[llvm-dev] [RFC] New Clang target selection options for ARM/AArch64

David Spickett via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 21 03:05:59 PDT 2018


Below is a document detailing changes we'd like to make to Clang/LLVM to improve the usability of the target options for ARM and AArch64.

To keep things simple the proposed changes are listed at the start and you can find the supporting examples at the end of the document.

I look forward to your feedback.

David Spickett.

RFC New Clang target feature selection options for ARM/AArch64

In this RFC we propose changes to ARM and AArch64 target selection. With the top level goals to:
- validate that given options make sense within architectural restrictions
- make option discovery and documentation easier
- unify the list of extensions that command lines and asm directives use
- bring the options closer to GCC's where appropriate

Current Options Comparison

                       | GCC           | Clang         |
                       | ARM | AArch64 | ARM | AArch64 |
| -march with '+<ext>' | Y   | Y       | Y   | Y       |
| checks extensions    | Y   | N       | N   | N       |
| .arch with '+<ext>'  | N   | N       | N   | Y       |
| .arch_extension      | Y   | Y       | Y   | N       |
| .fpu                 | Y   | N       | Y   | N       |
| -mfpu                | Y   | N       | Y   | N       |
| checks FPUs          | N   | n/a     | N   | n/a     |

Examples of each of these can be found at the end of this document.

Problems With the Current Options

- You cannot select all extensions through an assembly directive, since the AsmParser's list is a separate subset of the complete one in TargetParser.
- Combinations of options are not checked for compatibility.
- Many extensions are tied to their base architecture, though it is valid to add them individually to a previous v8.x-a architecture.
- Users need to work out what FPU they need for ARM, this should be implied by the selected arch and extensions.
- Discovery of valid extensions is difficult, both for the user and for the purposes of generating documentation.

Proposed solution

ARM and AArch64:
- Make the TargetParser the single source for extension names, removing the AsmParser tables.
- Reject unknown extension names with a diagnostic that includes a list of valid extensions for that architecture/CPU.
- Reject invalid combinations of architecture/CPU and extensions with an error diagnostic.
- Add independent subtarget features for each extension so that v8.x+1-a extensions can be used individually with earlier v8.x-a architectures where allowed.
- Emit a warning when a mandatory feature of the base architecture is enabled with '+extension', or disabled with '+noextension'. (and ignore the option)
- Errors caused by the solution above should be able to be downgraded to warnings with the usual -W* options. This applies only to cases where there is a reasonable interpretation of the options chosen.

- Allow all possible ARM extensions in the '.arch_extension' directive, without the '+' syntax
(allow them to be recognised, they could still be rejected for compatibility).
- Add an 'auto' value for -mfpu and make it the default. Meaning that the FPU is implied by mcpu/march. If mfpu is not auto, it should override other options and a warning should be emitted.
- Reject invalid mfpu and march/mcpu combinations with an error diagnostic.
- Reject invalid arch/cpu and extension combinations with an error diagnostic.

Optional features

- add the '.arch_extension' directive, with the same behaviour as ARM (no '+', one extension per directive). This brings Clang in line with GCC which has this directive for both architectures. Clang does however allow you to achieve the same thing by using '+' with '.arch'.

- Allow '+' in '.arch' and '.cpu'. GCC does not allow this, but it would make ARM/AArch64 more consistent within Clang.

Options Comparison With the Proposed Solution

Anything in brackets has changed from the previous table.

                       | GCC           | Clang             |
                       | ARM | AArch64 | ARM     | AArch64 |
| -march with '+<ext>' | Y   | Y       | Y       | Y       |
| checks extensions    | Y   | N       | (Y)     | (Y)     |
| .arch with '+<ext>'  | N   | N       | (Y)     | Y       | (optional)
| .arch_extension      | Y   | Y       | Y       | (Y)     | (optional)
| -mfpu                | Y   | N       | Y       | N       |
| .fpu                 | Y   | N       | Y       | N       |
| checks FPUs          | N   | n/a     | (Y)     | n/a     |


Use of Table-gen

The current implementation of TargetParser has a number of FIXME comments saying that it should be changed to use tablegen instead of pre processor macros. There are several advantages of porting TargetParser to tablegen:
- more readable than the current macros
- allows default/optional values more easily
- we can generate code and documentation from the same source
- easier to add new properties

- it requires a new tablegen backend to generate the include files
- additional indirection which could make debugging and future changes more difficult

We think the benefits outweigh the disadvantages in this case.

