[LLVM] Add exported visibility style for XCOFF

For the AIX linker, under default options, global or weak symbols which
have no visibility bits set to zero (i.e. no visibility, similar to ELF
default) are only exported if specified on an export list provided to
the linker. So AIX has an additional visibility style called
"exported" which indicates to the linker that the symbol should
be explicitly globally exported.

This change maps "dllexport" in the LLVM IR to correspond to XCOFF
exported as we feel this best models the intended semantic (discussion
on the discourse RFC thread: https://discourse.llvm.org/t/rfc-adding-exported-visibility-style-to-the-ir-to-model-xcoff-exported-visibility/61853)
and allows us to enable writing this visibility for the AIX target
in the assembly path.

Reviewed By: DiggerLin

Differential Revision: https://reviews.llvm.org/D123951

Author: daltenty

llvm/docs/LangRef.rst

@@ -481,7 +481,10 @@ styles:
     means that the declaration is visible to other modules and, in
     shared libraries, means that the declared entity may be overridden.
     On Darwin, default visibility means that the declaration is visible
-    to other modules. Default visibility corresponds to "external
+    to other modules. On XCOFF, default visibility means no explicit
+    visibility bit will be set and whether the symbol is visible
+    (i.e "exported") to other modules depends primarily on export lists
+    provided to the linker. Default visibility corresponds to "external
     linkage" in the language.
 "``hidden``" - Hidden style
     Two declarations of an object with hidden visibility refer to the
@@ -512,12 +515,15 @@ DLL storage class:
     symbol. On Microsoft Windows targets, the pointer name is formed by
     combining ``__imp_`` and the function or variable name.
 ``dllexport``
-    "``dllexport``" causes the compiler to provide a global pointer to a pointer
-    in a DLL, so that it can be referenced with the ``dllimport`` attribute. On
-    Microsoft Windows targets, the pointer name is formed by combining
-    ``__imp_`` and the function or variable name. Since this storage class
-    exists for defining a dll interface, the compiler, assembler and linker know
-    it is externally referenced and must refrain from deleting the symbol.
+    On Microsoft Windows targets, "``dllexport``" causes the compiler to provide
+    a global pointer to a pointer in a DLL, so that it can be referenced with the
+    ``dllimport`` attribute. the pointer name is formed by combining ``__imp_``
+    and the function or variable name. On XCOFF targets, ``dllexport`` indicates
+    that the symbol will be made visible to other modules using "exported"
+    visibility and thus placed by the linker in the loader section symbol table.
+    Since this storage class exists for defining a dll interface, the compiler,
+    assembler and linker know it is externally referenced and must refrain from
+    deleting the symbol.
 
 .. _tls_model:
 

llvm/include/llvm/MC/MCAsmInfo.h

@@ -430,6 +430,10 @@ class MCAsmInfo {
   /// hidden visibility.  Defaults to MCSA_Hidden.
   MCSymbolAttr HiddenVisibilityAttr = MCSA_Hidden;
 
+  /// This attribute, if not MCSA_Invalid, is used to declare a symbol as having
+  /// exported visibility.  Defaults to MCSA_Exported.
+  MCSymbolAttr ExportedVisibilityAttr = MCSA_Exported;
+
   /// This attribute, if not MCSA_Invalid, is used to declare an undefined
   /// symbol as having hidden visibility. Defaults to MCSA_Hidden.
   MCSymbolAttr HiddenDeclarationVisibilityAttr = MCSA_Hidden;
@@ -758,6 +762,8 @@ class MCAsmInfo {
 
   MCSymbolAttr getHiddenVisibilityAttr() const { return HiddenVisibilityAttr; }
 
+  MCSymbolAttr getExportedVisibilityAttr() const { return ExportedVisibilityAttr; }
+
   MCSymbolAttr getHiddenDeclarationVisibilityAttr() const {
     return HiddenDeclarationVisibilityAttr;
   }

llvm/include/llvm/MC/MCDirectives.h

@@ -31,6 +31,7 @@ enum MCSymbolAttr {
   MCSA_LGlobal,                 ///< .lglobl (XCOFF)
   MCSA_Extern,                  ///< .extern (XCOFF)
   MCSA_Hidden,                  ///< .hidden (ELF)
+  MCSA_Exported,                ///< .globl _foo, exported (XCOFF)
   MCSA_IndirectSymbol,          ///< .indirect_symbol (MachO)
   MCSA_Internal,                ///< .internal (ELF)
   MCSA_LazyReference,           ///< .lazy_reference (MachO)

llvm/lib/MC/MCAsmStreamer.cpp

@@ -762,6 +762,8 @@ bool MCAsmStreamer::emitSymbolAttribute(MCSymbol *Symbol,
   case MCSA_WeakDefAutoPrivate: OS << "\t.weak_def_can_be_hidden\t"; break;
   case MCSA_Cold:
     // Assemblers currently do not support a .cold directive.
+  case MCSA_Exported:
+    // Non-AIX assemblers currently do not support exported visibility.
     return false;
   }
 
