[cfe-dev] Behavior of -mcpu

[David Greene via cfe-dev]

Thanks.  Just to make sure I understand everything, for X86-64, using
-march will cause LLVM (via TargetTransformInfo) to use tuning
parameters appropriate to the subtarget given, whatever they may be,
along with the appropriate scheduling model, etc.?

And the same is true with -mcpu for AArch64 targets?

What about other targets?  Do we know what the rules are for them?

It would be great to have this all documented.  Is there any effort
around that?

                                 -David

Craig Topper <craig.topper at gmail.com> writes:

> According to gcc's documentation, -march implies -mtune for x86. Which
> matches what clang does without having -mtune. Their documentation
> also says -mcpu is a depcrated synonym for -mtune. So I'm not sure
> that -mcpu ever did -march and -mtune together on gcc at least for x86
>
> ~Craig
>
> On Fri, Mar 1, 2019 at 8:27 AM David Greene via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
>
>     Kristof Beyls <Kristof.Beyls at arm.com> writes:
>     
>     > Hi David, 
>     >
>     > I thought that mtune just wasn’t supported for any target in
>     clang,
>     > but I may be wrong.
>     > FWIW, the following thread discusses what would be needed to
>     implement
>     > support for mtune:
>     >
>     http://lists.llvm.org/pipermail/llvm-dev/2017-October/118344.html
>     > AFAIK, so far nobody has worked on implementing that proposal.
>     
>     Now I'm (more) confused. If -mtune doesn't work and -mcpu isn't
>     supported for x86-64 (I get a warning about an unused option) how
>     do
>     users compile for a target like skylake-avx512 and have the
>     optimizer
>     tune for it?
>     
>     -David
>     
>     > On 28 Feb 2019, at 21:42, David Greene via cfe-dev
>     > <cfe-dev at lists.llvm.org> wrote:
>     >
>     > 
>     > 
>     > Stefan Teleman <stefan.teleman at gmail.com> writes:
>     > 
>     > On Thu, Feb 28, 2019 at 2:09 PM David Greene via cfe-dev
>     > <cfe-dev at lists.llvm.org> wrote:
>     > 
>     > 
>     > Interesting. I am observing different behavior between -
>     > mcpu= and
>     > -march= -mtune= with aarch64-unknown-linux-gnu:
>     >
>     > We never supported -mtune=thunderx2t99.
>     > 
>     > -mcpu=thunderx2t99
>     > -march=armv8.1-a+lse
>     >
>     > This is problematic for users. If clang interprets command-line
>     > flags
>     > differently based on subtarget, how are users to know what to
>     pass
>     > to
>     > clang to get, say, the best performance, or best code size, or
>     > whatever?
>     > 
>     > Users don't expect to have to set up their build systems to pass
>     > different -mcpu/-march/-mtune flags based on what they are
>     > targeting.
>     > They want to do something like this and be done with it:
>     > 
>     > TARGET=$(call get_triple)
>     > ARCH=$(call get_arch)
>     > TUNE=$(call get_tune)
>     > 
>     > CFLAGS += --target ${TARGET} -march=${ARCH) -mtune=${TUNE}
>     > 
>     > They could, but probably won't be happy to, do:
>     > 
>     > ifeq (TARGET,aarch64-unknown-linux-gnu)
>     > ifeq (TUNE,thunderx2t99)
>     > # -mtune unsupported
>     > CFLAGS += -mcpu=${TUNE}
>     > else ifeq (TUNE,anothertarget)
>     > # -mcpu unsupported
>     > CFLAGS += -march=${ARCH} -mtune=${TUNE}
>     > else ifeq (TUNE,thirdtarget)
>     > CFLAGS += -mtune=${TUNE}
>     > endif
>     > else ifeq(TARGET,x86_64-unknown-linux-gnu)
>     > ifeq (TUNE,skylake-avx512)
>     > # -mtune unsupported
>     > CFLAGS += -mcpu=${TUNE}
>     > else ifeq (TUNE,broadwell)
>     > # -mcpu unsupported
>     > CFLAGS += -march=${ARCH} -mtune=${TUNE}
>     > endif
>     > else ifeq(TARGET,third-target-triple)
>     > # -mcpu and -mtune unsupported
>     > CFLAGS += -march=${ARCH}
>     > endif
>     > 
>     > It's hard enough for users to support building for multiple
>     > subtargets
>     > without having to deal with basic compiler behavior differences
>     > per
>     > subtarget.
>     > 
>     > Perhaps there is some capability of clang I'm not understanding.
>     > What's
>     > the expected/canonical way to tune compilation for a subtarget?
>     > 
>     > -David
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