[XCOFF] XCOFF constants, MCObjectFileInfo placeholder code for DWARF; NFC

[hubert.reinterpretcast]

Summary

This patch introduces the constants defined to identify DWARF sections in XCOFF into llvm/BinaryFormat/XCOFF.h and adds (NFC) placeholder code to llvm/lib/MC/MCObjectFileInfo.cpp where the DWARF sections for XCOFF are to be set up.

[jasonliu]

llvm/include/llvm/BinaryFormat/XCOFF.h

96

nit:
If we want to have the comments telling what each one is doing, do we want to consider taking the description from https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/filesreference/XCOFF.html
I find the description there is much clear than the short names we are using in the comments.

     Value   |     Macro	      | Description
		-----------------------------------------------
		0x10000  | SSUBTYP_DWINFO   | DWARF info section
		0x20000  | SSUBTYP_DWLINE   | DWARF line-number section
		0x30000  | SSUBTYP_DWPBNMS  | DWARF public names section
		0x40000  | SSUBTYP_DWPBTYP  | DWARF public types section
		0x50000  | SSUBTYP_DWARNGE  | DWARF aranges section
		0x60000  | SSUBTYP_DWABREV  | DWARF abbreviation section
		0x70000  | SSUBTYP_DWSTR    | DWARF strings section
		0x80000  | SSUBTYP_DWRNGES  | DWARF ranges section
    0x90000  | SSUBTYPE_DWLOC   | DWARF location lists section
		0xA0000  | SSUBTYPE_DWFRAME | DWARF frames section
	  0xB0000  | SSUBTYPE_DWMAC   | DWARF macros section

[hubert.reinterpretcast]

llvm/include/llvm/BinaryFormat/XCOFF.h

96

The "short names" here are precisely the second-half of the officially defined names. The descriptions from the AIX docs are a bad idea because they are ambiguous given DWARF 5's replacement of some of these with newer sections used for similar information and matching the same description.

[DiggerLin]

llvm/lib/MC/MCObjectFileInfo.cpp

841

what about to the initiation in the MCObjectFileInfo.h ?

as
MCSection *DwarfAbbrevSection = = nullptr;

[hubert.reinterpretcast]

llvm/lib/MC/MCObjectFileInfo.cpp

841

That's the common default that this code is supposed to eventually override. The patch description already says this patch is NFC.

[sfertile]

LGTM.

llvm/include/llvm/BinaryFormat/XCOFF.h

86

I didn't know there is a separator for numeric literals, that is pretty cool 👍 .

[jasonliu]

LGTM.

[DiggerLin]

LGTM

llvm/include/llvm/BinaryFormat/XCOFF.h

enum SectionTypeFlags {
   STYP_TDATA = 0x0400,
   STYP_TBSS = 0x0800,
   STYP_LOADER = 0x1000,
   STYP_DEBUG = 0x2000,
   STYP_TYPCHK = 0x4000,
   STYP_OVRFLO = 0x8000
 };
 
+/// Values for defining the section subtype of sections of type STYP_DWARF as
+/// they would appear in the (signed, 32-bit) s_flags field of the section
+/// header structure, contributing to the 16 most significant bits. Defined in
+/// the system header `scnhdr.h`.
+enum DwarfSectionSubtypeFlags : int32_t {
+  SSUBTYP_DWINFO = 0x1'0000,  ///< DWARF info section
+  SSUBTYP_DWLINE = 0x2'0000,  ///< DWARF line section
+  SSUBTYP_DWPBNMS = 0x3'0000, ///< DWARF pubnames section
+  SSUBTYP_DWPBTYP = 0x4'0000, ///< DWARF pubtypes section
+  SSUBTYP_DWARNGE = 0x5'0000, ///< DWARF aranges section
+  SSUBTYP_DWABREV = 0x6'0000, ///< DWARF abbrev section
+  SSUBTYP_DWSTR = 0x7'0000,   ///< DWARF str section
+  SSUBTYP_DWRNGES = 0x8'0000, ///< DWARF ranges section
+  SSUBTYP_DWLOC = 0x9'0000,   ///< DWARF loc section
+  SSUBTYP_DWFRAME = 0xA'0000, ///< DWARF frame section
+  SSUBTYP_DWMAC = 0xB'0000    ///< DWARF macinfo section
+};
+
 // STORAGE CLASSES, n_sclass field of syment.
 // The values come from `storclass.h` and `dbxstclass.h`.
 enum StorageClass : uint8_t {
   // Storage classes used for symbolic debugging symbols.
   C_FILE = 103,  // File name
   C_BINCL = 108, // Beginning of include file
   C_EINCL = 109, // Ending of include file
   C_GSYM = 128,  // Global variable

llvm/lib/MC/MCObjectFileInfo.cpp

ReadOnlySection = Ctx->getXCOFFSection(
       XCOFF::C_HIDEXT, SectionKind::getReadOnly());
 
   TOCBaseSection = Ctx->getXCOFFSection(
       "TOC", XCOFF::StorageMappingClass::XMC_TC0, XCOFF::XTY_SD,
       XCOFF::C_HIDEXT, SectionKind::getData());
 
   // The TOC-base always has 0 size, but 4 byte alignment.
   TOCBaseSection->setAlignment(Align(4));
+
+  // DWARF sections for XCOFF are not csects. They are special STYP_DWARF
+  // sections, and the individual DWARF sections are distinguished by their
+  // section subtype.
+  // TODO: Populate the DWARF sections appropriately.
+  DwarfAbbrevSection = nullptr;   // SSUBTYP_DWABREV
+  DwarfInfoSection = nullptr;     // SSUBTYP_DWINFO
+  DwarfLineSection = nullptr;     // SSUBTYP_DWLINE
+  DwarfFrameSection = nullptr;    // SSUBTYP_DWFRAME
+  DwarfPubNamesSection = nullptr; // SSUBTYP_DWPBNMS
+  DwarfPubTypesSection = nullptr; // SSUBTYP_DWPBTYP
+  DwarfStrSection = nullptr;      // SSUBTYP_DWSTR
+  DwarfLocSection = nullptr;      // SSUBTYP_DWLOC
+  DwarfARangesSection = nullptr;  // SSUBTYP_DWARNGE
+  DwarfRangesSection = nullptr;   // SSUBTYP_DWRNGES
+  DwarfMacinfoSection = nullptr;  // SSUBTYP_DWMAC
 }
 
 void MCObjectFileInfo::InitMCObjectFileInfo(const Triple &TheTriple, bool PIC,
                                             MCContext &ctx,
                                             bool LargeCodeModel) {
   PositionIndependent = PIC;
   Ctx = &ctx;