[gn] Rework debug configurations

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 8d02b1b4888b3e2756e119412cac22a066016368..81240d0cd524d63ff4d8519743efab2d2ef0b574 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -4,6 +4,7 @@
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
+import("//build/config/dcheck_always_on.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
@@ -47,6 +48,9 @@ declare_args() {
   # add a dependency on the ICU library.
   v8_enable_i18n_support = true
 
+  # Enable slow dchecks.
+  v8_enable_slow_dchecks = false
+
   # Interpreted regexp engine exists as platform-independent alternative
   # based where the regular expression is compiled to a bytecode.
   v8_interpreted_regexp = false
@@ -54,6 +58,9 @@ declare_args() {
   # Sets -dOBJECT_PRINT.
   v8_object_print = false
 
+  # Turns on compiler optimizations in V8 in Debug build.
+  v8_optimized_debug = true
+
   # With post mortem support enabled, metadata is embedded into libv8 that
   # describes various parameters of the VM for use by debuggers. See
   # tools/gen-postmortem-metadata.py for details.
@@ -130,6 +137,9 @@ config("features") {
 
   defines = []
 
+  if (!is_debug && dcheck_always_on) {
+    defines += [ "DEBUG" ]

[vogelheim, 2016/05/31 14:04:54]

readability nitpick, and not really sure about this:

It's a tad odd that DEBUG is sometimes defines here, and sometimes below (line
304). Can we move this check into the else branch of the if(is_debug) down
there? Then we'd have one big block that handles the DEBUG defines.

[Michael Achenbach, 2016/05/31 14:14:34]

Yea - can do. I wasn't really sure about what is a feauture and what's part of
toolchain. But I assume this is rather toolchain'ish stuff.

+  }
   if (v8_enable_disassembler) {
     defines += [ "ENABLE_DISASSEMBLER" ]
   }
@@ -294,7 +304,7 @@ config("toolchain") {
       "DEBUG",
       "TRACE_MAPS",
     ]
-    if (!v8_optimized_debug) {
+    if (v8_enable_slow_dchecks) {
       defines += [ "ENABLE_SLOW_DCHECKS" ]
     }
   }
@@ -315,7 +325,12 @@ template("v8_source_set") {
       ":toolchain",
     ]
 
-    if (!is_debug || v8_optimized_debug) {
+    # TODO(machenbach): This config doesn't affect executables and components.
+    # It should be moved to a common place.
+    if (is_debug && !v8_optimized_debug) {
+      configs -= [ "//build/config/compiler:default_optimization" ]
+      configs += [ "//build/config/compiler:no_optimize" ]
+    } else {
       configs -= [ "//build/config/compiler:default_optimization" ]
       configs += [ "//build/config/compiler:optimize_max" ]
     }