Optimize GetPathForVectorIcon*, save ~290 KB on disk

ash/resources/vector_icons/vector_icons.cc.template

Index: ash/resources/vector_icons/vector_icons.cc.template
diff --git a/ash/resources/vector_icons/vector_icons.cc.template b/ash/resources/vector_icons/vector_icons.cc.template
index e6f5f90101dbb63d73e8e912cae7b0bf7f998144..e48137d99f3a7818086385c94449c25d966f0f04 100644
--- a/ash/resources/vector_icons/vector_icons.cc.template
+++ b/ash/resources/vector_icons/vector_icons.cc.template
@@ -11,7 +11,7 @@
 #include "ui/gfx/vector_icon_types.h"
 
 #define PATH_ELEMENT_TEMPLATE(path_name, ...) \
-static gfx::PathElement path_name[] = {__VA_ARGS__};
+static constexpr gfx::PathElement path_name[] = {__VA_ARGS__};

[Evan Stade, 2017/01/10 17:30:26]

is this part actually necessary? I ask because I didn't find it necessary when
fixing a related problem with static initializers.

There are a few vector_icons.cc.template files out there ---
chrome/app/vector_icons/ and ash/resources/vector_icons/ both have them. Should
those be updated as well?

[brucedawson, 2017/01/10 18:31:54]

I meant to ask about that because changes to this file did not affect the build
at all. I decided to add constexpr because it was so obvious and simple and it
seemed better than leaving it as-is. Alternately the file could be deleted if
not used. Thoughts?
Yes. I'll look for them.

[Evan Stade, 2017/01/11 00:26:22]

the file is used, but I suspect you're not building Chrome OS (ash is only built
on ChromeOS these days). What I meant was that constexpr on this declaration
didn't seem to affect the static initializers; only the constexpr on the
PathElement ctors was necessary, so I wasn't sure if it was necessary for the
improvements listed in this CL.

[brucedawson, 2017/01/11 01:05:55]

Ah. Got it.

For VC++ putting constexpr on the array definitions is crucial. Without that
nothing changes.

Clearly this sort of thing is compiler dependent, but the great thing about
constexpr on the arrays is that it *guarantees* compile-time construction.
Anything less than that (including constexpr on the PathElement constructors)
is, at best, a hint.

So, constexpr on the PathElement constructors is necessary but not (necessarily)
sufficient.

It's good that the ChromeOS compiler figured this out without needing as much
constexpr.

There are some VC++ limitations that stop us from using constexpr as much as I
would like but I've put in a strongly worded feature request which they are
considering.

 
 #define VECTOR_ICON_TEMPLATE(icon_name, path_name, path_name_1x) \
 const gfx::VectorIcon icon_name = { path_name , path_name_1x };