To do this, we would need to move TargetParser to break the cyclic dependency of LLVMSupport -> llvm-tblgen -> LLVMSupport. There are 2 options for this:
1. create a new LLVMTargetParser library that contains all parsers for architectures that use it.
2. put the TargetParser for each backend in the library group for that backend. This requires one of:
    * Relaxing the requirement that target parsers must be built even if the backend is not.
    * Modifying the CMake scripts to build the target parsers even if the backend is not being built.

Option 1 is simpler but option 2 would allow us to make use of the existing tablegen files in the backends so it is preferred.

Using existing SubTarget features

If we go with option 2 above, we can reuse the existing subtarget features to work out any dependencies.

We have a prototype that took option 1 above. The command line is converted into a sequence of options and resolved by the LLVM backend. This means that Clang does not know exactly what will be enabled. It needs to know this to output the correct pre processor feature test macros.

Consider this AArch64 march:

The base arch is armv8.4-a, the crypto extension turns on AES/SHA2/SHA3/SM4. The nosha2 disables SHA2/SHA3 (since SHA3 is dependant on SHA2). Each of these features has an ACLE feature test macro, so Clang needs to know that nosha2 also disables SHA3.

New Errors and Warnings

Whether these are errors or warnings by default is up for debate. This is a suggestion to begin with.
(these apply to cmd lines and directives unless stated)

- unknown extension in an assembly directive (currently fails silently)
- extension incompatible with base arch, message shows the base arch it requires.
- extension requires another which is disabled later, message shows which one is required.
- extension requires another which is not enabled, message shows requirements.
- ARM mfpu option is not 'auto' and is incompatible with the base arch, message shows list of valid FPUs.

- ARM mfpu option is not auto and another option implies a different FPU than the mfpu value. The mfpu value will be used, and the message will show what was overridden.
- mandatory feature of the base arch is enabled with '+' (option is redundant so is ignored)
- mandatory feature of a base arch is disabled with '+no<feature>' (option makes no sense so the extension remains enabled)

Proposed diagnostic names: (in the same order as above)
- "target-feature" (top level group)
    - "incompatible-feature"
      - "extension-requirement-disabled"
    - "extension-requires"
    - "incompatible-fpu"
    - "implied-fpu-unused"
    - "mandatory-feature-ignored"
    - "mandatory-feature-disabled"

"Negative" Backend Features

There are a couple of features in ARM which remove capabilities rather than adding them. These are 'd16' (removes the top 16 D registers) and 'fp-only-sp' (removes double precision).
It would simplify the implementation if those were replaced with positive options. As in one that adds the top 16 D registers and one that enables double precision operations.

This is a relatively simple change to LLVM but it will effect a large number of tests and would be a breaking change for users of LLVM as a library.

.arch_extension Directive

Regardless of '.arch_extension' being added to AArch64, it has some issues that need to be addressed for the rest of these changes.

Extensions can now have different meanings based on the base architecture they apply to. For example on AArch64, 'crypto' means different things for v8.{1,2,3}-a than v8.4-a. The former adds 'sha2' and 'aes', the latter adds those and 'sm4' and 'sha3' on top.

We can handle this in a few of ways:
- Remove .arch_extension in favour of .arch. This conflicts with the option above to add it to AArch64 to bring us in line with GCC, and will break a lot of code written for older versions of Clang.
- Only accept options which do not vary with base architecture. For ARM, only the FPU options vary, and there is the .fpu directive for those. If we do decide to add .arch_extension to AArch64 this will mean that things like crypto will only be valid in .arch.
- Track the current base target, as implied by the command line or the last .arch/.cpu directive. This makes the directives as similar to the command lines as they can be without breaking backwards compatibility.

The last option makes the most sense to us, certainly if we want to add .arch_extension to AArch64 in a straightforward way.

ARM Assembly Directives

As discussed for AArch64 the ARM assembly directives ('.arch', '.cpu', '.fpu', '.arch_extension') should be updated to use the new target parser. Giving them access to a complete list of features.

