[AIX] In assembly file, create a dummy text renamed to an empty string (llvm#73052)

This works around an AIX assembler and linker bug. If the
-fno-integrated-as and -frecord-command-line options are used but
there's no actual code in the source file, the assembler creates an
object file with only an .info section. The AIX linker rejects such an
object file.

Author: stephenpeckham

llvm/include/llvm/MC/MCSectionXCOFF.h

@@ -115,6 +115,7 @@ class MCSectionXCOFF final : public MCSection {
   bool useCodeAlign() const override;
   bool isVirtualSection() const override;
   StringRef getSymbolTableName() const { return SymbolTableName; }
+  void setSymbolTableName(StringRef STN) { SymbolTableName = STN; }
   bool isMultiSymbolsAllowed() const { return MultiSymbolsAllowed; }
   bool isCsect() const { return CsectProp.has_value(); }
   bool isDwarfSect() const { return DwarfSubtypeFlags.has_value(); }

llvm/include/llvm/MC/MCSymbolXCOFF.h

@@ -52,9 +52,12 @@ class MCSymbolXCOFF : public MCSymbol {
 
   XCOFF::VisibilityType getVisibilityType() const { return VisibilityType; }
 
-  bool hasRename() const { return !SymbolTableName.empty(); }
+  bool hasRename() const { return HasRename; }
 
-  void setSymbolTableName(StringRef STN) { SymbolTableName = STN; }
+  void setSymbolTableName(StringRef STN) {
+    SymbolTableName = STN;
+    HasRename = true;
+  }
 
   StringRef getSymbolTableName() const {
     if (hasRename())
@@ -67,6 +70,7 @@ class MCSymbolXCOFF : public MCSymbol {
   MCSectionXCOFF *RepresentedCsect = nullptr;
   XCOFF::VisibilityType VisibilityType = XCOFF::SYM_V_UNSPECIFIED;
   StringRef SymbolTableName;
+  bool HasRename = false;
 };
 
 } // end namespace llvm

llvm/include/llvm/MC/MCXCOFFStreamer.h

@@ -33,10 +33,7 @@ class MCXCOFFStreamer : public MCObjectStreamer {
                                             MCSymbolAttr Visibility) override;
   void emitXCOFFRefDirective(const MCSymbol *Symbol) override;
   void emitXCOFFRenameDirective(const MCSymbol *Name,
-                                StringRef Rename) override {
-    report_fatal_error("emitXCOFFRenameDirective is not implemented yet on "
-                       "object generation path");
-  }
+                                StringRef Rename) override;
   void emitXCOFFExceptDirective(const MCSymbol *Symbol, const MCSymbol *Trap,
                                 unsigned Lang, unsigned Reason,
                                 unsigned FunctionSize, bool hasDebug) override;

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -93,6 +93,7 @@
 #include "llvm/MC/MCSectionCOFF.h"
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCSectionMachO.h"
+#include "llvm/MC/MCSectionXCOFF.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSubtargetInfo.h"
 #include "llvm/MC/MCSymbol.h"
@@ -444,7 +445,7 @@ bool AsmPrinter::doInitialization(Module &M) {
       .getModuleMetadata(M);
 
   // On AIX, we delay emitting any section information until
-  // after emitting the .file pseudo-op.  This allows additional
+  // after emitting the .file pseudo-op. This allows additional
   // information (such as the embedded command line) to be associated
   // with all sections in the object file rather than a single section.
   if (!TM.getTargetTriple().isOSBinFormatXCOFF())
@@ -496,8 +497,18 @@ bool AsmPrinter::doInitialization(Module &M) {
   // C_INFO symbol is preserved if any csect is kept by the linker.
   if (TM.getTargetTriple().isOSBinFormatXCOFF()) {
     emitModuleCommandLines(M);
-    // Now we can generate section information
+    // Now we can generate section information.
     OutStreamer->initSections(false, *TM.getMCSubtargetInfo());
+
+    // To work around an AIX assembler and/or linker bug, generate
+    // a rename for the default text-section symbol name.  This call has
+    // no effect when generating object code directly.
+    MCSection *TextSection =
+        OutStreamer->getContext().getObjectFileInfo()->getTextSection();
+    MCSymbolXCOFF *XSym =
+        static_cast<MCSectionXCOFF *>(TextSection)->getQualNameSymbol();
+    if (XSym->hasRename())
+      OutStreamer->emitXCOFFRenameDirective(XSym, XSym->getSymbolTableName());
   }
 
