allocator: add use_experimental_allocator_shim build flag

This CL introduces a new GYP/GN build flag
use_experimental_allocator_shim (default: false).
After this CL, the only side-effect of that flag is that tcmalloc
does not override anymore the libc symbols (malloc, new etc.).
This is in preparation of a unified shim layer, which will come
separately in upcoming CLs, which will take care of overriding
those symbols in Chrome.

Design doc: http://bit.ly/allocator-shim

BUG=550886

Review URL: https://codereview.chromium.org/1678893002

Cr-Commit-Position: refs/heads/master@{#374449}

base/allocator/BUILD.gn

@@ -47,13 +47,17 @@ config("allocator_shim_define") {
 }
 
 config("tcmalloc_flags") {
+  defines = []
   if (enable_debugallocation) {
-    defines = [
+    defines += [
       # Use debugallocation for Debug builds to catch problems early
       # and cleanly, http://crbug.com/30715 .
       "TCMALLOC_FOR_DEBUGALLOCATION",
     ]
   }
+  if (use_experimental_allocator_shim) {
+    defines += [ "TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC" ]
+  }
   if (is_clang) {
     cflags = [
       # tcmalloc initializes some fields in the wrong order.

base/allocator/allocator.gyp

@@ -20,6 +20,7 @@
     # e.g. for profiling (it's more rare to profile Debug builds,
     # but people sometimes need to do that).
     'disable_debugallocation%': 0,
+    'use_experimental_allocator_shim%': 0,
   },
   'targets': [
     # The only targets that should depend on allocator are 'base' and
@@ -340,6 +341,11 @@
                 '<(tcmalloc_dir)/src/profiler.cc',
               ],
             }],
+            ['use_experimental_allocator_shim==1', {
+              'defines': [
+                'TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC',
+              ],
+            }]
           ],
           'configurations': {
             'Debug_Base': {

build/config/allocator.gni

@@ -14,8 +14,15 @@ if (is_android || current_cpu == "mipsel" || is_mac || is_ios || is_asan ||
 declare_args() {
   # Memory allocator to use. Set to "none" to use default allocator.
   use_allocator = _default_allocator
+
+  # TODO(primiano): this should just become the default without having a flag,
+  # but we need to get there first. http://crbug.com/550886 .
+  # Causes all the allocations to be routed via allocator_shim.cc.
+  use_experimental_allocator_shim = false
 }
 
 assert(use_allocator == "none" || use_allocator == "tcmalloc")
 
 assert(!is_win || use_allocator == "none", "Tcmalloc doesn't work on Windows.")
+assert(!use_experimental_allocator_shim || is_linux,
+       "use_experimental_allocator_shim supported only on Linux")

third_party/tcmalloc/README.chromium

@@ -102,3 +102,4 @@ Modifications:
 - Conditionally define HAVE_VDSO_SUPPORT only on linux_x86 to avoid static initializers
 - Add TC_MALLOPT_IS_OVERRIDDEN_BY_TCMALLOC mallopt() arg
 - Added tc_malloc_skip_new_handler.
+- Added TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC which bypasses the libc_override logic.

third_party/tcmalloc/chromium/src/libc_override.h

@@ -59,12 +59,22 @@
 
 static void ReplaceSystemAlloc();  // defined in the .h files below
 
+#if defined(TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC)
+// TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC has the following semantic:
+//  - tcmalloc with all its tc_* (tc_malloc, tc_free) symbols is being built
+//    and linked as usual.
+//  - the default system allocator symbols (malloc, free, operator new) are NOT
+//    overridden. The embedded must take care of routing them to tc_* symbols.
+// This no-op #if block effectively prevents the inclusion of the
+// libc_override_* headers below.
+static void ReplaceSystemAlloc() {}
+
 // For windows, there are two ways to get tcmalloc.  If we're
 // patching, then src/windows/patch_function.cc will do the necessary
 // overriding here.  Otherwise, we doing the 'redefine' trick, where
 // we remove malloc/new/etc from mscvcrt.dll, and just need to define
 // them now.
-#if defined(_WIN32) && defined(WIN32_DO_PATCHING)
+#elif defined(_WIN32) && defined(WIN32_DO_PATCHING)
 void PatchWindowsFunctions();   // in src/windows/patch_function.cc
 static void ReplaceSystemAlloc() { PatchWindowsFunctions(); }