'.arch' and '.cpu' supporting the '+' syntax is mentioned as an optional goal above. This makes ARM/AArch64 consistent within Clang but breaks from GCC's features.

Current Command Line Option Examples

Clang ARM

Extensions can be used with '+<{no}extension>' syntax on march or mcpu, there is no checking that the combinations are valid. The FPU is selected with -mfpu and this is not validated either.

$ ./clang --target=arm-arm-none-eabi -march=armv8.2-a -mfpu=none -c /tmp/test.c -o /tmp/test.o
$ ./clang --target=arm-arm-none-eabi -mcpu=cortex-a53+dotprod -c /tmp/test.c -o /tmp/test.o
(can't use dotprod with v8-a)

$ ./clang --target=arm-arm-none-eabi -march=armv7-m -mfpu=neon-fp16 -c /tmp/test.c -o /tmp/test.o
(should be invalid but is allowed)


For GCC it is the same except that mfpu defaults to 'auto', meaning that the value is implied by other options. Extensions are checked for compatibility with the base architecture but FPUs are not.

$ ./arm-eabi-gcc -mcpu=cortex-a53 -mfpu=neon -c /tmp/test.c -o /tmp/test.o
$ ./arm-eabi-gcc -march=armv8-a -mfpu=auto -c /tmp/test.c -o /tmp/test.o

$ ./arm-eabi-gcc -march=armv8-a+dotprod -c /tmp/test.c -o /tmp/test.o
arm-eabi-gcc: error: 'armv8-a' does not support feature 'dotprod'
arm-eabi-gcc: note: valid feature names are: crc simd crypto nocrypto nofp

$ ./arm-eabi-gcc -march=armv7-m -mfpu=neon-fp16 -c /tmp/test.c -o /tmp/test.o
(same example given for Clang above, should be invalid)

Clang AArch64

The '+' syntax still applies but mfpu is replaced with '+' extensions.

$ ./clang --target=aarch64-arm-none-eabi -march=armv8.2-a -mfpu=none -c /tmp/test.c -o /tmp/test.o
clang-7: warning: argument unused during compilation: '-mfpu=none' [-Wunused-command-line-argument]
$ ./clang --target=aarch64-arm-none-eabi -march=armv8.2-a+nofp -c /tmp/test.c -o /tmp/test.o
$ ./clang --target=aarch64-arm-none-eabi -march=armv8-a+crypto -c /tmp/test.c -o /tmp/test.o

Dependencies within extensions are not checked. For example crypto requires simd, but it can be disabled in the same march option.

$ ./clang --target=aarch64-arm-none-eabi -march=armv8-a+crypto+nosimd -c /tmp/test.c -o /tmp/test.o

Dependencies between an extension and the base arch are not checked either. Dot product cannot be used with v8.0-a but it is allowed.

$ ./clang --target=aarch64-arm-none-eabi -march=armv8-a+dotprod -c /tmp/test.c -o /tmp/test.o

GCC AArch64

For GCC AArch64 mfpu is also dropped in favour of '+' extensions.

$ ./aarch64-elf-gcc -march=armv8.2-a -mfpu=none -c /tmp/test.c -o /tmp/test.o
aarch64-elf-gcc: error: unrecognized command line option '-mfpu=none'; did you mean '-gz=none'?

Extensions are rejected if not recognised but not checked for compatibility. Hence the Clang crypto/simd example above is allowed with GCC too.

$ ./aarch64-elf-gcc -march=armv8.2-a+food -c /tmp/test.c -o /tmp/test.o
cc1: error: invalid feature modifier in '-march=armv8.2-a+food'
$ ./aarch64-elf-gcc -march=armv8.2-a+dotprod -c /tmp/test.c -o /tmp/test.o
$ ./aarch64-elf-gcc -march=armv8-a+dotprod -c /tmp/test.c -o /tmp/test.o
(should not be allowed)
$ ./aarch64-elf-gcc -march=armv8-a+crypto+nosimd -c /tmp/test.c -o /tmp/test.o
(should not be allowed)

Current Assembly Directive Examples

Clang .arch/.arch_extension

AArch64 uses .arch and '+' syntax, ARM uses .arch_extension/.arch and does not support '+' syntax in either.

