llvm dev - Mar 2018 - [cfe-dev] When to use '-mcpu' versus '-march'

Thanks Eric,

 

After the original reply to my query I had a good look at the GCC documentation
for these options, and what I discovered is that “there is no consensus” in GCC.
Basically, saying do what GCC does was a non-answer as it clarified nothing.

 

X86 has deprecated ‘-mcpu’ in favour of ‘-mtune’, and it uses ‘-mtune’ to mean
that the scheduling, etc. should be biased in favour of more performant code for
the processor identified by ‘-mtune’, but that the code is still 100% functional
on all other processors in the family identified by ‘-march’.

 

MIPS and ARM seem to have a very different notion of what they mean, and at the
end of my examination of these options, I came to the conclusion that there is
no common convention for usage and it all seems very haphazard.

 

With the processor that I am targeting, the notion of tuning schedules for a
particular variant is meaningless, because the schedule is critical in the
absence of interlocked instruction execution (and VLIW).  This effectively means
that ‘-mtune’ is not appropriate to my platform and leaves me with the toss-up
of ‘-mcpu’ versus ‘-march’, and to tell you the truth, I can see little reason
to choose one over the other.

 

Most of the time, I want to differentiate based on ISA versions.  That is, the
code should be generated for one version of the ISA or another, though for the
same basic architecture (ISA can vary a lot from version to version).  ARM seems
to use ‘-march’ and ‘-mcpu’ more or less interchangeably, and uses ‘+’ to add
features to the value specified.

 

The majority of GCC target only use ‘-mcpu’, so I think that in the absence of
any real and meaningful best practice, that I will just stick with the triples
and ‘-mcpu’.

 

All the best,

 

            MartinO

 

From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Eric
Christopher via cfe-dev
Sent: 20 March 2018 17:50
To: Richard Hadsell <hadsell at blueskystudios.com>
Cc: Clang Dev <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] When to use '-mcpu' versus '-march'

 

Hi Richard,

On Tue, Mar 20, 2018 at 9:33 AM Richard Hadsell <hadsell at
blueskystudios.com <mailto:hadsell at blueskystudios.com> > wrote:

Does clang differ from g++ in this respect?

 

It shouldn't, we strive for compatibility here.

 

This is from the man page for g++ 4.8.5 for Intel x86 and x86-64 processors:

 

This is because this is for the in-tree x86 target. ARM is different. Mips is
more like what I said. mn10300 is more like what I said. Power is different
again, and in a different way.

 

However, Martin was asking about a non-specific out of tree target and the
advice there should still hold.

 

-eric

 

        -march=cpu-type
            Generate instructions for the machine type cpu-type.  In contrast to
-mtune=cpu-type, which merely tunes the generated code
            for the specified cpu-type, -march=cpu-type allows GCC to generate
code that may not run at all on processors other than the
            one indicated.  Specifying -march=cpu-type implies -mtune=cpu-type.
...
        -mtune=cpu-type
            Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions.
            While picking a specific cpu-type schedules things appropriately for
that particular chip, the compiler does not generate any
            code that cannot run on the default machine type unless you use a
-march=cpu-type option.  ...
...
        -mcpu=cpu-type
            A deprecated synonym for -mtune.



On 03/20/2018 12:40 AM, Eric Christopher via cfe-dev
wrote:
> Hi Martin,
>
> Sorry about the delay here, but I've got some advice even though a lot
of things aren't solid anywhere.
>
> a) Does this match an out of tree target for gcc? If so, I'd match
that.
> b) Typically -march/-mtune (though the latter isn't supported in llvm
at the moment, see an earlier post from me to llvm-dev on how to support that)
are what I'd use for new ports. We used it in a number of new/out of tree
ports in gcc as well.
> c) -mcpu is occastionally used for "-march+-mtune" when -march by
itself is generally "this architecture, but generic tuning".
>
--

Dick Hadsell                    203-992-6320 <tel:(203)%20992-6320>   Fax:
203-992-6001 <tel:(203)%20992-6001>
Reply-to:                       hadsell at blueskystudios.com <mailto:hadsell
at blueskystudios.com>
Blue Sky Studioshttp://www.blueskystudios.com
<http://www.blueskystudios.com>
1 American Lane, Greenwich, CT 06831-2560

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Hi Martin,

On Tue, Mar 20, 2018 at 1:18 PM Martin J. O'Riordan <MartinO at
theheart.ie>
wrote:
> Thanks Eric,
>
>
>
> After the original reply to my query I had a good look at the GCC
> documentation for these options, and what I discovered is that “there is no
> consensus” in GCC.  Basically, saying do what GCC does was a non-answer as
> it clarified nothing.
>
>
>
That's why I made the comments I did on a generic new target which is the
suggested consensus.

