llvm dev - Apr 2008 - [LLVMdev] flag_unit_at_a_time and pass scheduling in llvm-gcc

Hi Devang,
> You can do inlining even when flag_unit_at_a_time is off. And one can  
> enable unit-at-a-time without enabling any optimizations.  The unit-at- 
> a-time is not meant to select optimization passes, though it may  
> influence selection.
this flag is used quite a bit in llvm-backend.cpp, for example:

    if (flag_unit_at_a_time) {
      PM->add(createGlobalOptimizerPass());       // Optimize out global vars
      PM->add(createGlobalDCEPass());             // Remove unused fns and
globs
      PM->add(createIPConstantPropagationPass()); // IP Constant Propagation
      PM->add(createDeadArgEliminationPass());    // Dead argument
elimination
    }

I thought I understood why but it seems that I don't :)

Ciao,

Duncan.

On Apr 16, 2008, at 1:02 PM, Duncan Sands wrote:
> Hi Devang,
>
>> You can do inlining even when flag_unit_at_a_time is off. And one can
>> enable unit-at-a-time without enabling any optimizations.  The unit- 
>> at-
>> a-time is not meant to select optimization passes, though it may
>> influence selection.
>
> this flag is used quite a bit in llvm-backend.cpp, for example:
>
>    if (flag_unit_at_a_time) {
>      PM->add(createGlobalOptimizerPass());       // Optimize out  
> global vars
>      PM->add(createGlobalDCEPass());             // Remove unused  
> fns and globs
>      PM->add(createIPConstantPropagationPass()); // IP Constant  
> Propagation
>      PM->add(createDeadArgEliminationPass());    // Dead argument  
> elimination
>    }
>
> I thought I understood why but it seems that I don't :)
IMO, we should avoid using flag_unit_at_a_time here.

-
Devang

Hi Devang,
> > this flag is used quite a bit in llvm-backend.cpp, for example:
> >
> >    if (flag_unit_at_a_time) {
> >      PM->add(createGlobalOptimizerPass());       // Optimize out  
> > global vars
> >      PM->add(createGlobalDCEPass());             // Remove unused  
> > fns and globs
> >      PM->add(createIPConstantPropagationPass()); // IP Constant  
> > Propagation
> >      PM->add(createDeadArgEliminationPass());    // Dead argument  
> > elimination
> >    }
> >
> > I thought I understood why but it seems that I don't :)
> 
> IMO, we should avoid using flag_unit_at_a_time here.
given DannyB's explanation that this flag exists in gcc so that glibc
works properly in spite of abusing ASM, perhaps this logic in llvm-backend
also exists to ensure that glibc works?

Ciao,

Duncan.