[clang][BFloat] Avoid redefining bfloat16_t in arm_neon.h

[dim]

Summary

As of https://reviews.llvm.org/D79708, clang-tblgen generates arm_neon.h,
arm_sve.h and arm_bf16.h, and all those generated files will contain a
typedef of bfloat16_t. However, arm_neon.h and arm_sve.h include
arm_bf16.h immediately before their own typedef:

#include <arm_bf16.h>
typedef __bf16 bfloat16_t;

With a recent version of clang (I used 16.0.1) this results in warnings:

/usr/lib/clang/16/include/arm_neon.h:38:16: error: redefinition of typedef 'bfloat16_t' is a C11 feature [-Werror,-Wtypedef-redefinition]

Since arm_bf16.h is very likely supposed to be the one true place where
bfloat16_t is defined, I propose to delete the duplicate typedefs from the
generated arm_neon.h and arm_sve.h.

[dim]

Also fix up arm_sve.h.

[simonbutcher]

I can't see anything wrong with this patch and it looks pretty straightforward to me, but is it necessary?

'-Wtypedef-redefinition' is documented as not applying to system headers, and I couldn't reproduce the issue unless I changed the headers to not be system headers.

[dim]

In D148822#4305188, @simonbutcher wrote:

I can't see anything wrong with this patch and it looks pretty straightforward to me, but is it necessary?

'-Wtypedef-redefinition' is documented as not applying to system headers, and I couldn't reproduce the issue unless I changed the headers to not be system headers.

I think this due to the way some parts of FreeBSD get compiled, where we definitely use -Wsystem-headers. It is likely that this warning (which was turned into an error because parts of the tree also get compiled with -Werror) came through because of that. In any case, there is no need to redefine bfloat16_t, at least not here. Another option is to add yet another define like __bfloat_16_defined and test for that, but it only makes things uglier.

[sdesmalen]

I agree it makes sense to remove the typedef if they are also defined in arm_bf16.h.

[simonbutcher]

we definitely use -Wsystem-headers.

Fair enough, that hadn't occurred to me. The change LGTM.

clang/utils/TableGen/NeonEmitter.cpp

void NeonEmitter::run(raw_ostream &OS) {
 
   OS << "#if !defined(__ARM_NEON)\n";
   OS << "#error \"NEON support not enabled\"\n";
   OS << "#else\n\n";
 
   OS << "#include <stdint.h>\n\n";
 
   OS << "#include <arm_bf16.h>\n";
-  OS << "typedef __bf16 bfloat16_t;\n";
 
   // Emit NEON-specific scalar typedefs.
   OS << "typedef float float32_t;\n";
   OS << "typedef __fp16 float16_t;\n";
 
   OS << "#ifdef __aarch64__\n";
   OS << "typedef double float64_t;\n";
   OS << "#endif\n\n";