[clang-offload-wrapper] Add standard notes for ELF offload images

The patch adds ELF notes into SHT_NOTE sections of ELF offload images
passed to clang-offload-wrapper.

The new notes use a null-terminated "LLVMOMPOFFLOAD" note name.
There are currently three types of notes:

VERSION: a string (not null-terminated) representing the ELF offload
image structure. The current version '1.0' does not put any restrictions
on the structure of the image. If we ever need to come up with a common
structure for ELF offload images (e.g. to be able to analyze the images
in libomptarget in some standard way), then we will introduce new versions.

PRODUCER: a vendor specific name of the producing toolchain.
Upstream LLVM uses "LLVM" (not null-terminated).

PRODUCER_VERSION: a vendor specific version of the producing toolchain.
Upstream LLVM uses LLVM_VERSION_STRING with optional <space> LLVM_REVISION.

All three notes are not mandatory currently.

Differential Revision: https://reviews.llvm.org/D99551

Author: Vyacheslav Zakharin

clang/test/Driver/Inputs/empty-elf-template.yaml

@@ -0,0 +1,5 @@
+--- !ELF
+FileHeader:
+  Class: ELFCLASS[[BITS]]
+  Data:  ELFDATA2[[ENCODING]]
+  Type:  ET_REL

clang/test/Driver/clang-offload-wrapper.c

@@ -19,9 +19,10 @@
 //
 // Check bitcode produced by the wrapper tool.
 //
-// RUN: clang-offload-wrapper -target=x86_64-pc-linux-gnu -o %t.wrapper.bc %t.tgt
+// RUN: clang-offload-wrapper -target=x86_64-pc-linux-gnu -o %t.wrapper.bc %t.tgt 2>&1 | FileCheck %s --check-prefix ELF-WARNING
 // RUN: llvm-dis %t.wrapper.bc -o - | FileCheck %s --check-prefix CHECK-IR
 
+// ELF-WARNING: is not an ELF image, so notes cannot be added to it.
 // CHECK-IR: target triple = "x86_64-pc-linux-gnu"
 
 // CHECK-IR-DAG: [[ENTTY:%.+]] = type { i8*, i8*, i{{32|64}}, i32, i32 }
@@ -53,3 +54,24 @@
 // CHECK-IR:   ret void
 
 // CHECK-IR: declare void @__tgt_unregister_lib([[DESCTY]]*)
+
+// Check that clang-offload-wrapper adds LLVMOMPOFFLOAD notes
+// into the ELF offload images:
+// RUN: yaml2obj %S/Inputs/empty-elf-template.yaml -o %t.64le -DBITS=64 -DENCODING=LSB
+// RUN: clang-offload-wrapper -target=x86_64-pc-linux-gnu -o %t.wrapper.elf64le.bc %t.64le
+// RUN: llvm-dis %t.wrapper.elf64le.bc -o - | FileCheck %s --check-prefix OMPNOTES
+// RUN: yaml2obj %S/Inputs/empty-elf-template.yaml -o %t.64be -DBITS=64 -DENCODING=MSB
+// RUN: clang-offload-wrapper -target=x86_64-pc-linux-gnu -o %t.wrapper.elf64be.bc %t.64be
+// RUN: llvm-dis %t.wrapper.elf64be.bc -o - | FileCheck %s --check-prefix OMPNOTES
+// RUN: yaml2obj %S/Inputs/empty-elf-template.yaml -o %t.32le -DBITS=32 -DENCODING=LSB
+// RUN: clang-offload-wrapper -target=x86_64-pc-linux-gnu -o %t.wrapper.elf32le.bc %t.32le
+// RUN: llvm-dis %t.wrapper.elf32le.bc -o - | FileCheck %s --check-prefix OMPNOTES
+// RUN: yaml2obj %S/Inputs/empty-elf-template.yaml -o %t.32be -DBITS=32 -DENCODING=MSB
+// RUN: clang-offload-wrapper -target=x86_64-pc-linux-gnu -o %t.wrapper.elf32be.bc %t.32be
+// RUN: llvm-dis %t.wrapper.elf32be.bc -o - | FileCheck %s --check-prefix OMPNOTES
+
+// There is no clean way for extracting the offload image
+// from the object file currently, so try to find
+// the inserted ELF notes in the device image variable's
+// initializer:
+// OMPNOTES: @{{.+}} = internal unnamed_addr constant [{{[0-9]+}} x i8] c"{{.*}}LLVMOMPOFFLOAD{{.*}}LLVMOMPOFFLOAD{{.*}}LLVMOMPOFFLOAD{{.*}}"

clang/tools/clang-offload-wrapper/CMakeLists.txt

@@ -1,4 +1,4 @@
-set(LLVM_LINK_COMPONENTS BitWriter Core Support TransformUtils)
+set(LLVM_LINK_COMPONENTS BitWriter Core Object Support TransformUtils)
 
 add_clang_tool(clang-offload-wrapper
   ClangOffloadWrapper.cpp