Read staticBump from dylink section

This allows us to stop depending wasm-emscripten-finalize to
write staticBump which in turn will allow us to stop exporting
`__data_end`.

As a side effect of this the staticBump information is no
longer available in the normal linking mode which means we
can't include dynamic_base in EMSCRIPTEN_METADATA anymore.  One
more nail in the coffin for EMSCRIPTEN_METADATA. See #12231

emscripten.py

@@ -21,6 +21,7 @@
 from tools import diagnostics
 from tools import shared
 from tools import gen_struct_info
+from tools import webassembly
 from tools.shared import WINDOWS, asstr, path_from_root, exit_with_error, asmjs_mangle, treat_as_user_function
 from tools.toolchain_profiler import ToolchainProfiler
 
@@ -141,8 +142,6 @@ def read_proxied_function_signatures(asmConsts):
 
     shared.Settings.PROXIED_FUNCTION_SIGNATURES = read_proxied_function_signatures(metadata['asmConsts'])
 
-  metadata['staticBump'] = align_memory(metadata['staticBump'])
-
   shared.Settings.BINARYEN_FEATURES = metadata['features']
   if shared.Settings.RELOCATABLE:
     # When building relocatable output (e.g. MAIN_MODULE) the reported table
@@ -197,20 +196,11 @@ def compile_settings(temp_files):
 
 class Memory():
   def __init__(self, metadata):
-    # Note: if RELOCATABLE, then only relative sizes can be computed, and we don't
-    #       actually write out any absolute memory locations ({{{ STACK_BASE }}}
-    #       does not exist, etc.)
-
-    # Memory layout:
-    #  * first the static globals
-    self.static_bump = metadata['staticBump']
-    #  * then the stack (up on fastcomp, down on upstream)
-    self.stack_low = align_memory(shared.Settings.GLOBAL_BASE + self.static_bump)
-    self.stack_high = align_memory(self.stack_low + shared.Settings.TOTAL_STACK)
-    self.stack_base = self.stack_high
-    self.stack_max = self.stack_low
-    #  * then dynamic memory begins
-    self.dynamic_base = align_memory(self.stack_high)
+    stack_low = align_memory(shared.Settings.GLOBAL_BASE + metadata['staticBump'])
+    stack_high = align_memory(stack_low + shared.Settings.TOTAL_STACK)
+    self.stack_base = stack_high
+    self.stack_max = stack_low
+    self.dynamic_base = align_memory(stack_high)
 
 
 def apply_memory(js, memory):
@@ -319,10 +309,6 @@ def emscript(infile, outfile_js, memfile, temp_files, DEBUG):
 
   update_settings_glue(metadata, DEBUG)
 
-  memory = Memory(metadata)
-  logger.debug('stack_base: %d, stack_max: %d, dynamic_base: %d, static bump: %d', memory.stack_base, memory.stack_max, memory.dynamic_base, memory.static_bump)
-  shared.Settings.LEGACY_DYNAMIC_BASE = memory.dynamic_base
-
   if not outfile_js:
     logger.debug('emscript: skipping js compiler glue')
     return
@@ -358,8 +344,17 @@ def emscript(infile, outfile_js, memfile, temp_files, DEBUG):
     pre += '\n' + global_initializers + '\n'
 
   if shared.Settings.RELOCATABLE:
+    memory = Memory(metadata)
+    logger.debug('stack_base: %d, stack_max: %d, dynamic_base: %d', memory.stack_base, memory.stack_max, memory.dynamic_base)
+    memsize = webassembly.parse_dylink_section(infile)[0]
+    # TODO(sbc): Remove staticBump completely from metadata once this assertion
+    # has proved its not needed.
+    assert(memsize == metadata['staticBump'])
+    metadata['staticBump'] = align_memory(memsize)
+
     pre = apply_memory(pre, memory)
     post = apply_memory(post, memory)
+
   pre = apply_static_code_hooks(pre) # In regular runtime, atinits etc. exist in the preamble part
   post = apply_static_code_hooks(post) # In MINIMAL_RUNTIME, atinit exists in the postamble part
 

src/settings_internal.js

@@ -62,13 +62,6 @@ var EMBIND = 0;
 // Whether the main() function reads the argc/argv parameters.
 var MAIN_READS_PARAMS = 1;
 
