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Robinson, Paul via llvm-dev

2015-Nov-18 16:04 UTC

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

Well, 'optnone' is already not identical to -O0, and given the nature of
things, probably can't be; but I am persuaded that it's reasonable for
it to honor the -fast-isel option as a debugging tactic.  I'll take an AI to
make this happen.
Thanks,
--paulr
P.S. One nit, the "O0 + optnone" case should not have an asterisk, the
FastISel flag is not manipulated if the opt level is already zero. Does not
affect the strength of your argument, of course.

From: Daniel Sanders [mailto:Daniel.Sanders at imgtec.com]
Sent: Wednesday, November 18, 2015 2:19 AM
To: Robinson, Paul; llvm-dev at lists.llvm.org
Subject: RE: Mips unconditionally uses fast-isel?

> -----Original Message-----
> From: Robinson, Paul [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 behaviour.



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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Eric Christopher via llvm-dev

2015-Nov-18 17:48 UTC

head link

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

I'd have figured optnone was "no optimizations" not "use the
entire O0 code
path"?

At least that seemed to be the intent when you added it Paul?

-eric

On Wed, Nov 18, 2015 at 8:05 AM Robinson, Paul via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Well, 'optnone' is already not identical to –O0, and given the
nature of
> things, probably can't be; but I am persuaded that it's reasonable
for it
> to honor the –fast-isel option as a debugging tactic.  I'll take an AI
to
> make this happen.
>
> Thanks,
>
> --paulr
>
> P.S. One nit, the "O0 + optnone" case should not have an
asterisk, the
> FastISel flag is not manipulated if the opt level is already zero. Does not
> affect the strength of your argument, of course.
>
>
>
> *From:* Daniel Sanders [mailto:Daniel.Sanders at imgtec.com]
> *Sent:* Wednesday, November 18, 2015 2:19 AM
> *To:* Robinson, Paul; llvm-dev at lists.llvm.org
>
>
> *Subject:* RE: Mips unconditionally uses fast-isel?
>
>
>
> > -----Original Message-----
>
> > From: Robinson, Paul [mailto:Paul_Robinson at playstation.sony.com
> <Paul_Robinson at playstation.sony.com>]
>
> > Sent: 17 November 2015 22:58
>
> > To: Daniel Sanders; 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 behaviour.
>
>
>
> 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.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Robinson, Paul via llvm-dev

2015-Nov-18 18:42 UTC

head link

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

The driving goal of 'optnone' is to have an easy way for programmers to
get an "-O0 like" debugging experience for selected functions, without
making them build everything with –O0.
To that end, we turn off as much optimization as we reasonably can, but in the
context of a pipeline that is generally expecting optimizations to be enabled,
in practice we can't exactly match –O0 because "things break" if
we turn off everything.  Luckily, exactly matching –O0 isn't a requirement.

Contemplating the "things break" situation, I am entirely willing to
believe (it may have actually happened) that problems can occur when using
FastISel in a pipeline that isn't expecting it.  So, for purposes of
diagnosing these 'optnone' problems, it seems useful to be able to force
'optnone' not to use FastISel.  This is a different motivation than
Daniel expressed, which is a more principled idea coming from the "optnone
should exactly match –O0" misconception; but the conclusion (that we should
respect an explicit –fast-isel=false within 'optnone' functions) is the
same.
--paulr

From: Eric Christopher [mailto:echristo at gmail.com]
Sent: Wednesday, November 18, 2015 9:48 AM
To: Robinson, Paul; Daniel Sanders; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Mips unconditionally uses fast-isel?

I'd have figured optnone was "no optimizations" not "use the
entire O0 code path"?

At least that seemed to be the intent when you added it Paul?

-eric

On Wed, Nov 18, 2015 at 8:05 AM Robinson, Paul via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Well, 'optnone' is already not identical to –O0, and given the nature of
things, probably can't be; but I am persuaded that it's reasonable for
it to honor the –fast-isel option as a debugging tactic.  I'll take an AI to
make this happen.
Thanks,
--paulr
P.S. One nit, the "O0 + optnone" case should not have an asterisk, the
FastISel flag is not manipulated if the opt level is already zero. Does not
affect the strength of your argument, of course.

From: Daniel Sanders [mailto:Daniel.Sanders at
imgtec.com<mailto:Daniel.Sanders at imgtec.com>]
Sent: Wednesday, November 18, 2015 2:19 AM
To: Robinson, Paul; llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>

Subject: RE: Mips unconditionally uses fast-isel?

> -----Original Message-----
> From: Robinson, Paul [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 behaviour.



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.
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Daniel Sanders via llvm-dev

2015-Nov-19 11:44 UTC

head link

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

> Well, 'optnone' is already not identical to -O0, and given the
nature of things, probably can't be
It's not important to the topic but I'm curious about this since, in
instruction selection at least, they both set SelectionDAGISel::OptLevel to
CodeGenOpt::None and MCCodeGenInfo::OptLevel by one means or another. Could you
point me at a difference between the two? I assume it's outside of
instruction selection.
> P.S. One nit, the "O0 + optnone" case should not have an
asterisk, the FastISel flag is not manipulated if the opt level is already zero.
Does not affect the strength of your argument, of course.
Bugs aside, you're right. I made this table using the Mips target so it was
switching from O2 to O0.

