Add functions for access emscripten stack layout information (#11437)

This change adds emscripten_stack_get_base/free/end/current.

This is a cut down version of #11162 which uses wasm assembly
rather than relying on binaryen changes.

Also remove old emscripten_get_stack_base/emscripten_get_stack_top
from src/library.js.  emscripten_get_stack_top never worked anyway
since its was not marked as asm and therefore not referencing
the arm-internal version of STACKTOP.

src/library.js

@@ -4547,14 +4547,6 @@ LibraryManager.library = {
     });
   },
 
-  emscripten_get_stack_top: function() {
-    return STACKTOP;
-  },
-
-  emscripten_get_stack_base: function() {
-    return STACK_BASE;
-  },
-
   _readAsmConstArgsArray: '=[]',
   $readAsmConstArgs__deps: ['_readAsmConstArgsArray'],
   $readAsmConstArgs: function(sigPtr, buf) {

src/library_stack.js

@@ -5,15 +5,23 @@
  */
 
 mergeInto(LibraryManager.library, {
-#if WASM_BACKEND == 0
+  emscripten_stack_get_base: function() {
+    return STACK_BASE;
+  },
+  emscripten_stack_get_end: function() {
+    // TODO(sbc): rename STACK_MAX -> STACK_END?
+    return STACK_MAX;
+  },
+
+#if !WASM_BACKEND
   $abortStackOverflow__deps: ['$stackSave'],
 #endif
   $abortStackOverflow__import: true,
   $abortStackOverflow: function(allocSize) {
     abort('Stack overflow! Attempted to allocate ' + allocSize + ' bytes on the stack, but stack has only ' + (STACK_MAX - stackSave() + allocSize) + ' bytes available!');
   },
 
-#if WASM_BACKEND == 0
+#if !WASM_BACKEND
   $stackAlloc__asm: true,
   $stackAlloc__sig: 'ii',
 #if ASSERTIONS || STACK_OVERFLOW_CHECK >= 2
@@ -43,5 +51,18 @@ mergeInto(LibraryManager.library, {
     top = top|0;
     STACKTOP = top;
   },
+
+  // With the wasm backend, these functions are implemented as native
+  // functions in compiler-rt/stack_ops.s
+  emscripten_stack_get_current__asm: true,
+  emscripten_stack_get_current__sig: 'i',
+  emscripten_stack_get_current: function() {
+    return STACKTOP|0;
+  },
+  emscripten_stack_get_free__asm: true,
+  emscripten_stack_get_free__sig: 'i',
+  emscripten_stack_get_free: function() {
+    return (STACK_MAX|0) - (STACKTOP|0);
+  }
 #endif
 });

system/include/emscripten/stack.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2020 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#pragma once
+
+// API that gives access to introspecting the Wasm data stack. Build with
+// -lstack.js to use this API.
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Returns the starting address of the wasm stack. This is the address
+// that the stack pointer would point to when no bytes are in use on the stack.
+uintptr_t emscripten_stack_get_base(void);
+
+// Returns the end address of the wasm stack. This is the address that the stack
+// pointer would point to when the whole stack is in use.  (the address pointed
+// to by the end is not part of the stack itself) Note that in fastcomp, the
+// stack grows up, whereas in wasm backend, it grows down.  So with wasm
+// backend, the address returned by emscripten_stack_get_end() is smaller than
+// emscripten_stack_get_base().
+uintptr_t emscripten_stack_get_end(void);
+
+// Returns the current stack pointer.
+uintptr_t emscripten_stack_get_current(void);
+
+// Returns the number of free bytes left on the stack.
+size_t emscripten_stack_get_free(void);
+
+#ifdef __cplusplus
+}
+#endif

system/lib/compiler-rt/stack_ops.s

@@ -1,6 +1,8 @@
 .globl stackSave
 .globl stackRestore
 .globl stackAlloc
+.globl emscripten_stack_get_current
+.globl emscripten_stack_get_free
 
