llvm dev - May 2016 - LLVM issuse:AArch64 TargetParser

Hi,

A64 versus A32/T32 code generation is controlled by the -target option which I
don’t believe is under discussion here.

James
On 18 May 2016, at 13:17, Bruce Hoult <bruce at hoult.org<mailto:bruce at
hoult.org>> wrote:

Note that armv8a modifies the A32 and T32 instruction sets, and is therefore an
important -march option for 32 bit code. Therefore armv8a can not be used to
imply aarch64.

For example:

- IT predication instruction with more than a single controlled 16 bit
instruction, or an instruction that modifies PC is deprecated. All old uses of
IT work by default, but a control bit enables the OS to cause a trap on use of a
deprecated use.

- new A32 and T32 Load Acquire/Store Release instructions

- new A32 and T32 scalar FP instructions including conditional select, min, max,
new rounding modes for FP to int conversion, conversion between half precision
and double precision

- new A32 and T32 vector forms of the above FP operations

- new A32 and T32 cryptographic instructions

- changes to barrier instructions

Users can reasonably expect -march=armv8a for 32 bit code to potentially
generate the new FP instructions and restrict the use of the IT instruction.


On Thu, May 5, 2016 at 1:11 PM, Jojo Ma via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hi everyone,

I'm a member engineer of linaro's llvm team,coming from Spreadtrum.I am
a new person on LLVM.Now I'm writing a Target Parser for AArch64,so options
parsing of AArch64 about cpu & arch & fpu can be summary to one place.

In the TargetParser,we assume "aarch64" and "arm64" are
synonyms of armv8a(as they are only for armv8a,people usually do this). So after
using AArch64TargetParser in options parsing in Clang,using
"-march=aarch64" and "-march=arm64" are both valid.

I saw Kevin has a different view on this in his submission:r213353.

+// RUN: %clang -target aarch64 -march=arm64 -### -c %s 2>&1 | FileCheck
-check-prefix=CHECK-ARCH-NOT-ACCEPT %s
+// RUN: %clang -target aarch64 -march=aarch64 -### -c %s 2>&1 |
FileCheck -check-prefix=CHECK-ARCH-NOT-ACCEPT %s
+// CHECK-ARCH-NOT-ACCEPT: error: the clang compiler does not support

So I failed on the regression testing.So I think we should reach a consensus on
this issue.
Would you mind letting me know what you think of this issuse?

Thank you very much!Looking forward to your reply!
I enclose the attachments for your reference.

Best Regards,
Jojo.ma<http://Jojo.ma>

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On 18 May 2016 at 13:21, James Molloy via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> A64 versus A32/T32 code generation is controlled by the -target option
which
> I don’t believe is under discussion here.
Indeed, that is the case. The -target option picks the back-end, and
-march chooses the support level, same as GCC.

Just to be clear:

  "-target armv8" implies AArch32, so you can't use
"-march=aarch64" with it.
  "-target aarch64" is strictly AArch64, so you can't use
"-march=armv7" or lower with it.

However, the first case above is an *aberration* and the recommended usage is:

  "-target arm -march=armv8a" which will select the "ARMv8A
AArch32
ARM instruction set".
  "-target aarch64 -march=armv8a" which will select the "ARMv8A
AArch64 instruction set".

Also, please do *not* use:

  "-target thumb -march=armv8a", even though it works today in LLVM,
it's another aberration.

Instead, use:

  "-target arm -march=armv8a -mthumb". This will correctly choose the
"ARMv8A AArch32 Thumb2 instruction set".

cheers,
--renato

Thanks!

Just for completeness, if you want to force T16, should you use

 "-target arm -march=armv6 -mthumb -mfloat-abi=soft"

Or is there a better way?

(the last option needed because T16 has no access to FP registers, so you'd
better either not be using FP arguments/results at all or else compile
everything else you link to with softfp as well)


On Wed, May 18, 2016 at 3:31 PM, Renato Golin <renato.golin at linaro.org>
wrote:
> On 18 May 2016 at 13:21, James Molloy via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hi,
> >
> > A64 versus A32/T32 code generation is controlled by the -target option
> which
> > I don’t believe is under discussion here.
>
> Indeed, that is the case. The -target option picks the back-end, and
> -march chooses the support level, same as GCC.
>
> Just to be clear:
>
>   "-target armv8" implies AArch32, so you can't use
"-march=aarch64" with
> it.
>   "-target aarch64" is strictly AArch64, so you can't use
> "-march=armv7" or lower with it.
>
> However, the first case above is an *aberration* and the recommended usage
> is:
>
>   "-target arm -march=armv8a" which will select the "ARMv8A
AArch32
> ARM instruction set".
>   "-target aarch64 -march=armv8a" which will select the
"ARMv8A
> AArch64 instruction set".
>
> Also, please do *not* use:
>
>   "-target thumb -march=armv8a", even though it works today in
LLVM,
> it's another aberration.
>
> Instead, use:
>
>   "-target arm -march=armv8a -mthumb". This will correctly choose
the
> "ARMv8A AArch32 Thumb2 instruction set".
>
> cheers,
> --renato
>
> --
> This message has been scanned for viruses and
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> believed to be clean.
>
>
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