> X86 has deprecated ‘-mcpu’ in favour of ‘-mtune’, and it uses ‘-mtune’ to
> mean that the scheduling, etc. should be biased in favour of more
> performant code for the processor identified by ‘-mtune’, but that the
> code is still 100% functional on all other processors in the family
> identified by ‘-march’.
>
>
>
This is correct.

> MIPS and ARM seem to have a very different notion of what they mean, and
> at the end of my examination of these options, I came to the conclusion
> that there is no common convention for usage and it all seems very
> haphazard.
>
>
>
MIPS is very specific to using -march and -mtune. -mcpu is an alias to
(-march/-mtune).

This is what I suggest to people wanting to have new ports.

> With the processor that I am targeting, the notion of tuning schedules for
> a particular variant is meaningless, because the schedule is critical in
> the absence of interlocked instruction execution (and VLIW).  This
> effectively means that ‘-mtune’ is not appropriate to my platform and
> leaves me with the toss-up of ‘-mcpu’ versus ‘-march’, and to tell you
> the truth, I can see little reason to choose one over the other.
>
>
>
-mtune is used for more than scheduling.

> Most of the time, I want to differentiate based on ISA versions.  That is,
> the code should be generated for one version of the ISA or another, though
> for the same basic architecture (ISA can vary a lot from version to
> version).  ARM seems to use ‘-march’ and ‘-mcpu’ more or less
> interchangeably, and uses ‘+’ to add features to the value specified.
>
>
>
That's not quite true either and is subtle behavior. Overall I think the
ARM strategy is the worst compiler argument method to copy.

> The majority of GCC target only use ‘-mcpu’, so I think that in the
> absence of any real and meaningful best practice, that I will just stick
> with the triples and ‘-mcpu’.
>
>
>
I'd really suggest you not do this, but since it's going to be an out of
tree target you, may, of course do what you want.

-eric

All the best,
>
>
>
>             MartinO
>
>
>
> *From:* cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] *On Behalf Of
*Eric
> Christopher via cfe-dev
> *Sent:* 20 March 2018 17:50
> *To:* Richard Hadsell <hadsell at blueskystudios.com>
> *Cc:* Clang Dev <cfe-dev at lists.llvm.org>
> *Subject:* Re: [cfe-dev] When to use '-mcpu' versus
'-march'
>
>
>
> Hi Richard,
>
> On Tue, Mar 20, 2018 at 9:33 AM Richard Hadsell <
> hadsell at blueskystudios.com> wrote:
>
> Does clang differ from g++ in this respect?
>
>
>
> It shouldn't, we strive for compatibility here.
>
>
>
> This is from the man page for g++ 4.8.5 for Intel x86 and x86-64
> processors:
>
>
>
> This is because this is for the in-tree x86 target. ARM is different. Mips
> is more like what I said. mn10300 is more like what I said. Power is
> different again, and in a different way.
>
>
>
> However, Martin was asking about a non-specific out of tree target and the
> advice there should still hold.
>
>
>
> -eric
>
>
>
>         -march=cpu-type
>             Generate instructions for the machine type cpu-type.  In
> contrast to -mtune=cpu-type, which merely tunes the generated code
>             for the specified cpu-type, -march=cpu-type allows GCC to
> generate code that may not run at all on processors other than the
>             one indicated.  Specifying -march=cpu-type implies
> -mtune=cpu-type.
> ...
>         -mtune=cpu-type
>             Tune to cpu-type everything applicable about the generated
> code, except for the ABI and the set of available instructions.
>             While picking a specific cpu-type schedules things
> appropriately for that particular chip, the compiler does not generate any
>             code that cannot run on the default machine type unless you
> use a -march=cpu-type option.  ...
> ...
>         -mcpu=cpu-type
>             A deprecated synonym for -mtune.
>
>
>
> On 03/20/2018 12:40 AM, Eric Christopher via cfe-dev wrote:
> > Hi Martin,
> >
> > Sorry about the delay here, but I've got some advice even though a
lot
> of things aren't solid anywhere.
> >
> > a) Does this match an out of tree target for gcc? If so, I'd match
that.
> > b) Typically -march/-mtune (though the latter isn't supported in
llvm at
> the moment, see an earlier post from me to llvm-dev on how to support that)
> are what I'd use for new ports. We used it in a number of new/out of
tree
> ports in gcc as well.
> > c) -mcpu is occastionally used for "-march+-mtune" when
-march by itself
> is generally "this architecture, but generic tuning".
> >
>
> --
>
> Dick Hadsell                    203-992-6320 <(203)%20992-6320>  Fax:
> 203-992-6001 <(203)%20992-6001>
> Reply-to:                       hadsell at blueskystudios.com
> Blue Sky Studioshttp://www.blueskystudios.com
> 1 American Lane, Greenwich, CT 06831
>
<https://maps.google.com/?q=1+American+Lane,+Greenwich,+CT+06831&entry=gmail&source=g>
> -2560
>
>
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Thanks very much Eric for taking the time to carefully explain this to me.

