Add a global v8_enable_allocation_folding build-time flag

When this flag is enabled, allocation folding behaviour depends
on the --turbo-allocation-folding runtime flag.

When it's disabled, --turbo-allocation-folding is ignored.

This flag will be used later to control the
CodeStubAssembler::InlineAllocate behaviour.

Change-Id: Iea7bbafd8454571dda7d56349b3dc63d3b54ba99
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2878754Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Wenyu Zhao <wenyu.zhao@anu.edu.au>
Cr-Commit-Position: refs/heads/master@{#74858}

BUILD.gn

@@ -318,6 +318,10 @@ declare_args() {
 
   # Allow for JS promise hooks (instead of just C++).
   v8_allow_javascript_in_promise_hooks = false
+
+  # Enable allocation folding globally (sets -dV8_ALLOCATION_FOLDING).
+  # When it's disabled, the --turbo-allocation-folding runtime flag will be ignored.
+  v8_enable_allocation_folding = true
 }
 
 # Derived defaults.
@@ -868,6 +872,9 @@ config("features") {
   if (v8_allow_javascript_in_promise_hooks) {
     defines += [ "V8_ALLOW_JAVASCRIPT_IN_PROMISE_HOOKS" ]
   }
+  if (v8_enable_allocation_folding) {
+    defines += [ "V8_ALLOCATION_FOLDING" ]
+  }
 }
 
 config("toolchain") {

src/flags/flag-definitions.h

@@ -418,6 +418,16 @@ DEFINE_NEG_IMPLICATION(enable_third_party_heap, concurrent_marking)
 DEFINE_BOOL_READONLY(enable_third_party_heap, V8_ENABLE_THIRD_PARTY_HEAP_BOOL,
                      "Use third-party heap")
 
+#ifdef V8_ALLOCATION_FOLDING
+#define V8_ALLOCATION_FOLDING_BOOL true
+#else
+#define V8_ALLOCATION_FOLDING_BOOL false
+#endif
+
+DEFINE_BOOL_READONLY(enable_allocation_folding, V8_ALLOCATION_FOLDING_BOOL,
+                     "Use allocation folding globally")
+DEFINE_NEG_NEG_IMPLICATION(enable_allocation_folding, turbo_allocation_folding)
+
 #ifdef V8_DISABLE_WRITE_BARRIERS
 #define V8_DISABLE_WRITE_BARRIERS_BOOL true
 #else