In both arches, the list of possible extensions is not complete since it is separate from the one in TargetParser. So there is no way to enable dotprod (amongst other things) with a directive.

(example is using AArch64)
.arch armv8.2-a # error: instruction requires: dotprod
udot v0.2s, v1.8b, v2.8b

.arch armv8.2-a+dotprod # error: instruction requires: dotprod
udot v0.2s, v1.8b, v2.8b

ARM uses the .arch_extension directive which is one extension per use, with no '+'.

.arch armv7-a #error: instruction requires: crc armv8
CRC32B r0, r1, r2

.arch armv8-a+crc #error: Unknown arch name
CRC32B r0, r1, r2

.arch armv8-a # no error
.arch_extension crc
CRC32B r0, r1, r2

You can see here that though ARM march/mcpu would understand +crc, the assembly directive does not.

ARM does check validity of extensions provided with '.arch_extension'.

.arch armv7-a
.arch_extension crc
CRC32B r0, r1, r2

main.s:20:17: error: architectural extension 'crc' is not allowed for the current base architecture
.arch_extension crc

AArch64 only rejects known extensions that aren't supported at all.

.arch armv8-a+pan # unsupported architectural extension: pan

Neither ARM or AArch64 know about the inter dependencies between extensions. So the example from the command lines applies here too.

(example is using AArch64)
.arch armv8-a+crypto+nosimd # no error/warning, crypto requires simd

GCC .arch/.arch_extension

GCC is more consistent across the two arches, both use .arch and .arch_extension. Neither understand the '+' syntax.

.arch armv8-a+crc # invalid

.arch armv8-a # valid
.arch_extension crc

.arch_extension crc # valid
.arch_extension crc+crypto #invalid

For extensions that vary based on base architecture, GCC tracks the last known arch.

Clang .fpu

.fpu is only available for ARM. Values are not checked for compatibility, only rejected if completely unknown.

./clang --target=aarch64-arm-none-eabi -march=armv8-a -c /tmp/test.s -o /tmp/test.o
/tmp/test.s:1:1: error: unknown directive
.fpu neon

$ ./clang --target=arm-arm-none-eabi -march=armv7-m -c /tmp/test.s -o /tmp/test.o
/tmp/test.s:1:6: error: Unknown FPU name
.fpu clearly-not-valid

(same example as 'Clang ARM' command lines, should be invalid)
$ cat /tmp/test.s
.fpu neon-fp16
$ ./clang --target=arm-arm-none-eabi -march=armv7-m -c /tmp/test.s -o /tmp/test.o

GCC .fpu

.fpu is provided for ARM only and the FPU names are not checked against the base arch or CPU.

This is correctly rejected from a command line:
$ ./arm-eabi-gcc -march=armv6zk+neon -c /tmp/test.s -o /tmp/test.o
arm-eabi-gcc: error: 'armv6zk' does not support feature 'neon'
arm-eabi-gcc: note: valid feature names are: fp nofp vfpv2

Whereas the directive is accepted:
$ cat /tmp/test.s
.fpu neon
$ ./arm-eabi-gcc -march=armv6zk -c /tmp/test.s -o /tmp/test.o

For AArch64 .fpu is removed in favour of .arch_extension. Instead of directly selecting an FPU it is implied by the extensions used.

$ cat /tmp/test.s
.fpu neon
$ ./aarch64-elf-gcc -march=armv8-a+simd -c /tmp/test.s -o /tmp/test.o
/tmp/test.s: Assembler messages:
/tmp/test.s:1: Error: unknown pseudo-op: `.fpu'

$ cat /tmp/test.s
.arch_extension simd
$ ./aarch64-elf-gcc -march=armv8-a -c /tmp/test.s -o /tmp/test.o


Crypto extension requires SIMD: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500e/CJHDEBAF.html
GCC ARM options: https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html
GCC ARM directives: https://sourceware.org/binutils/docs/as/ARM-Directives.html
GCC AArch64 options: https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html
GCC AArch64 directives: https://sourceware.org/binutils/docs/as/AArch64-Directives.html

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