[LLVMdev] where are my phi-nodes?

[Fariborz Jahanian]

On Sep 13, 2010, at 10:46 AM, John McCall wrote:

> On Sep 13, 2010, at 1:15 AM, Duncan Sands wrote:
>>> I compiled with "llvm-gcc -emit-llvm test.c -S -o test.ll".  
>>> Attached to this
>>> message are the source and the resulting ll output.
>>
>> compile with optimization.  The llvm-gcc front-end "cheats" and  
>> stores/loads all
>> values to/from memory, avoiding the need to construct phi nodes;  
>> instead it lets
>> the optimizers do that.  For what it's worth the dragonegg plugin  
>> produces phi
>> nodes directly, even at -O0.  I don't know what clang does.
>
> If you're talking about variables, clang emits stores and loads, of  
> course.  C/C++
> variables specify their semantics in terms of objects in memory;   
> SSA conversion
> is an optimization which does break (obviously invalid) programs.   
> Any attempt
> by the frontend to emit variables in true SSA form is just  
> duplicating this
> optimization — in the case of dragonegg, because the SSA conversion is
> implicit in how gcc creates the IR it lowers from.

Makes sense. However, grep'ing on CreatePHI in clang's CodeGen module  
I see several
places where clang's 'front-end' is doing this.  What is the rational  
for that?

- Fariborz

>
>
> John.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev