Handle local commons for XCOFF object file writing

[jasonliu]

Summary

Adds support for emitting common local global symbols to an XCOFF object file.
Local commons are emitted into the .bss section with a storage class of
C_HIDEXT.

[hubert.reinterpretcast]

llvm/lib/MC/XCOFFObjectWriter.cpp

240

"CSect" would match how the function is named.

llvm/test/CodeGen/PowerPC/aix-xcoff-lcomm.ll

5

There should be a follow-up patch to this (dependent on D65240) that checks the symbol table entries.

19

There is no matching RUN line.

[hubert.reinterpretcast]

llvm/lib/MC/XCOFFObjectWriter.cpp

240

I guess it's "CSECT" now.

242

There's no Add in the base patch anymore. This needs a rebase.

llvm/test/CodeGen/PowerPC/aix-xcoff-lcomm.ll

5

D65240 might land before this, in which case, this patch should be updated for the test changes.

[jasonliu]

Moving forward this patch for David.

[jasonliu]

Rebased the patch on master.

[sfertile]

LGTM.

[hubert.reinterpretcast]

LGTM.

[hubert.reinterpretcast]

Whereas commons cannot be placed into the same section, local commons can be. While it is true that there are no csect members defined in the symbol table for a csect for local commons, the separate label names are available in the assembly. In the object file, these are offsets from the csect. How references are done via the TOC is a matter of choice.

llvm/lib/MC/MCXCOFFStreamer.cpp

 //===- lib/MC/MCXCOFFStreamer.cpp - XCOFF Object Output -------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file assembles .s files and emits XCOFF .o object files.
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/MC/MCXCOFFStreamer.h"
+#include "llvm/BinaryFormat/XCOFF.h"
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCCodeEmitter.h"
 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCSymbolXCOFF.h"
+#include "llvm/MC/MCXCOFFStreamer.h"
 #include "llvm/Support/TargetRegistry.h"
 
 using namespace llvm;
 
 MCXCOFFStreamer::MCXCOFFStreamer(MCContext &Context,
                                  std::unique_ptr<MCAsmBackend> MAB,
                                  std::unique_ptr<MCObjectWriter> OW,
                                  std::unique_ptr<MCCodeEmitter> Emitter)
MCStreamer *llvm::createXCOFFStreamer(MCContext &Context,
                                       bool RelaxAll) {
   MCXCOFFStreamer *S = new MCXCOFFStreamer(Context, std::move(MAB),
                                            std::move(OW), std::move(CE));
   if (RelaxAll)
     S->getAssembler().setRelaxAll(true);
   return S;
 }
 
-void MCXCOFFStreamer::EmitXCOFFLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
-                                            unsigned ByteAlign) {
-  report_fatal_error("Emission of local commons not implemented yet.");
+void MCXCOFFStreamer::EmitXCOFFLocalCommonSymbol(MCSymbol *Symbol,
+                                                 uint64_t Size,
+                                                 unsigned ByteAlignment) {
+  EmitCommonSymbol(Symbol, Size, ByteAlignment);
 }

llvm/lib/MC/XCOFFObjectWriter.cpp

case XCOFF::XMC_PR:
       break;
     case XCOFF::XMC_RW:
       if (XCOFF::XTY_CM == MCSec->getCSectType()) {
         BSSCsects.emplace_back(MCSec);
         WrapperMap[MCSec] = &BSSCsects.back();
         break;
       }
       report_fatal_error("Unhandled mapping of read-write csect to section.");
+    case XCOFF::XMC_BS:
+      assert(XCOFF::XTY_CM == MCSec->getCSectType() &&
+             "Mapping invalid csect. CSECT with bss storage class must be "
+             "common type.");
+      BSSCsects.emplace_back(MCSec);
+      WrapperMap[MCSec] = &BSSCsects.back();
+      break;
     default:
       report_fatal_error("Unhandled mapping of csect to section.");
     }
   }
 
   for (const MCSymbol &S : Asm.symbols()) {
     // Nothing to do for temporary symbols.
     if (S.isTemporary())

llvm/test/CodeGen/PowerPC/aix-xcoff-lcomm.ll

 ; RUN: llc -mtriple powerpc-ibm-aix-xcoff < %s | FileCheck %s
 ; RUN: llc -mtriple powerpc64-ibm-aix-xcoff < %s | FileCheck %s
 
+; RUN: llc -mtriple powerpc-ibm-aix-xcoff -filetype=obj -o %t.o < %s
+; RUN: llvm-readobj --section-headers --file-header %t.o | \
+; RUN: FileCheck --check-prefix=OBJ %s
+
+; RUN: not llc -mtriple powerpc64-ibm-aix-xcoff -filetype=obj < %s 2>&1 | \
+; RUN: FileCheck --check-prefix=OBJ64 %s
+; OBJ64: LLVM ERROR: 64-bit XCOFF object files are not supported yet.
+
 @a = internal global i32 0, align 4
 @b = internal global i64 0, align 8
 @c = internal global i16 0, align 2
 
 ; CHECK:      .lcomm a,4,a,2
 ; CHECK-NEXT: .lcomm b,8,b,3
 ; CHECK-NEXT: .lcomm c,2,c,1
+
+; OBJ:      File: {{.*}}aix-xcoff-lcomm.ll.tmp.o
+; OBJ-NEXT: Format: aixcoff-rs6000
+; OBJ-NEXT: Arch: powerpc
+; OBJ-NEXT: AddressSize: 32bit
+; OBJ-NEXT: FileHeader {
+; OBJ-NEXT:   Magic: 0x1DF
+; OBJ-NEXT:   NumberOfSections: 1
+; OBJ-NEXT:   TimeStamp:
+; OBJ-NEXT:   SymbolTableOffset: 0x3C
+; OBJ-NEXT:   SymbolTableEntries: 6
+; OBJ-NEXT:   OptionalHeaderSize: 0x0
+; OBJ-NEXT:   Flags: 0x0
+; OBJ-NEXT: }
+; OBJ-NEXT: Sections [
+; OBJ-NEXT:   Section {
+; OBJ-NEXT:     Index: 1
+; OBJ-NEXT:     Name: .bss
+; OBJ-NEXT:     PhysicalAddress: 0x0
+; OBJ-NEXT:     VirtualAddress: 0x0
+; OBJ-NEXT:     Size: 0x14
+; OBJ-NEXT:     RawDataOffset: 0x0
+; OBJ-NEXT:     RelocationPointer: 0x0
+; OBJ-NEXT:     LineNumberPointer: 0x0
+; OBJ-NEXT:     NumberOfRelocations: 0
+; OBJ-NEXT:     NumberOfLineNumbers: 0
+; OBJ-NEXT:     Type: STYP_BSS (0x80)
+; OBJ-NEXT:   }
+; OBJ-NEXT: ]