Add TABLE_BASE setting for controlling the offset of static table slots. (#20149)

See https://reviews.llvm.org/D158892

Fixes: #20097

Author: sbc100

ChangeLog.md

@@ -41,6 +41,9 @@ See docs/process.md for more on how version tagging works.
   the existing behaviour for `MINIMAL_RUNTIME` and `STRICT` mode.
   If you use GLFW you now need to explictly opt into it using `-sUSE_GLFW` or
   `-lglfw`. (#19939)
+- A new settings `TABLE_BASE` was introduced that can be used to place static
+  function addresses (table slots) at a certain offset.  This defaults to 1
+  which is the previously fixed value. (#20149)
 
 3.1.45 - 08/23/23
 -----------------

src/library.js

@@ -3520,12 +3520,12 @@ addToLibrary({
   // tell the memory segments where to place themselves
   __memory_base: "new WebAssembly.Global({'value': '{{{ POINTER_WASM_TYPE }}}', 'mutable': false}, {{{ to64(GLOBAL_BASE) }}})",
   // the wasm backend reserves slot 0 for the NULL function pointer
-  __table_base: "new WebAssembly.Global({'value': '{{{ POINTER_WASM_TYPE }}}', 'mutable': false}, {{{ to64(1) }}})",
+  __table_base: "new WebAssembly.Global({'value': '{{{ POINTER_WASM_TYPE }}}', 'mutable': false}, {{{ to64(TABLE_BASE) }}})",
 #if MEMORY64 == 2
   __memory_base32: "new WebAssembly.Global({'value': 'i32', 'mutable': false}, {{{ GLOBAL_BASE }}})",
 #endif
 #if MEMORY64
-  __table_base32: 1,
+  __table_base32: {{{ TABLE_BASE }}},
 #endif
   // To support such allocations during startup, track them on __heap_base and
   // then when the main module is loaded it reads that value and uses it to

src/settings.js

@@ -248,6 +248,11 @@ var ALLOW_TABLE_GROWTH = false;
 // [link]
 var GLOBAL_BASE = 1024;
 
+// Where where table slots (function addresses) are allocated.
+// This must be at least 1 to reserve the zero slot for the null pointer.
+// [link]
+var TABLE_BASE = 1;
+
 // Whether closure compiling is being run on this output
 // [link]
 var USE_CLOSURE_COMPILER = false;

test/test_other.py

@@ -13725,3 +13725,12 @@ def test_no_minify(self):
     self.run_process([EMCC, test_file('hello_world.c'), '--pre-js=pre.js', '-Oz', '--minify=0'])
     content = read_file('a.out.js')
     self.assertContained(comment, content)
+
+  def test_table_base(self):
+    create_file('test.c', r'''
+    #include <stdio.h>
+    int main() {
+      printf("addr = %p\n", &printf);
+    }''')
+    self.do_runf('test.c', 'addr = 0x1\n')
+    self.do_runf('test.c', 'addr = 0x400\n', emcc_args=['-sTABLE_BASE=1024'])

tools/building.py

@@ -232,8 +232,11 @@ def lld_flags_for_executable(external_symbols):
 
   if settings.STACK_FIRST:
     cmd.append('--stack-first')
-  elif not settings.RELOCATABLE:
-    cmd.append('--global-base=%s' % settings.GLOBAL_BASE)
+
+  if not settings.RELOCATABLE:
+    cmd.append('--table-base=%s' % settings.TABLE_BASE)
+    if not settings.STACK_FIRST:
+      cmd.append('--global-base=%s' % settings.GLOBAL_BASE)
 
   return cmd