Disable -ftrivial-auto-var-init=pattern on variable

This variable significantly slows down indexed-getter.html of blink_perf.bindings on linux-perf
https://pinpoint-dot-chromeperf.appspot.com/job/136cce54620000

V8_STACK_UNINITIALIZED macro is the same as http://crrev.com/c/1974951

Bug: chromium:977230

Change-Id: I3e7e91804e13b856d6b2ba0a5d67f0354636c510
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2046872
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Auto-Submit: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66278}

src/base/compiler-specific.h

@@ -105,4 +105,45 @@
 #define V8_NOEXCEPT
 #endif
 
+#if defined(__clang__)
+
+#if __has_attribute(uninitialized)
+// Attribute "uninitialized" disables -ftrivial-auto-var-init=pattern for
+// the specified variable.
+// Library-wide alternative is
+// 'configs -= [ "//build/config/compiler:default_init_stack_vars" ]' in .gn
+// file.
+//
+// See "init_stack_vars" in build/config/compiler/BUILD.gn and
+// http://crbug.com/977230
+// "init_stack_vars" is enabled for non-official builds and we hope to enable it
+// in official build in 2020 as well. The flag writes fixed pattern into
+// uninitialized parts of all local variables. In rare cases such initialization
+// is undesirable and attribute can be used:
+//   1. Degraded performance
+// In most cases compiler is able to remove additional stores. E.g. if memory is
+// never accessed or properly initialized later. Preserved stores mostly will
+// not affect program performance. However if compiler failed on some
+// performance critical code we can get a visible regression in a benchmark.
+//   2. memset, memcpy calls
+// Compiler may replace some memory writes with memset or memcpy calls. This is
+// not -ftrivial-auto-var-init specific, but it can happen more likely with the
+// flag. It can be a problem if code is not linked with C run-time library.
+//
+// Note: The flag is security risk mitigation feature. So in future the
+// attribute uses should be avoided when possible. However to enable this
+// mitigation on the most of the code we need to be less strict now and minimize
+// number of exceptions later. So if in doubt feel free to use attribute, but
+// please document the problem for someone who is going to cleanup it later.
+// E.g. platform, bot, benchmark or test name in patch description or next to
+// the attribute.
+#define V8_STACK_UNINITIALIZED __attribute__((uninitialized))
+#else  // No attribute uninitialized
+#define V8_STACK_UNINITIALIZED
+#endif  // attribute uninitialized
+
+#else  // Not clang
+#define V8_STACK_UNINITIALIZED
+#endif  // clang
+
 #endif  // V8_BASE_COMPILER_SPECIFIC_H_

src/ic/ic.cc

@@ -2794,8 +2794,10 @@ RUNTIME_FUNCTION(Runtime_LoadElementWithInterceptor) {
 
   Handle<InterceptorInfo> interceptor(receiver->GetIndexedInterceptor(),
                                       isolate);
-  PropertyCallbackArguments arguments(isolate, interceptor->data(), *receiver,
-                                      *receiver, Just(kDontThrow));
+  // Initialization significantly slows down indexed-getter.html of
+  // blink_perf.bindings on linux-perf. https://crbug.com/977230
+  V8_STACK_UNINITIALIZED PropertyCallbackArguments arguments(
+      isolate, interceptor->data(), *receiver, *receiver, Just(kDontThrow));
   Handle<Object> result = arguments.CallIndexedGetter(interceptor, index);
 
   RETURN_FAILURE_IF_SCHEDULED_EXCEPTION(isolate);