[llvm-dev] Mips unconditionally uses fast-isel?

[Pete Cooper via llvm-dev]

Hi Daniel, Paul

Thanks for looking in to this.  I think its great to fix the inconsistency here.

...
> On Nov 18, 2015, at 2:19 AM, Daniel Sanders via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> > -----Original Message-----
> > From: Robinson, Paul [mailto:Paul_Robinson at playstation.sony.com <mailto:Paul_Robinson at playstation.sony.com>]
> > Sent: 17 November 2015 22:58
> > To: Daniel Sanders; llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> > Subject: RE: Mips unconditionally uses fast-isel?
> > 
> > > > The other thing that might work, is having TargetMachine remember how
> > > > the fast-isel option got set, and make OptLevelChanger do the right
> > > > thing. But that seems like a hack to work around Mips not obeying the
> > > > specified optimization level, honestly.
> > >
> > > I think we should do that as well. I don't think it's right that optnone
> > > enables Fast ISel even when it's been explicitly disabled. It should do
> > > the same checks as addPassesToGenerateCode() does.
> > 
> > Hm?  What you're asking for is that "-O2" and "-O2 -fast-isel=none" are
> > identical, unless you have an 'optnone' function. Do you really have a
> > use-case for controlling the codegen path for an 'optnone' function?
> > The whole point of 'optnone' is to avoid optimizations.
> > --paulr
> > 
>  
> No, that's already true. -O2 doesn't try to enable Fast ISel (unless the optnone attribute is given) so –fast-isel=false has no effect.
>  
> I'm saying that optnone means 'use –O0 for this function' and that optnone should
> respect non-default values of the -fast-isel flag like –O0 does. This is the behaviour I'd expect:
>  
> -fast-isel=false
> -fast-isel=default
> -fast-isel=true
> -O0
> SelectionDAG
> FastISel
> FastISel
> -O0 + optnone attribute
> SelectionDAG*
> FastISel
> FastISel
> -O1 + optnone attribute
> SelectionDAG*
> FastISel
> FastISel
> -O2 + optnone attribute
> SelectionDAG*
> FastISel
> FastISel
> -O3 + optnone attribute
> SelectionDAG*
> FastISel
> FastISel
> -O1
> SelectionDAG
> SelectionDAG
> FastISel
> -O2
> SelectionDAG
> SelectionDAG
> FastISel
> -O3
> SelectionDAG
> SelectionDAG
> FastISel
> The cells marked with '*' differ from the current behavior.
This table is excellent.  Any chance you can find a good place in code to put it, or even somewhere in the docs?

I did look to see if http://llvm.org/docs/WritingAnLLVMBackend.html#instruction-selector had a section on fast-isel but it surprisingly doesn’t.  If it did, I think this table would have been good to put there to explain when a backend will get which selector.

Cheers,
Pete
>  
> In terms of code, I think this part of OptLevelChanger::OptLevelChanger():
>    if (NewOptLevel == CodeGenOpt::None) {
>       DEBUG(dbgs() << "\nEnable FastISel for Function "
>             << IS.MF->getFunction()->getName() << "\n");
>       IS.TM.setFastISel(true);
>     }
> Should be:
>     if (NewOptLevel == CodeGenOpt::None) {
>       DEBUG(dbgs() << "\nEnable FastISel for Function "
>             << IS.MF->getFunction()->getName() << "\n");
>       IS.TM.setFastISel(EnableFastISelOption != cl::BOU_FALSE);
>     }
> Where EnableFastISelOption has the same value as the global in LLVMTargetMachine.cpp
>  
> The main reason I'm asking for this is that I think it's weird to for optnone to use a different code generator
> than  –O0. These hidden overrides exist to help us debug code generation problems and, faced with a code
> generation bug, –fast-isel=false is useful for quickly determining whether it's in FastISel or somewhere else.
> The current behaviour allows optnone to overrule the hidden option to force-disable FastISel which will give
> misleading guidance for bugs that lie in functions with optnone.
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