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

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)
@@ -7925,7 +7927,25 @@ bool AArch64AsmParser::parseDirectiveAeabiSubSectionHeader(SMLoc L) {
     return true;
   }
   Parser.Lex();
-  // consume a comma
+
+  // 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 subsection does not exists, report error.
+    if (!getTargetStreamer().hasSubsection(SubsectionName)) {
+      Error(Parser.getTok().getLoc(),
+            "Could not switch to subsection '" + SubsectionName +
+                "' using subsection name, subsection has not been defined");
+      return true;
+    }
+    // Since subsection exists, no need to call
+    // AArch64TargetStreamer::emitAttributesSubsection(...)
+    getTargetStreamer().activateAttributesSubsection(SubsectionName);
+    return false;
+  }
+
+  // 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()) {

llvm/lib/Target/AArch64/MCTargetDesc/AArch64TargetStreamer.cpp

@@ -153,17 +153,24 @@ MCTargetStreamer *llvm::createAArch64NullTargetStreamer(MCStreamer &S) {
   return new AArch64TargetStreamer(S);
 }
 
+bool AArch64TargetStreamer::hasSubsection(StringRef SubSectionName) {
+  for (MCELFStreamer::AttributeSubSection &SubSection : AttributeSubSections) {
+    if (SubSection.VendorName == SubSectionName)
+      return true;
+  }
+  return false;
+}
+
 void AArch64TargetStreamer::emitAttributesSubsection(
     StringRef VendorName, AArch64BuildAttributes::SubsectionOptional IsOptional,
     AArch64BuildAttributes::SubsectionType ParameterType) {
 
-  // If exists, return.
-  for (MCELFStreamer::AttributeSubSection &SubSection : AttributeSubSections) {
-    if (VendorName == SubSection.VendorName) {
-      activateAttributesSubsection(VendorName);
-      return;
-    }
+  // If exists, activate and return.
+  if (hasSubsection(VendorName)) {
+    activateAttributesSubsection(VendorName);
+    return;
   }
+
   // else, add the subsection
   MCELFStreamer::AttributeSubSection AttSubSection;
   AttSubSection.VendorName = VendorName;

llvm/lib/Target/AArch64/MCTargetDesc/AArch64TargetStreamer.h

@@ -117,6 +117,7 @@ class AArch64TargetStreamer : public MCTargetStreamer {
                          MCELFStreamer::AttributeSubSection &AttSubSection);
 
   SmallVector<MCELFStreamer::AttributeSubSection, 64> AttributeSubSections;
+  bool hasSubsection(StringRef SubSectionName);
 
 private:
   std::unique_ptr<AssemblerConstantPools> ConstantPools;

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