@@ -904,6 +906,9 @@ void MCAsmStreamer::emitXCOFFSymbolLinkageWithVisibility(
   case MCSA_Protected:
     OS << ",protected";
     break;
+  case MCSA_Exported:
+    OS << ",exported";
+    break;
   default:
     report_fatal_error("unexpected value for Visibility type");
   }

llvm/lib/MC/MCELFStreamer.cpp

@@ -215,6 +215,7 @@ bool MCELFStreamer::emitSymbolAttribute(MCSymbol *S, MCSymbolAttr Attribute) {
   case MCSA_WeakDefAutoPrivate:
   case MCSA_Invalid:
   case MCSA_IndirectSymbol:
+  case MCSA_Exported:
     return false;
 
   case MCSA_NoDeadStrip:

llvm/lib/MC/MCMachOStreamer.cpp

@@ -358,6 +358,7 @@ bool MCMachOStreamer::emitSymbolAttribute(MCSymbol *Sym,
   case MCSA_Weak:
   case MCSA_Local:
   case MCSA_LGlobal:
+  case MCSA_Exported:
     return false;
 
   case MCSA_Global:

llvm/lib/MC/MCWasmStreamer.cpp

@@ -121,6 +121,7 @@ bool MCWasmStreamer::emitSymbolAttribute(MCSymbol *S, MCSymbolAttr Attribute) {
   case MCSA_Invalid:
   case MCSA_IndirectSymbol:
   case MCSA_Protected:
+  case MCSA_Exported:
     return false;
 
   case MCSA_Hidden:

llvm/lib/MC/MCXCOFFStreamer.cpp

@@ -56,6 +56,9 @@ bool MCXCOFFStreamer::emitSymbolAttribute(MCSymbol *Sym,
   case llvm::MCSA_Protected:
     Symbol->setVisibilityType(XCOFF::SYM_V_PROTECTED);
     break;
+  case llvm::MCSA_Exported:
+    Symbol->setVisibilityType(XCOFF::SYM_V_EXPORTED);
+    break;
   default:
     report_fatal_error("Not implemented yet.");
   }

llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp

@@ -1898,10 +1898,15 @@ void PPCAIXAsmPrinter::emitLinkage(const GlobalValue *GV,
 
   MCSymbolAttr VisibilityAttr = MCSA_Invalid;
   if (!TM.getIgnoreXCOFFVisibility()) {
+    if (GV->hasDLLExportStorageClass() && !GV->hasDefaultVisibility())
+      report_fatal_error(
+          "Cannot not be both dllexport and non-default visibility");
     switch (GV->getVisibility()) {
 
-    // TODO: "exported" and "internal" Visibility needs to go here.
+    // TODO: "internal" Visibility needs to go here.
     case GlobalValue::DefaultVisibility:
+      if (GV->hasDLLExportStorageClass())
+        VisibilityAttr = MAI->getExportedVisibilityAttr();
       break;
     case GlobalValue::HiddenVisibility:
       VisibilityAttr = MAI->getHiddenVisibilityAttr();

llvm/test/CodeGen/PowerPC/aix-xcoff-exported-nondefault.ll

@@ -0,0 +1,15 @@
+; RUN: not --crash llc -verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff 2>&1 < %s | \
+; RUN:   FileCheck %s
+; RUN: not --crash llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff 2>&1 < %s |\
+; RUN:   FileCheck %s
+
+; RUN: not --crash llc -verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff \
+; RUN:     -filetype=obj -o %t.o < %s 2>&1 | \
+; RUN:   FileCheck %s
+
+; RUN: not --crash llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff \
+; RUN:     -filetype=obj -o %t.o 2>&1 < %s 2>&1 | \
+; RUN:   FileCheck %s
+
+; CHECK: LLVM ERROR: Cannot not be both dllexport and non-default visibility
+@b_e = hidden dllexport global i32 0, align 4