[LLVMdev] missed optimizations

[Nuno Lopes]

Hi,

As a follow up of this thread I've made a patch that implements a simple 
approach to propagate the function return values as described previously.

It can transform e.g. the following program:

define i32 @f(...) nounwind {
(...)
 %cond = select i1 %tobool, i32 2, i32 3  ; <i32> [#uses=1]
 ret i32 %cond
}

define i32 @g(...) nounwind {
entry:
 %call = call i32 (...)* @f()  ; <i32> [#uses=1]
 switch i32 %call, label %sw.default [
  i32 5, label %sw.bb
  i32 2, label %sw.bb1
  i32 3, label %sw.bb2
 ]

sw.bb:  ; preds = %entry
 ret i32 -1

sw.bb1:  ; preds = %entry
 ret i32 3

sw.bb2:  ; preds = %entry
 ret i32 55

sw.default:  ; preds = %entry
 ret i32 0
}


into:

define i32 @g(...) nounwind {
entry:
 %call = call i32 (...)* @f()  ; <i32> [#uses=1]
 %cond = icmp eq i32 %call, 2  ; <i1> [#uses=1]
 %retval = select i1 %cond, i32 3, i32 55  ; <i32> [#uses=1]
 ret i32 %retval
}


This kind of transformation isn't currently done by LLVM (note that here 
this pass is only removing case statements. other transformations are not of 
my responsibility :).
The patch is available at 
http://web.ist.utl.pt/nuno.lopes/llvm_function_ret_infer.txt
I would love to ear some feedback, so that this pass can eventually be 
merged to the LLVM tree. In particular I'm not sure if there's a better way 
to track the possible return values of each function (e.g. is there any code 
that I can reuse?)


Thanks,
Nuno