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

Tue Mar 20 23:30:22 PDT 2018

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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