[LLVM][AArch64] Build attributes: Support switching to a defined subsection by name only (#154159)

The AArch64 build attribute specification now allows switching to an
already-defined subsection using its name alone, without repeating the
optionality and type parameters.

This patch updates the parser to support that behavior.

Spec reference: https://github.com/ARM-software/abi-aa/pull/230/files

Author: sivan-shani

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

@@ -7909,9 +7909,11 @@ bool AArch64AsmParser::parseDirectiveSEHSavePReg(SMLoc L) {
 }
 
 bool AArch64AsmParser::parseDirectiveAeabiSubSectionHeader(SMLoc L) {
-  // Expecting 3 AsmToken::Identifier after '.aeabi_subsection', a name and 2
-  // parameters, e.g.: .aeabi_subsection (1)aeabi_feature_and_bits, (2)optional,
-  // (3)uleb128 separated by 2 commas.
+  // Handle parsing of .aeabi_subsection directives
+  // - On first declaration of a subsection, expect exactly three identifiers
+  //   after `.aeabi_subsection`: the subsection name and two parameters.
+  // - When switching to an existing subsection, it is valid to provide only
+  //   the subsection name, or the name together with the two parameters.
   MCAsmParser &Parser = getParser();
 
   // Consume the name (subsection name)
@@ -7925,16 +7927,38 @@ bool AArch64AsmParser::parseDirectiveAeabiSubSectionHeader(SMLoc L) {
     return true;
   }
   Parser.Lex();
-  // consume a comma
+
+  std::unique_ptr<MCELFStreamer::AttributeSubSection> SubsectionExists =
+      getTargetStreamer().getAttributesSubsectionByName(SubsectionName);
+  // Check whether only the subsection name was provided.
+  // If so, the user is trying to switch to a subsection that should have been
+  // declared before.
+  if (Parser.getTok().is(llvm::AsmToken::EndOfStatement)) {
+    if (SubsectionExists) {
+      getTargetStreamer().emitAttributesSubsection(
+          SubsectionName,
+          static_cast<AArch64BuildAttributes::SubsectionOptional>(
+              SubsectionExists->IsOptional),
+          static_cast<AArch64BuildAttributes::SubsectionType>(
+              SubsectionExists->ParameterType));
+      return false;
+    }
+    // If subsection does not exists, report error.
+    else {
+      Error(Parser.getTok().getLoc(),
+            "Could not switch to subsection '" + SubsectionName +
+                "' using subsection name, subsection has not been defined");
+      return true;
+    }
+  }
+
+  // Otherwise, expecting 2 more parameters: consume a comma
   // parseComma() return *false* on success, and call Lex(), no need to call
   // Lex() again.
   if (Parser.parseComma()) {
     return true;
   }
 
-  std::unique_ptr<MCELFStreamer::AttributeSubSection> SubsectionExists =
-      getTargetStreamer().getAttributesSubsectionByName(SubsectionName);
-
   // Consume the first parameter (optionality parameter)
   AArch64BuildAttributes::SubsectionOptional IsOptional;
   // options: optional/required

llvm/test/MC/AArch64/build-attributes-asm-aeabi-switch_using_name_only.s

@@ -0,0 +1,39 @@
+// RUN: llvm-mc -triple=aarch64 %s -o - | FileCheck %s --check-prefix=ASM
+// RUN: llvm-mc -triple=aarch64 -filetype=obj %s -o - | llvm-readelf --hex-dump=.ARM.attributes - | FileCheck %s --check-prefix=ELF
+
+// ASM: .aeabi_subsection aeabi_pauthabi, required, uleb128
+// ASM: .aeabi_subsection aeabi_feature_and_bits, optional, uleb128
+// ASM: .aeabi_attribute	0, 1 // Tag_Feature_BTI
+// ASM: .aeabi_attribute	2, 1 // Tag_PAuth_Schema
+// ASM: .aeabi_attribute	1, 1 // Tag_PAuth_Platform
+// ASM: .aeabi_attribute	2, 1 // Tag_Feature_GCS
+// ASM: .aeabi_attribute	1, 0 // Tag_Feature_PAC
+// ASM: .aeabi_attribute	7, 1
+// ASM: .aeabi_attribute	7, 0
+
+// ELF: Hex dump of section '.ARM.attributes':
+// ELF-NEXT: 0x00000000 411b0000 00616561 62695f70 61757468 A....aeabi_pauth
+// ELF-NEXT: 0x00000010 61626900 00000201 01010700 25000000 abi.........%...
+// ELF-NEXT: 0x00000020 61656162 695f6665 61747572 655f616e aeabi_feature_an
+// ELF-NEXT: 0x00000030 645f6269 74730001 00000102 01010007 d_bits..........
+// ELF-NEXT: 0x00000040 01
+
+
+.aeabi_subsection aeabi_pauthabi, required, uleb128
+.aeabi_subsection aeabi_feature_and_bits, optional, uleb128
+.aeabi_attribute Tag_Feature_BTI, 1
+.aeabi_subsection aeabi_feature_and_bits
+.aeabi_subsection aeabi_pauthabi
+.aeabi_attribute Tag_PAuth_Schema, 1
+.aeabi_subsection aeabi_pauthabi
+.aeabi_attribute Tag_PAuth_Platform, 1
+.aeabi_subsection aeabi_pauthabi
+.aeabi_subsection aeabi_feature_and_bits
+.aeabi_attribute Tag_Feature_GCS, 1
+.aeabi_subsection aeabi_pauthabi
+.aeabi_subsection aeabi_feature_and_bits
+.aeabi_attribute Tag_Feature_PAC, 0
+.aeabi_subsection aeabi_feature_and_bits
+.aeabi_attribute 7, 1
+.aeabi_subsection aeabi_pauthabi
+.aeabi_attribute 7, 0