llvm dev - Nov 2015 - Mips unconditionally uses fast-isel?

I was mucking around in FastISel, and was surprised to see the test
  llvm/test/CodeGen/Mips/emergency-spill-slot-near-fp.ll
failed.  This was surprising because it specifies -fast-isel=false.

Does the Mips code generator use fast-isel even when you ask it not to?
Thanks,
--paulr

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Robinson, Paul via llvm-dev
> Sent: 16 November 2015 22:19
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] Mips unconditionally uses fast-isel?
> 
> I was mucking around in FastISel, and was surprised to see the test
>   llvm/test/CodeGen/Mips/emergency-spill-slot-near-fp.ll
> failed.  This was surprising because it specifies -fast-isel=false.
> 
> Does the Mips code generator use fast-isel even when you ask it not to?
> Thanks,
> --paulr
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
This seems to be an all-targets bug. There's code in OptLevelChanger
(lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp) that forces Fast ISel to be
enabled for every -O0 function.

> > I was mucking around in FastISel, and was surprised to see the test
> >   llvm/test/CodeGen/Mips/emergency-spill-slot-near-fp.ll
> > failed.  This was surprising because it specifies -fast-isel=false.
> >
> > Does the Mips code generator use fast-isel even when you ask it not
to?
> > Thanks,
> > --paulr
>
> This seems to be an all-targets bug. There's code in OptLevelChanger
> (lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp) that forces Fast ISel to
> be enabled for every -O0 function.
Well.... the test specifies -O0 so even though the function is marked
'optnone' I'd expect OptLevelChanger to be a no-op, because it
checks
whether the old and new OptLevel are the same. Which they should be.

If I use -debug-only=isel I see it is changing from -O2 to -O0, which
makes me think the "real" bug is that llc is ignoring -O0.
--paulr