ARM Clang: couldn't allocate output register for constraint 'w'

Question

uint8_t* dataPtr;
uint8x8x4_t dataVec;
__asm__ __volatile__( "vldmia %1, %h0" : "=w"( dataVec ) : "r"( dataPtr ) );

Above ARM inline assembly code works fine when compiled with Android NDK using GCC toolchain for armeabi-v7a ABI. However, I get following compiler error when I switch to Clang

error: couldn't allocate output register for constraint 'w'

According to LLVM docs, constraint 'w' can be used for SIMD register on ARM target.

Am I missing something? Has anyone encountered this issue? Is it a bug in LLVM?

Thanks

Answer 1

As mentioned in the LLVM Docs:

The constraint codes are, in general, expected to behave the same way they do in GCC. LLVM’s support is often implemented on an ‘as-needed’ basis, to support C inline asm code which was supported by GCC. A mismatch in behavior between LLVM and GCC likely indicates a bug in LLVM.

Maybe it's a good idea for you to report this on https://bugs.llvm.org/