[Object] Add ELF section type for offloading objects

Currently we use the `.llvm.offloading` section to store device-side
objects inside the host, creating a fat binary. The contents of these
sections is currently determined by the name of the section while it
should ideally be determined by its type. This patch adds the new
`SHT_LLVM_OFFLOADING` section type to the ELF section types. Which
should make it easier to identify this specific data format.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D129052

llvm/docs/Extensions.rst

@@ -450,6 +450,18 @@ Example:
    .uleb128  .LBB_END0_1-.LBB0_1          # BB_1 size
    .byte     y                            # BB_1 metadata
 
+``SHT_LLVM_OFFLOADING`` Section (offloading data)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+This section stores the binary data used to perform offloading device linking
+and execution, creating a fat binary. This section is emitted during compilation
+of offloading languages such as OpenMP or CUDA. If the data is intended to be
+used by the device linker only, it should use the ``SHF_EXCLUDE`` flag so it is
+automatically stripped from the final executable or shared library.
+
+The binary data stored in this section conforms to a custom binary format used
+for storing offloading metadata. This format is effectively a string table
+containing metadata accompanied by a device image.
+
 CodeView-Dependent
 ------------------
 

llvm/include/llvm/BinaryFormat/ELF.h

@@ -990,6 +990,7 @@ enum : unsigned {
                   // backward-compatibility).
   SHT_LLVM_CALL_GRAPH_PROFILE = 0x6fff4c09, // LLVM Call Graph Profile.
   SHT_LLVM_BB_ADDR_MAP = 0x6fff4c0a,        // LLVM Basic Block Address Map.
+  SHT_LLVM_OFFLOADING = 0x6fff4c0b,         // LLVM device offloading data.
   // Android's experimental support for SHT_RELR sections.
   // https://android.googlesource.com/platform/bionic/+/b7feec74547f84559a1467aca02708ff61346d2a/libc/include/elf.h#512
   SHT_ANDROID_RELR = 0x6fffff00,   // Relocation entries; only offsets.

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -501,6 +501,9 @@ static unsigned getELFSectionType(StringRef Name, SectionKind K) {
   if (hasPrefix(Name, ".preinit_array"))
     return ELF::SHT_PREINIT_ARRAY;
 
+  if (hasPrefix(Name, ".llvm.offloading"))
+    return ELF::SHT_LLVM_OFFLOADING;
+
   if (K.isBSS() || K.isThreadBSS())
     return ELF::SHT_NOBITS;
 

llvm/lib/MC/MCParser/ELFAsmParser.cpp

@@ -660,6 +660,8 @@ EndStmt:
       Type = ELF::SHT_LLVM_SYMPART;
     else if (TypeName == "llvm_bb_addr_map")
       Type = ELF::SHT_LLVM_BB_ADDR_MAP;
+    else if (TypeName == "llvm_offloading")
+      Type = ELF::SHT_LLVM_OFFLOADING;
     else if (TypeName.getAsInteger(0, Type))
       return TokError("unknown section type");
   }

llvm/lib/MC/MCSectionELF.cpp

@@ -167,6 +167,8 @@ void MCSectionELF::printSwitchToSection(const MCAsmInfo &MAI, const Triple &T,
     OS << "llvm_bb_addr_map";
   else if (Type == ELF::SHT_LLVM_BB_ADDR_MAP_V0)
     OS << "llvm_bb_addr_map_v0";
+  else if (Type == ELF::SHT_LLVM_OFFLOADING)
+    OS << "llvm_offloading";
   else
     report_fatal_error("unsupported type 0x" + Twine::utohexstr(Type) +
                        " for section " + getName());

llvm/lib/Object/ELF.cpp

@@ -297,6 +297,7 @@ StringRef llvm::object::getELFSectionTypeName(uint32_t Machine, unsigned Type) {
     STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_PART_PHDR);
     STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_BB_ADDR_MAP_V0);
     STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_BB_ADDR_MAP);
+    STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_OFFLOADING);
     STRINGIFY_ENUM_CASE(ELF, SHT_GNU_ATTRIBUTES);
     STRINGIFY_ENUM_CASE(ELF, SHT_GNU_HASH);
     STRINGIFY_ENUM_CASE(ELF, SHT_GNU_verdef);

llvm/lib/ObjectYAML/ELFYAML.cpp

@@ -656,6 +656,7 @@ void ScalarEnumerationTraits<ELFYAML::ELF_SHT>::enumeration(
   ECase(SHT_LLVM_PART_PHDR);
   ECase(SHT_LLVM_BB_ADDR_MAP_V0);
   ECase(SHT_LLVM_BB_ADDR_MAP);
+  ECase(SHT_LLVM_OFFLOADING);
   ECase(SHT_GNU_ATTRIBUTES);
   ECase(SHT_GNU_HASH);
   ECase(SHT_GNU_verdef);

llvm/test/CodeGen/X86/offload_sections.ll

@@ -4,5 +4,5 @@
 @llvm.embedded.object = private constant [1 x i8] c"\00", section ".llvm.offloading"
 @llvm.compiler.used = appending global [1 x ptr] [ptr @llvm.embedded.object], section "llvm.metadata"
 
-; CHECK-ELF: .section	.llvm.offloading,"e"
+; CHECK-ELF: .section	.llvm.offloading,"e",@llvm_offloading
 ; CHECK-COFF: .section	.llvm.offloading,"dr"

llvm/test/MC/AsmParser/llvm_section_types.s

@@ -13,6 +13,8 @@
 .byte 1
 .section    .section6,"",@llvm_dependent_libraries
 .byte 1
+.section    .section7,"",@llvm_offloading
+.byte 1
 
 # CHECK:        Name: .section1
 # CHECK-NEXT:   Type: SHT_LLVM_BB_ADDR_MAP
@@ -26,3 +28,5 @@
 # CHECK-NEXT:   Type: SHT_LLVM_SYMPART
 # CHECK:        Name: .section6
 # CHECK-NEXT:   Type: SHT_LLVM_DEPENDENT_LIBRARIES
+# CHECK:        Name: .section7
+# CHECK-NEXT:   Type: SHT_LLVM_OFFLOADING

llvm/test/ObjectYAML/ELF/sht-offloading.yaml

@@ -0,0 +1,28 @@
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-readobj --sections %t | FileCheck -check-prefix=OBJ %s
+# RUN: obj2yaml %t | FileCheck -check-prefix=YAML %s
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+Sections:
+  - Name:            .llvm.offloading
+    Type:            SHT_LLVM_OFFLOADING
+    Flags:           [ SHF_EXCLUDE ]
+
+#      OBJ: Sections [
+#      OBJ:   Section {
+#      OBJ:     Index: 1
+# OBJ-NEXT:     Name: .llvm.offloading (1)
+# OBJ-NEXT:     Type: SHT_LLVM_OFFLOADING (0x6FFF4C0B)
+# OBJ-NEXT:     Flags [ (0x80000000)
+# OBJ-NEXT:       SHF_EXCLUDE (0x80000000)
+# OBJ-NEXT:     ]
+
+
+#      YAML: Sections:
+# YAML-NEXT:   - Name:            .llvm.offloading
+# YAML-NEXT:     Type:            SHT_LLVM_OFFLOADING
+# YAML-NEXT:     Flags:           [ SHF_EXCLUDE ]