[vm] Merge text and data sections in ELF snapshots.

To do this, BitsContainer is changed to be a more rope-like
representation of section portions. In addition to containing
most of the same information stored per-section previously, each
portion also has a section-relative offset that is calculated when
it is added. Thus, merging two compatible BitsContainer sections
is just adding the portions from the second to the first, tweaking
the section-relative offset for each.

Other changes in this CL:

* Create PseudoSections subclasses for the elf header, program
  header table, and section header table, so we can treat them
  more uniformly with the other parts of the ELF snapshot.

* We now only allocate as much BSS space in the snapshot as is needed
  for any text sections in the snapshot, instead of always allocating
  a big enough BSS space for both VM and isolate, even for deferred
  snapshots where there is no VM isolate.

* We already separated segment and section alignment in previous CLs,
  so the fact that our own ELF loader needs load segments to be
  page-aligned no longer means that the sections within those segments
  also needs to be. Thus, we align individual instructions sections to
  kMaxObjectAlignment, like readonly data sections, since both hold a
  single Image object. This removes unnecessary intra-section padding.

TEST=Tests that check DWARF information and trybots that use ELF
     snapshots.

Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-linux-debug-simarm_x64-try,vm-kernel-precomp-linux-debug-x64-try,vm-kernel-precomp-linux-release-simarm-try,vm-kernel-precomp-linux-release-simarm64-try,vm-kernel-precomp-dwarf-linux-product-x64-try
Change-Id: If0315c8b7b0f31481b676a8901f49cd3a44b5561
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206365
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Tess Strickland <[email protected]>

Author: sstrickl

runtime/bin/elf_loader.cc

@@ -6,7 +6,6 @@
 #include <bin/file.h>
 #include <platform/elf.h>
 #include <platform/globals.h>
-#include <vm/bss_relocs.h>
 #include <vm/cpu.h>
 #include <vm/virtual_memory.h>
 
@@ -239,8 +238,6 @@ class LoadedElf {
   const char* dynamic_string_table_ = nullptr;
   const dart::elf::Symbol* dynamic_symbol_table_ = nullptr;
   uword dynamic_symbol_count_ = 0;
-  uword* vm_bss_ = nullptr;
-  uword* isolate_bss_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(LoadedElf);
 };
@@ -468,20 +465,11 @@ bool LoadedElf::ReadSections() {
       dynamic_symbol_table_ = reinterpret_cast<const dart::elf::Symbol*>(
           base_->start() + header.memory_offset);
       dynamic_symbol_count_ = header.file_size / sizeof(dart::elf::Symbol);
-    } else if (strcmp(name, ".bss") == 0) {
-      auto const bss_size =
-          (BSS::kVmEntryCount + BSS::kIsolateEntryCount) * kWordSize;
-      CHECK_ERROR(header.memory_offset != 0, ".bss must be loaded.");
-      CHECK_ERROR(header.file_size >= bss_size,
-                  ".bss does not have enough space.");
-      vm_bss_ = reinterpret_cast<uword*>(base_->start() + header.memory_offset);
-      isolate_bss_ = vm_bss_ + BSS::kVmEntryCount;
     }
   }
 
   CHECK_ERROR(dynamic_string_table_ != nullptr, "Couldn't find .dynstr.");
   CHECK_ERROR(dynamic_symbol_table_ != nullptr, "Couldn't find .dynsym.");
-  CHECK_ERROR(vm_bss_ != nullptr, "Couldn't find .bss.");
   return true;
 }