[llvm] [LLVM][Clang][AArch64] Implement AArch64 build attributes (PR #118771)

[via llvm-commits]

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@@ -294,6 +379,58 @@ AArch64ELFStreamer &AArch64TargetELFStreamer::getStreamer() {
   return static_cast<AArch64ELFStreamer &>(Streamer);
 }
 
+void AArch64TargetELFStreamer::emitSubsection(unsigned Vendor,
+                                              unsigned IsMandatory,
+                                              unsigned ParameterType) {
+  StringRef VendorAsStr = ARMBuildAttrs::vendorToStr(Vendor);
+
+  // If exists, return.
+  for (MCELFStreamer::AttributeSubSection &SubSection : AttributeSubSections) {
+    if (SubSection.Vendor == VendorAsStr) {
+      llvm_unreachable("AArch64 build attributes subsection already exists");
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sivan-shani wrote:

Clarification: 
> It is valid and possible to emit attribute to any subsection at any time
Referring to the ELF mechanism. Asm attributes need to be emitted in order since they are being printed directly. 

https://github.com/llvm/llvm-project/pull/118771