[LLVMdev] Fwd: Create superblock in LLVM IR

[Tomas Minac]

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.

         *.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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