-// Computed during emscripten for the purpose of writing to emscripten_metadata
-// section.
-// TODO(sbc): Remove this.  If emscripten doesn't need it then neither should
-// any other loader.
-// See https://github.com/emscripten-core/emscripten/issues/12231
-var LEGACY_DYNAMIC_BASE = -1;
-
 // List of functions implemented in compiled code; received from the backend.
 var IMPLEMENTED_FUNCTIONS = [];
 

tools/webassembly.py

@@ -26,7 +26,7 @@
 # change, increment EMSCRIPTEN_ABI_MINOR if EMSCRIPTEN_ABI_MAJOR == 0
 # or the ABI change is backwards compatible, otherwise increment
 # EMSCRIPTEN_ABI_MAJOR and set EMSCRIPTEN_ABI_MINOR = 0.
-EMSCRIPTEN_ABI_MAJOR, EMSCRIPTEN_ABI_MINOR = (0, 28)
+EMSCRIPTEN_ABI_MAJOR, EMSCRIPTEN_ABI_MINOR = (0, 29)
 
 
 def toLEB(x):
@@ -62,7 +62,6 @@ def add_emscripten_metadata(wasm_file):
 
   mem_size = shared.Settings.INITIAL_MEMORY // WASM_PAGE_SIZE
   global_base = shared.Settings.GLOBAL_BASE
-  dynamic_base = shared.Settings.LEGACY_DYNAMIC_BASE
 
   logger.debug('creating wasm emscripten metadata section with mem size %d' % mem_size)
   name = b'\x13emscripten_metadata' # section name, including prefixed size
@@ -83,7 +82,7 @@ def add_emscripten_metadata(wasm_file):
     toLEB(mem_size) +
     toLEB(0) +
     toLEB(global_base) +
-    toLEB(dynamic_base) +
+    toLEB(0) +
     # dynamictopPtr, always 0 now
     toLEB(0) +
 
@@ -109,13 +108,9 @@ def add_emscripten_metadata(wasm_file):
     f.write(orig[8:])
 
 
-def add_dylink_section(wasm_file, needed_dynlibs):
-  # A wasm shared library has a special "dylink" section, see tools-conventions repo.
-  # This function adds this section to the beginning on the given file.
-
+def parse_dylink_section(wasm_file):
   wasm = open(wasm_file, 'rb').read()
   section_name = b'\06dylink' # section name, including prefixed size
-  file_header = wasm[:8]
 
   # Read the existing section data
   offset = 8
@@ -132,8 +127,17 @@ def add_dylink_section(wasm_file, needed_dynlibs):
   table_size, offset = readLEB(section, offset)
   table_align, offset = readLEB(section, offset)
 
+  return (mem_size, mem_align, table_size, table_align, section_end)
+
+
+def add_dylink_section(wasm_file, needed_dynlibs):
+  # A wasm shared library has a special "dylink" section, see tools-conventions repo.
+  # This function adds this section to the beginning on the given file.
+
+  mem_size, mem_align, table_size, table_align, section_end = parse_dylink_section(wasm_file)
   logger.debug('creating wasm dynamic library with mem size %d, table size %d, align %d' % (mem_size, table_size, mem_align))
 
+  section_name = b'\06dylink' # section name, including prefixed size
   contents = (toLEB(mem_size) + toLEB(mem_align) +
               toLEB(table_size) + toLEB(0))
 
@@ -164,6 +168,10 @@ def add_dylink_section(wasm_file, needed_dynlibs):
     contents += toLEB(len(dyn_needed))
     contents += dyn_needed
 
+  orig = open(wasm_file, 'rb').read()
+  file_header = orig[:8]
+  file_remainder = orig[section_end:]
+
   section_size = len(section_name) + len(contents)
   with open(wasm_file, 'wb') as f:
     # copy magic number and version
@@ -174,4 +182,4 @@ def add_dylink_section(wasm_file, needed_dynlibs):
     f.write(section_name)
     f.write(contents)
     # copy rest of binary
-    f.write(wasm[section_end:])
+    f.write(file_remainder)