[AArch64] Always add PURECODE flag to empty .text if "+execute-only" is set (#132196)

Previously, the `SHF_AARCH64_PURECODE` section flag wasn't added to the
implicitly created `.text` section if the module didn't contain any
functions, because no other section had the flag set.

Now, the `SHF_AARCH64_PURECODE` is always added if the "+execute-only"
target feature is set for the module during compilation.

Author: Il-Capitano

llvm/lib/Target/AArch64/AArch64TargetObjectFile.cpp

@@ -16,6 +16,7 @@
 #include "llvm/IR/Module.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCValue.h"
 using namespace llvm;
@@ -27,6 +28,14 @@ void AArch64_ELFTargetObjectFile::Initialize(MCContext &Ctx,
   // AARCH64 ELF ABI does not define static relocation type for TLS offset
   // within a module.  Do not generate AT_location for TLS variables.
   SupportDebugThreadLocalLocation = false;
+
+  // Make sure the implicitly created empty .text section has the
+  // SHF_AARCH64_PURECODE flag set if the "+execute-only" target feature is
+  // present.
+  if (TM.getMCSubtargetInfo()->hasFeature(AArch64::FeatureExecuteOnly)) {
+    auto *Text = cast<MCSectionELF>(TextSection);
+    Text->setFlags(Text->getFlags() | ELF::SHF_AARCH64_PURECODE);
+  }
 }
 
 void AArch64_ELFTargetObjectFile::emitPersonalityValueImpl(

llvm/test/CodeGen/AArch64/execute-only-empty.ll

@@ -0,0 +1,13 @@
+; RUN: llc -filetype=obj -mtriple=aarch64 -mattr=+execute-only %s -o %t.o
+; RUN: llvm-readobj -S %t.o | FileCheck %s
+
+; CHECK:         Name: .text
+; CHECK-NEXT:    Type: SHT_PROGBITS
+; CHECK-NEXT:    Flags [
+; CHECK-NEXT:      SHF_AARCH64_PURECODE
+; CHECK-NEXT:      SHF_ALLOC
+; CHECK-NEXT:      SHF_EXECINSTR
+; CHECK-NEXT:    ]
+; CHECK-NEXT:    Address:
+; CHECK-NEXT:    Offset:
+; CHECK-NEXT:    Size: 0