 30, 2008, at 10:42 PM, David A. Greene wrote:
>>>> SSA form, it is reasonable to say "give me the first def" and  
>>>> expect
>>>> it to be the only def.  For multiply defined values like physregs,
>>>> this is not true, because the reg can have multiple defs.
>>> Gotcha.  This is exactly what I want.  Thanks for the explanation.
>>> For non-SSA values, is there some indication of which defs reach  
>>> which
>>> uses?  I don't need this right now but I can imagine using it in the
>>> future.
>> The reg def/kill/dead flags are all that there is.
> I just discovered that def_itterator (and presumably, reg_iterator)  
> doesn't
> include implicit defs, for example at function calls for caller-save  
> physical
> registers.  Guh.  I'm not sure if it should or not, but it's certainly
> necessary information in some cases.  Is this expected behavior, or an
> oversight?

MachineRegisterInfo tracks virtual register only.

I also wish it would track physical register defs and uses as well. It  
can be used to simplify a lot of code (in livevariable, etc.). Chris,  
do you think that's feasible?


>                                            -Dave
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