 .globaltype __stack_pointer, i32
 
@@ -30,3 +32,27 @@ stackAlloc:
   global.set __stack_pointer
   local.get 1
   end_function
+
+emscripten_stack_get_current:
+  .functype emscripten_stack_get_current () -> (i32)
+  global.get __stack_pointer
+  end_function
+
+.functype emscripten_stack_get_end () -> (i32)
+.functype emscripten_stack_get_base () -> (i32)
+.globaltype __stack_end, i32
+__stack_end:
+
+emscripten_stack_get_free:
+  # set __stack_base/__stack_end on first call
+  .functype emscripten_stack_get_free () -> (i32)
+  global.get __stack_end
+  i32.eqz
+  if
+    call emscripten_stack_get_end
+    global.set __stack_end
+  end_if
+  global.get __stack_pointer
+  global.get __stack_end
+  i32.sub
+  end_function

tests/core/test_stack_get_free.c

@@ -0,0 +1,47 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <alloca.h>
+#include <emscripten.h>
+#include <emscripten/stack.h>
+#include <assert.h>
+#include <string.h>
+
+void __attribute__((noinline)) DoSomething(char *addr) {
+  memset(addr, 42, 13);
+}
+
+void TestStackValidity() {
+  uintptr_t base = emscripten_stack_get_base();
+  uintptr_t end = emscripten_stack_get_end();
+
+  uintptr_t used = abs((intptr_t)base - (intptr_t)emscripten_stack_get_current());
+  uintptr_t free = abs((intptr_t)end - (intptr_t)emscripten_stack_get_current());
+  uintptr_t free2 = emscripten_stack_get_free();
+  uintptr_t total = abs((intptr_t)end - (intptr_t)base);
+  assert(used + free == total);
+  assert(free == free2);
+}
+
+int increment = 256 * 1024;
+
+int main() {
+  TestStackValidity();
+
+  uintptr_t origFree = emscripten_stack_get_free();
+  uintptr_t prevFree = emscripten_stack_get_free();
+  printf("Stack used: %u\n", origFree - emscripten_stack_get_free());
+  for(int i = 0; i < 10; ++i) {
+    int increment_noopt = emscripten_random() >= 0 ? increment : 2;
+    char *p = alloca(increment_noopt);
+    DoSomething(p);
+    uintptr_t free = emscripten_stack_get_free();
+    assert(prevFree - free == increment);
+    prevFree = free;
+    // Print something from the allocationed region to prevent whole program
+    // optimizations from elminiating the alloca completely.
+    printf("Val: %d\n", p[10]);
+    printf("Stack used: %u\n", origFree - emscripten_stack_get_free());
+    TestStackValidity();
+  }
+  return 0;
+}

tests/core/test_stack_get_free.out

@@ -0,0 +1,21 @@
+Stack used: 0
+Val: 42
+Stack used: 262144
+Val: 42
+Stack used: 524288
+Val: 42
+Stack used: 786432
+Val: 42
+Stack used: 1048576
+Val: 42
+Stack used: 1310720
+Val: 42
+Stack used: 1572864
+Val: 42
+Stack used: 1835008
+Val: 42
+Stack used: 2097152
+Val: 42
+Stack used: 2359296
+Val: 42
+Stack used: 2621440

tests/test_core.py

@@ -8664,13 +8664,20 @@ def test_emscripten_math(self):
     self.do_run_in_out_file_test('tests', 'core', 'test_emscripten_math')
 
   # Tests that users can pass custom JS options from command line using
-  # the -jsDfoo=val syntax. (https://github.com/emscripten-core/emscripten/issues/10580)
+  # the -jsDfoo=val syntax:
+  # See https://github.com/emscripten-core/emscripten/issues/10580.
   def test_custom_js_options(self):
     self.emcc_args += ['--js-library', path_from_root('tests', 'core', 'test_custom_js_settings.js'), '-jsDCUSTOM_JS_OPTION=1']
     self.do_run_in_out_file_test('tests', 'core', 'test_custom_js_settings')
 
     self.assertContained('cannot change built-in settings values with a -jsD directive', self.expect_fail([EMCC, '-jsDWASM=0']))
 
+  # Tests <emscripten/stack.h> API
+  def test_emscripten_stack(self):
+    self.emcc_args += ['-lstack.js']
+    self.set_setting('TOTAL_STACK', 4 * 1024 * 1024)
+    self.do_run_in_out_file_test('tests', 'core', 'test_stack_get_free')
+
 
 # Generate tests for everything
 def make_run(name, emcc_args, settings=None, env=None):