[vm] Output accompaning relocatable object for Mach-O snapshots.

In order to appropriately generate .dSYMs for a snapshot and allow
the strip tool to be used on the snapshot afterwards, add a mode that
not only outputs the snapshot (sans DWARF information) but also
an associated relocatable object file that contains the program's DWARF
information. That allows dsymutil to retrieve the DWARF information
from the relocatable object when run on the snapshot prior to stripping.

To specify that the relocatable object file should be output, use
the new --macho-object command line argument to gen_snapshot
to specify where the object file should be written.

This CL also adds an additional command line argument,
--macho-reduce-padding, which reduces the alignment used for segments
and the text/const sections in Mach-O outputs from 16KB to 64 bytes.
The larger padding is needed for some uses, like non-native loading
of Mach-O objects, but can be elided for other uses like Flutter builds.

TEST=vm/dart/use_dwarf_stack_traces_flag_test
     vm/dart/use_macho_reduce_padding_flag_test

Change-Id: I2bf4bacb70c41299b8b6fdb7635c2374acf7a07d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/457420
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>

runtime/platform/mach_o.h

@@ -86,7 +86,7 @@ static constexpr uint32_t MH_CIGAM_64 = 0xcffaedfe;
 
 // Filetypes for the Mach-O header.
 
-// A relocatable object file (e.g., an executable).
+// A relocatable object file that has all sections in a single unnamed segment.
 static constexpr uint32_t MH_OBJECT = 0x1;
 // A dynamically bound shared library.
 static constexpr uint32_t MH_DYLIB = 0x6;
@@ -167,7 +167,7 @@ struct segment_command {
   uint32_t nsects;
   //
   uint32_t flags;
-  // section_command[]
+  // section[]
 };
 
 // Contains the same fields as segment_command, but the starting memory
@@ -184,7 +184,7 @@ struct segment_command_64 {
   vm_prot_t initprot;
   uint32_t nsects;
   uint32_t flags;
-  // section_command_64[]
+  // section_64[]
 };
 
 struct section {
@@ -271,6 +271,12 @@ static constexpr char SECT_DEBUG_ABBREV[] = "__debug_abbrev";
 // table and code signature.
 static constexpr char SEG_LINKEDIT[] = "__LINKEDIT";
 
+// Segment/section names used for relocatable object files.
+static constexpr char SEG_UNNAMED[] = "";
+
+static constexpr char SEG_LD[] = "__LD";
+static constexpr char SECT_COMPACT_UNWIND[] = "__compact_unwind";
+
 struct symtab_command {
   uint32_t cmd;  // LC_SYMTAB
   uint32_t cmdsize;
@@ -654,6 +660,89 @@ struct unwind_info_header {
   // ... regular and compressed second level pages ...
 };
 
+// Relocation information in relocatable objects. The reloff field in
+// the section and section_64 structs gives the starting file offset for
+// the section's relocation information, and nreloc gives the number of
+// structs found starting from that offset.
+struct relocation_info {
+  // The "address" of the relocation entry is the offset into the
+  // corresponding section.
+  int32_t address;
+  // The metadata contains the following bit fields, from low to high:
+  // 0-23:  The index of the section or symbol on which this relation entry
+  //        is based. Note that, as with other parts of the Mach-O format,
+  //        section indices are 1-based, while symbol indices are 0-based.
+  // 24:    Whether or not this relocation is PC-relative.
+  // 25-26: log2(n), where n is the size of the relocation entry in bytes.
+  // 27:    Whether or not this relocation is "external". An external
+  //        relocation is based on a symbol in the symbol table. If false,
+  //        then the relocation is based on a section instead.
+  // 28-31: The type of the relocation entry, see the RELOC_TYPE_*
+  //        constants below.
+  uint32_t metadata;
+};
+
+// The number of low bits used to store the symbol or section index in
+// the relocation entry.
+static constexpr uint32_t RELOC_METADATA_INDEX_BITS = 24;
+
+// The size of the relocation in the section contents.
+static constexpr uint32_t RELOC_SIZE_BYTE = 0 << 25;
+static constexpr uint32_t RELOC_SIZE_2BYTES = 1 << 25;
+static constexpr uint32_t RELOC_SIZE_4BYTES = 2 << 25;
+static constexpr uint32_t RELOC_SIZE_8BYTES = 3 << 25;
+
+// This bit is set if the index in the payload is the index of a symbol
+// in the symbol table. It is unset if the index in the payload is the
+// (1-based) index of a section.
+static constexpr uint32_t RELOC_EXTERN = 1 << 27;
+
+// For our purposes, the MachOWriter only emits two types of relocation entries:
+// UNSIGNED and SUBTRACTOR. The numeric encoding of these types are
+// platform dependent, but the semantics are the same for both X64 and ARM64:
+//
+// Consider a relocation comprised of the following parts:
+//   (Target + TOffset) - (Source + SOffset)
+// at the offset ROffset in section Section with virtual address
+//   RAddress = Section.addr + ROffset
+// in the relocatable object.
+//
+// An UNSIGNED relocation entry specifies Target. It refers either to a section
+// or a symbol via the stored index.
+//
+// A SUBTRACTOR relocation entry specifies Source. It always refers to a symbol,
+// never a section. Additionally, SUBTRACTOR relocation entries are always found
+// immediately before the corresponding UNSIGNED relocation entries.
+//
+// The offsets are combined into a single addend, which is stored in the section
+// contents at the offset of the relocation. If the relocation entries are
+// symbol based, then
+//   addend = TOffset - SOffset
+// and for section-based relocation entries (which have no Source/SOffset),
+//   addend = RAddress + TOffset
+// That is, the virtual address of the relocation is included in the addend for
+// section-based relocation entries.
+
+// X64-specific constants for relocation types.
+
+// A relocation specifying a section or symbol that is the target
+// of the relocation.
+static constexpr uint32_t RELOC_TYPE_X64_UNSIGNED = 0 << 28;
+// A relocation specifying a symbol that is the source of the relocation.
+// Note that in the list of the relocations, the source comes immediately
+// _before_ the target (UNSIGNED) entry that it is subtracted from.
+static constexpr uint32_t RELOC_TYPE_X64_SUBTRACTOR = 5 << 28;
+
+// ARM64-specific constants for relocation types.
+
+// A relocation specifying a section or symbol that is the target
+// of the relocation.
+static constexpr uint32_t RELOC_TYPE_ARM64_UNSIGNED = 0 << 28;
+// A relocation specifying a symbol that is the source of the relocation.
+// Note that in the list of the relocations, the source comes immediately
+// _before_ the target (UNSIGNED) entry that it is subtracted from.
+static constexpr uint32_t RELOC_TYPE_ARM64_SUBTRACTOR = 1 << 28;
+
 #pragma pack(pop)
 
 }  // namespace mach_o