llvm dev - Apr 2013 - [LLVMdev] llvm ir vs dag

It seems to me that a lot of the transformations done on DAG could more 
easily and cleanly be done in the IR.

It could require some ability to extend IR for targets but this could be 
useful anyway.
With inline assembly, one can already place assembly in the IR.

Some things that could be done in IR:

1) call and return lowering and related functions
2) atomic
3) soft float
4) ... many others

This could eliminate a lot of messy coding takes place in target 
lowering and DAG to DAG.

The average person doing porting should not, IMO, be exposed to many of 
the details of chains, glue and other complexities and idiosyncrasies of 
the Selection DAG.

The main idea is to eliminate or shrink considerably XXXTargetLowering  
and XXXISelDagToDag.

There could also be some Machine IR added to the current IR that is not 
target specific.

My 2c.

Reed

Hi Reed,

There was some discussion at the DevMeeting last year about this.  The consensus
was that:

- It would be great if SelectionDAG could go away and the translations be done
via progressive replacement in the IR of LLVM instructions with machine
instructions and LLVM registers to machine registers.

- If someone did this work, there are lots of people willing to review and
comment on it.

- No one has all three of the time, expertise, and inclination to actually do
it.

Or, to put it more succinctly: patches welcome.

David

On 16 Apr 2013, at 01:45, reed kotler <rkotler at mips.com> wrote:
> It seems to me that a lot of the transformations done on DAG could more
easily and cleanly be done in the IR.
> 
> It could require some ability to extend IR for targets but this could be
useful anyway.
> With inline assembly, one can already place assembly in the IR.
> 
> Some things that could be done in IR:
> 
> 1) call and return lowering and related functions
> 2) atomic
> 3) soft float
> 4) ... many others
> 
> This could eliminate a lot of messy coding takes place in target lowering
and DAG to DAG.
> 
> The average person doing porting should not, IMO, be exposed to many of the
details of chains, glue and other complexities and idiosyncrasies of the
Selection DAG.
> 
> The main idea is to eliminate or shrink considerably XXXTargetLowering  and
XXXISelDagToDag.
> 
> There could also be some Machine IR added to the current IR that is not
target specific.
> 
> My 2c.
> 
> Reed
> 
> 
> 
> 
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