   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();

llvm/lib/MC/MCObjectFileInfo.cpp

@@ -933,10 +933,16 @@ void MCObjectFileInfo::initXCOFFMCObjectFileInfo(const Triple &T) {
   // the ABI or object file format, but various tools rely on the section
   // name being empty (considering named symbols to be "user symbol names").
   TextSection = Ctx->getXCOFFSection(
-      "", SectionKind::getText(),
+      "..text..", // Use a non-null name to work around an AIX assembler bug...
+      SectionKind::getText(),
       XCOFF::CsectProperties(XCOFF::StorageMappingClass::XMC_PR, XCOFF::XTY_SD),
       /* MultiSymbolsAllowed*/ true);
 
+  // ... but use a null name when generating the symbol table.
+  MCSectionXCOFF *TS = static_cast<MCSectionXCOFF *>(TextSection);
+  TS->getQualNameSymbol()->setSymbolTableName("");
+  TS->setSymbolTableName("");
+
   DataSection = Ctx->getXCOFFSection(
       ".data", SectionKind::getData(),
       XCOFF::CsectProperties(XCOFF::StorageMappingClass::XMC_RW, XCOFF::XTY_SD),

llvm/lib/MC/MCStreamer.cpp

@@ -1190,10 +1190,7 @@ void MCStreamer::emitXCOFFSymbolLinkageWithVisibility(MCSymbol *Symbol,
 }
 
 void MCStreamer::emitXCOFFRenameDirective(const MCSymbol *Name,
-                                          StringRef Rename) {
-  llvm_unreachable("emitXCOFFRenameDirective is only supported on "
-                   "XCOFF targets");
-}
+                                          StringRef Rename) {}
 
 void MCStreamer::emitXCOFFRefDirective(const MCSymbol *Symbol) {
   llvm_unreachable("emitXCOFFRefDirective is only supported on XCOFF targets");

llvm/lib/MC/MCXCOFFStreamer.cpp

@@ -96,6 +96,13 @@ void MCXCOFFStreamer::emitXCOFFRefDirective(const MCSymbol *Symbol) {
   DF->getFixups().push_back(Fixup);
 }
 
+void MCXCOFFStreamer::emitXCOFFRenameDirective(const MCSymbol *Name,
+                                               StringRef Rename) {
+  const MCSymbolXCOFF *Symbol = cast<const MCSymbolXCOFF>(Name);
+  if (!Symbol->hasRename())
+    report_fatal_error("Only explicit .rename is supported for XCOFF.");
+}
+
 void MCXCOFFStreamer::emitXCOFFExceptDirective(const MCSymbol *Symbol,
                                                const MCSymbol *Trap,
                                                unsigned Lang, unsigned Reason,

llvm/test/CodeGen/PowerPC/aix-alias.ll

@@ -55,15 +55,15 @@ entry:
 ; ASM-NEXT:    .csect fun[DS]
 ; ASM-NEXT:  fun_weak:                               # @fun
 ; ASM-NEXT:  fun_hidden:
-; ASM:         .csect [PR],5
+; ASM:         .csect ..text..[PR],5
 ; ASM-NEXT:  .fun:
 ; ASM-NEXT:  .fun_weak:
 ; ASM-NEXT:  .fun_hidden:
 ; ASM-NEXT:  # %bb.0:
 ; ASM-NEXT:    li 3, 0
 ; ASM-NEXT:    blr
 ; ASM-NEXT:                                          # -- End function
-; ASM:       .csect [PR],5
+; ASM:       .csect ..text..[PR],5
 ; ASM-NEXT:  .test:
 ; ASM-NEXT:  # %bb.0:                                # %entry
 ; ASM:         bl .fun

llvm/test/CodeGen/PowerPC/aix-emit-tracebacktable-clobber-register.ll

@@ -99,7 +99,7 @@ entry:
 ; COMMON-NEXT:  .align  2
 ; COMMON-NEXT:  .vbyte  4, 0
 ; COMMON-NEXT:  .vbyte  4, 0
-; CHECK-ASM-NEXT:   .csect [PR],5
+; CHECK-ASM-NEXT:   .csect ..text..[PR],5
 ; CHECK-FUNC-NEXT:  .csect .foov[PR],5
 ; COMMON-NEXT:                                         # -- End function
 ; COMMON:       .toc

llvm/test/CodeGen/PowerPC/aix-extern-weak.ll

@@ -41,7 +41,7 @@ declare extern_weak void @foo_ext_weak(ptr)
 ; BIT64-NEXT:     .vbyte	8, .main                   # @main
 ; BIT64-NEXT:     .vbyte	8, TOC[TC0]
 ; BIT64-NEXT:     .vbyte	8, 0
-; COMMON-NEXT:    .csect  [PR]
+; COMMON-NEXT:    .csect  ..text..[PR]
 ; COMMON-NEXT:    .main:
 
 ; COMMON:         .csect  .data[RW]