I'll try to fix that everything-is-O2 bug today/tomorrow. It will probably
be tomorrow due to other commitments.
> This is a different motivation than Daniel expressed, which is a more
principled idea coming from the "optnone should exactly match -O0"
misconception
It doesn't change anything but just for the record: While I do think that
this should be true as far as possible, my opinion on -fast-isel=false is more
rooted in the idea that emergency overrides shouldn't be overridable by
normal use. As an analogy, circuit breakers shouldn't be overridable by a
light switch.

From: Robinson, Paul [mailto:Paul_Robinson at playstation.sony.com]
Sent: 18 November 2015 16:04
To: Daniel Sanders; llvm-dev at lists.llvm.org
Subject: RE: Mips unconditionally uses fast-isel?

Well, 'optnone' is already not identical to -O0, and given the nature of
things, probably can't be; but I am persuaded that it's reasonable for
it to honor the -fast-isel option as a debugging tactic.  I'll take an AI to
make this happen.
Thanks,
--paulr
P.S. One nit, the "O0 + optnone" case should not have an asterisk, the
FastISel flag is not manipulated if the opt level is already zero. Does not
affect the strength of your argument, of course.

From: Daniel Sanders [mailto:Daniel.Sanders at imgtec.com]
Sent: Wednesday, November 18, 2015 2:19 AM
To: Robinson, Paul; llvm-dev at lists.llvm.org
Subject: RE: Mips unconditionally uses fast-isel?

> -----Original Message-----
> From: Robinson, Paul [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 behaviour.



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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Robinson, Paul via llvm-dev

2015-Nov-19 15:30 UTC

head link

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

> > Well, 'optnone' is already not identical to -O0, and given the
nature of things, probably can't be
> It's not important to the topic but I'm curious about this since,
in instruction selection at least, they both set SelectionDAGISel::OptLevel to
CodeGenOpt::None and MCCodeGenInfo::OptLevel by one means or another. Could you
point me at a difference between the two? I assume it's outside of
instruction selection.
Without actually doing the research...  There are interactions between DAG
construction, DAG Combine in particular, possibly CodeGenPrepare, and ISel,
which I don't really understand. We tried to turn off all of DAG Combine,
IIRC, but that totally didn't work, so instead we turn off individual bits
and pieces.
We made a distinct effort to find all the IR and MachineFunction passes that
don't run at O0 and turn them off individually (Chandler was particularly
insistent that the pass manager should not be aware of 'optnone') and
it's always possible that we missed some; those cases should be properly
considered bugs.  And of course new passes get added from time to time, and
typically people don't think about 'optnone' when they're doing
that... again those are properly speaking bugs.

Regarding the "circuit-breaker" analogy, 'optnone' itself is a
kind of user-visible circuit breaker, and making it not break the entire circuit
could be considered a little strange.  But as I said, it does seem like a useful
debugging tactic for 'optnone' itself.
--paulr

From: Daniel Sanders [mailto:Daniel.Sanders at imgtec.com]
Sent: Thursday, November 19, 2015 3:45 AM
To: Robinson, Paul; llvm-dev at lists.llvm.org
Subject: RE: Mips unconditionally uses fast-isel?
> Well, 'optnone' is already not identical to -O0, and given the
nature of things, probably can't be
It's not important to the topic but I'm curious about this since, in
instruction selection at least, they both set SelectionDAGISel::OptLevel to
CodeGenOpt::None and MCCodeGenInfo::OptLevel by one means or another. Could you
point me at a difference between the two? I assume it's outside of
instruction selection.
> P.S. One nit, the "O0 + optnone" case should not have an
asterisk, the FastISel flag is not manipulated if the opt level is already zero.
Does not affect the strength of your argument, of course.
Bugs aside, you're right. I made this table using the Mips target so it was
switching from O2 to O0.

I'll try to fix that everything-is-O2 bug today/tomorrow. It will probably
be tomorrow due to other commitments.
> This is a different motivation than Daniel expressed, which is a more
principled idea coming from the "optnone should exactly match -O0"
misconception
It doesn't change anything but just for the record: While I do think that
this should be true as far as possible, my opinion on -fast-isel=false is more
rooted in the idea that emergency overrides shouldn't be overridable by
normal use. As an analogy, circuit breakers shouldn't be overridable by a
light switch.

From: Robinson, Paul [mailto:Paul_Robinson at playstation.sony.com]
Sent: 18 November 2015 16:04
To: Daniel Sanders; llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>
Subject: RE: Mips unconditionally uses fast-isel?

Well, 'optnone' is already not identical to -O0, and given the nature of
things, probably can't be; but I am persuaded that it's reasonable for
it to honor the -fast-isel option as a debugging tactic.  I'll take an AI to
make this happen.
Thanks,
--paulr
P.S. One nit, the "O0 + optnone" case should not have an asterisk, the
FastISel flag is not manipulated if the opt level is already zero. Does not
affect the strength of your argument, of course.

From: Daniel Sanders [mailto:Daniel.Sanders at imgtec.com]
Sent: Wednesday, November 18, 2015 2:19 AM
To: Robinson, Paul; llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>
Subject: RE: Mips unconditionally uses fast-isel?

> -----Original Message-----
> From: Robinson, Paul [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 behaviour.



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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