 

So if I am the author of the backend for a new processor technology, or willing
to modernise my existing implementation, you would recommend that the ‘-mcpu’
option is deprecated and probably best not used at all, or perhaps just as a
synonym for ‘-march + -mtune’?

 

The first part of the target triple guides the overall high level processor
architecture - essentially the CPU - and that I would be better using ‘-march’
to specify the variants of this CPU architecture for groups of variants that are
not binary compatible with each other; effectively my ISA generations.  And that
‘-mtune’ is used to tune the code-generation for performance of a particular
version of my processor from a single group that are otherwise binary
compatible.

 

For the past few years, our processor has had essentially the same conceptual
hardware architecture, but each generation has had significant changes to the
ISA.  Sometimes this is just instructions added or removed, but sometimes it is
changes in the binary encoding of instructions, or the assembly syntax used for
expressing the instructions, or the scheduling constraints.  I expect that
‘-march’ would be appropriate for differentiating between these incompatible
variations of the architecture.

 

At the moment I can only think of one use for ‘-mtune’ though for my target, and
that would be to allow fine-tuning of the instructions chosen to handle some
kinds of hardware bug fixes; though this would be a rare scenario as usually HW
fixes require different code generation, not just schedule tuning.  At the
moment my backend is not cache-aware, but if I did implement the prefetch ISDs,
then ‘-mtune’ would I suppose be appropriate for this as the caches can vary
from one configuration to another.

 

When you say “-mtune is used for more than scheduling”, what kinds of use-case
would be good examples?

 

Would this be a reasonable summary?  My original question was because I found
that I could not determine a consistent usage of these options in GCC supported
targets to guide my own decisions.

 

Of course, for targets common to LLVM and GCC, it is essential that LLVM follows
the GCC uses of these switches for compatibility; but it is guidance for new
targets that I think is missing that could establish best practices for both
compiler development communities.

 

All the best and thanks again,

 

            MartinO

 

From: Eric Christopher [mailto:echristo at gmail.com] 
Sent: 20 March 2018 20:38
To: Martin J. O'Riordan <MartinO at theheart.ie>
Cc: Richard Hadsell <hadsell at blueskystudios.com>; LLVM Developers
<llvm-dev at lists.llvm.org>
Subject: Re: [cfe-dev] When to use '-mcpu' versus '-march'

 

Hi Martin,

On Tue, Mar 20, 2018 at 1:18 PM Martin J. O'Riordan <MartinO at
theheart.ie <mailto:MartinO at theheart.ie> > wrote:

After the original reply to my query I had a good look at the GCC documentation
for these options, and what I discovered is that “there is no consensus” in GCC.
Basically, saying do what GCC does was a non-answer as it clarified nothing.

That's why I made the comments I did on a generic new target which is the
suggested consensus.

X86 has deprecated ‘-mcpu’ in favour of ‘-mtune’, and it uses ‘-mtune’ to mean
that the scheduling, etc. should be biased in favour of more performant code for
the processor identified by ‘-mtune’, but that the code is still 100% functional
on all other processors in the family identified by ‘-march’.

This is correct.

MIPS and ARM seem to have a very different notion of what they mean, and at the
end of my examination of these options, I came to the conclusion that there is
no common convention for usage and it all seems very haphazard.

MIPS is very specific to using -march and -mtune. -mcpu is an alias to
(-march/-mtune).

 

This is what I suggest to people wanting to have new ports.

With the processor that I am targeting, the notion of tuning schedules for a
particular variant is meaningless, because the schedule is critical in the
absence of interlocked instruction execution (and VLIW).  This effectively means
that ‘-mtune’ is not appropriate to my platform and leaves me with the toss-up
of ‘-mcpu’ versus ‘-march’, and to tell you the truth, I can see little reason
to choose one over the other.

-mtune is used for more than scheduling.

Most of the time, I want to differentiate based on ISA versions.  That is, the
code should be generated for one version of the ISA or another, though for the
same basic architecture (ISA can vary a lot from version to version).  ARM seems
to use ‘-march’ and ‘-mcpu’ more or less interchangeably, and uses ‘+’ to add
features to the value specified.

That's not quite true either and is subtle behavior. Overall I think the ARM
strategy is the worst compiler argument method to copy.

The majority of GCC target only use ‘-mcpu’, so I think that in the absence of
any real and meaningful best practice, that I will just stick with the triples
and ‘-mcpu’.

I'd really suggest you not do this, but since it's going to be an out of
tree target you, may, of course do what you want.

 

-eric

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