[cfe-dev] Question about thumb2 define in clang for ARMv8

[Peter Smith via cfe-dev]

Hello,

My turn for the double post, my mail client stripped off the list on reply-all and I’d assumed it was llvm-dev. My apologies, resending back on cfe-dev

This is likely down to the default triple and CPU that clang uses.

An arm-linux-gnueabihf-gcc defaults to Thumb architecture v7-A which supports Thumb2. Whereas clang defaults to Arm state and architecture v4t which does not support Thumb2.

On a relatively recent build of clang, but I think this goes back some way:
clang --target=arm-linux-gnueabihf -mthumb -march=armv7-a -dM -E - < /dev/null | grep thumb
#define __thumb2__ 1
#define __thumb__ 1

Can you give that a try?

Peter


From: cfe-dev <cfe-dev-bounces at lists.llvm.org> On Behalf Of Mikhail R. Gadelha via cfe-dev
Sent: 10 December 2021 15:32
To: clang mailing-list <cfe-dev at lists.llvm.org>
Subject: [cfe-dev] Question about thumb2 define in clang for ARMv8

(Sorry if this is a double post, I sent this email a few days ago from an e-mail that's not subscribed to the mailing list and it seems to be stuck in moderation).

Hi everyone,

tl;dr: GCC defines __thumb2__ in ARMv8 and clang doesn't. Is that a bug? Is there a better way to check if thumb2 is supported? Maybe __ARM_ARCH_ISA_THUMB?

~

I recently started to work with Webkit in ARMv8 and decided to give clang a try building the javascript engine (called JSC) used by WebKit.

However, it turns out the JIT was always disabled because of a CMake script that checks if the compiler defines either thumb2 or __thumb2__:

https://github.com/WebKit/WebKit/blob/363701084a50c009bec41164cdb843df66d769d1/Source/cmake/OptionsCommon.cmake#L31

I've tested with some other versions of clang but none seem to define it: https://godbolt.org/z/WjGh3eTzv

While __thumb2__ is defined in GCC:

$ gcc -dM -E  main.c  | grep thumb
#define __thumb2__ 1
#define __thumb__ 1

clang doesn't define __thumb__ nor __thumb2__.

I managed to get clang with __thumb2__ defined by:
1. Changing the triplet: I eventually got a lot of compilation errors and gave up.
2. Added "-D__thumb__=1" to CMAKE_CXX_FLAGS :P and managed to fully build JSC. There were some regressions in a place I was investigating a bug but nothing major.

Anyway, I guess my questions are:
1. Is there a reason why __thumb2__ isn't defined in clang? Do we want it to be defined in clang?
2. Should we change WebKit to check if "__ARM_ARCH_ISA_THUMB" is 2 as per [0], instead of __thumb2__? (both GCC and clang seem to define it to 2 in the hardware I'm testing).

Thanks in advance.
Mikhail.

[0] https://github.com/llvm/llvm-project/blob/8ee55ab6c06ec9f769f1086e3761c92286d56339/clang/lib/Basic/Targets/ARM.cpp#L709

--

Mikhail R. Gadelha.
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