[LLVMdev] Create superblock in LLVM IR

[Andrew Trick]

On Sep 4, 2012, at 9:05 AM, Tomas Minac <minac.tomas at gmail.com> wrote:

> 
> Hello,
> 
> I am member of research team on our Faculty of Information Technology (university VUT in Brno Czech Republic). We use LLVM for many projects and actually we want to create system, which will be able compile and profile aplication for VLIW processors in few iterations.
> We want to load profile information in 'opt' part of LLVM and use it for create superblocks. Note, when superblocks will be created, we want to use llc for our specials backends.

I suggest looking at the problem upside-down. What optimization do you need to enable on superblocks, and how will changing the IR help? Any single predecessor blocks laid out sequentially looks like a superblock to me. What do you gain by changing the IR? Maybe you can write a self-contained MachineIR pass that does the optimization you need, or extend an existing pass.

-Andy

>         *.ll -> opt  -> llc -> *.asm
>               |
>      profile information    
> 
> We are thinking about creating superblocks in two ways:
>     1. Simply join basic blocks and branch instrunction in the join point replace by some intrinsic instrunction.
>             Example:
> define i32 @foo(i32 %a, i32 %b) nounwind uwtable {
> entry:
>         ...
>   br i1 %cond, label %if.then, label %if.else
> if.then:                                          ; preds = %entry
>         ...
>   br label %if.end
> if.else:                                          ; preds = %entry
>         ...
>   br label %if.end
> if.end:                                           ; preds = %if.else, %if.then
>         ...
>   ret i32 %add7
>         It will be transformed to:
> 
> define i32 @foo(i32 %a, i32 %b) nounwind uwtable {
> entry:
>         ...
>     spec.instrinsic.instr.with.label.on.if.else()
>         ...
>   br label %if.end
> if.else:                                          ; preds = %entry
>         ...
>   br label %if.end
> if.end:                                           ; preds = %if.else, %if.then
>         ...
>   ret i32 %add7
> 
>     2. Create class named SuperBlock, which will be very similar to BasicBlock class and has vector of contained BasicBlocks. Of course, in this case, we will have to create MachineSuperBlock class, because we want to use llc, how I noted above. Result can looks like that:
>     Example:
> define i32 @foo(i32 %a, i32 %b) nounwind uwtable {
> superblock.entry.start:
> entry:
>         ...
>   br i1 %cond, label %if.then, label %if.else
> if.then:                                          ; preds = %entry
>         ...
>   br label %if.end
> superblock.entry.end:
> superblock.if.else.start:
> if.else:                                          ; preds = %entry
>         ...
>   br label %if.end
> 
> superblock.if.else.end:
> superblock.if.end.start:
>  if.end:                                           ; preds = %if.else, %if.then
>         ...
>   ret i32 %add7
> superblock.if.end.end:
>     
> What do you think which way will bring less problems for additional processing with llc?
> 
> I hope and looking for some more advices or any notes to our drafts I will be thankfull.
> 
> Regards,
> Tomas Minac
> 
> 
> 
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