llvm dev - Aug 2016 - Inconsistency in -march option between llc and clang

Hi

On AArch64 (ARM 64-Bit Platform), I see there is an inconsistency in the 
values accepted by -march option between clang and llc.

---
$ llc -march=aarch64 -mattr=help 2>&1 | grep error
$ #No Error

$ llc -march=armv8a -mattr=help 2>&1 | grep error
llc: error: invalid target 'armv8a'.

$ clang -march=armv8a -E - < /dev/null  2>&1 | grep error
$ #No Error

$ clang -march=aarch64 -E - < /dev/null  2>&1 | grep error
clang-4.0: error: the clang compiler does not support '-march=aarch64'
---

llc accepts aarch64 (rejects armv8a) while clang accepts armv8a (rejects 
aarch64).

Shall I file this as bug (and assign it to myself)? Or am I missing 
something obvious?

Thanks a lot
--
Supra

On 29 August 2016 at 06:17, Suprateeka R Hegde via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> On AArch64 (ARM 64-Bit Platform), I see there is an inconsistency in the
> values accepted by -march option between clang and llc.
This is because Clang has a triple which provides the primary context
for interpreting -march. armv8a is a valid -march in both AArch32 and
AArch64 modes.

llc, on the other hand, uses -march as an abbreviated way to choose
the primary triple. It accepts a more limited set of options (arm,
thumb, arm64 or aarch64). It's not supposed to be a user-facing tool
though, so as long as there's a way to get the configuration you want
giving it a sophisticated interface isn't a priority (and arguably not
even desirable).

For llc, you probably want to use things like "-mattr=+v8.1a" to
specify architecture revisions (v8a is the default, since it's the
first 64-bit ARM version). And use either a full triple like
"-mtriple=aarch64-linux-gnu", or "-march=aarch64" if you
don't care
what OS gets targeted (be careful there: tests vary subtly between iOS
and Linux and can break depending on the host machine if you're not
careful).

Cheers.

Tim.

I am OK with this explanation -- not to polish the user-interface of llc 
  as it is not supposed to be a user-facing tool.

But this needs to be documented well. Package developers need to very 
well understand this, Today I see that llc is used quite a lot of places 
specially in system software development. For instance, POCL (Portable 
OpenCL) uses clang and llc both. But tries to use the same value for 
-march and fails.

--
Supra

On 29-Aug-2016 08:35 PM, Tim Northover wrote:
> On 29 August 2016 at 06:17, Suprateeka R Hegde via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> On AArch64 (ARM 64-Bit Platform), I see there is an inconsistency in
the
>> values accepted by -march option between clang and llc.
>
> This is because Clang has a triple which provides the primary context
> for interpreting -march. armv8a is a valid -march in both AArch32 and
> AArch64 modes.
>
> llc, on the other hand, uses -march as an abbreviated way to choose
> the primary triple. It accepts a more limited set of options (arm,
> thumb, arm64 or aarch64). It's not supposed to be a user-facing tool
> though, so as long as there's a way to get the configuration you want
> giving it a sophisticated interface isn't a priority (and arguably not
> even desirable).
>
> For llc, you probably want to use things like "-mattr=+v8.1a" to
> specify architecture revisions (v8a is the default, since it's the
> first 64-bit ARM version). And use either a full triple like
> "-mtriple=aarch64-linux-gnu", or "-march=aarch64" if
you don't care
> what OS gets targeted (be careful there: tests vary subtly between iOS
> and Linux and can break depending on the host machine if you're not
> careful).
>
> Cheers.
>
> Tim.
>