Bug 1499507 - Fold the 'profiler is active' check into the 'JSContext has a non-null PseudoStack' check. r=sfink

This eliminates a few instructions from every profiler label and saves code size.
We have around 9000 WebIDL constructors + methods + getters + setters which all
have an inlined instance of this code.

This change reduces the binary size on Linux x64 by around 160KB.

Here's a diff of the impact on the code generated for Attr_Binding::get_specified
in the Mac build:

    movq       %rsp, %rbp
    pushq      %r15
    pushq      %r14
    pushq      %r12
    pushq      %rbx
    subq       $0x10, %rsp
    movq       %rcx, %r14
    movq       %rdx, %r15
-   movq       __ZN7mozilla8profiler6detail12RacyFeatures18sActiveAndFeaturesE@GOT, %rax ; __ZN7mozilla8profiler6detail12RacyFeatures18sActiveAndFeaturesE@GOT
-   movl       (%rax), %eax
-   testl      %eax, %eax
-   js         loc_xxxxx
-
-   movq       $0x0, -40(%rbp)
-   jmp        loc_xxxxx
-
-   movq       0x78(%rdi), %rbx
+   movq       0x80(%rdi), %rbx
    movq       %rbx, -40(%rbp)
    testq      %rbx, %rbx
    je         loc_xxxxx

    movl       0x10(%rbx), %r12d
    cmpl       %r12d, (%rbx)
    jbe        loc_xxxxx

Differential Revision: https://phabricator.services.mozilla.com/D9192

--HG--
extra : moz-landing-system : lando

Author: mstange

js/public/ProfilingStack.h

@@ -471,12 +471,16 @@ class GeckoProfilerThread
 
     ProfilingStack*         profilingStack_;
 
+    // Same as profilingStack_ if the profiler is currently active, otherwise null.
+    ProfilingStack*         profilingStackIfEnabled_;
+
   public:
     GeckoProfilerThread();
 
     uint32_t stackPointer() { MOZ_ASSERT(infraInstalled()); return profilingStack_->stackPointer; }
     ProfilingStackFrame* stack() { return profilingStack_->frames; }
     ProfilingStack* getProfilingStack() { return profilingStack_; }
+    ProfilingStack* getProfilingStackIfEnabled() { return profilingStackIfEnabled_; }
 
     /*
      * True if the profiler infrastructure is setup.  Should be true in builds
@@ -485,7 +489,8 @@ class GeckoProfilerThread
      */
     bool infraInstalled() { return profilingStack_ != nullptr; }
 
-    void setProfilingStack(ProfilingStack* profilingStack);
+    void setProfilingStack(ProfilingStack* profilingStack, bool enabled);
+    void enable(bool enable) { profilingStackIfEnabled_ = enable ? profilingStack_ : nullptr; }
     void trace(JSTracer* trc);
 
     /*

js/public/RootingAPI.h

@@ -866,8 +866,8 @@ class RootingContext
 
     // Gecko profiling metadata.
     // This isn't really rooting related. It's only here because we want
-    // GetContextProfilingStack to be inlineable into non-JS code, and we
-    // didn't want to add another superclass of JSContext just for this.
+    // GetContextProfilingStackIfEnabled to be inlineable into non-JS code, and
+    // we didn't want to add another superclass of JSContext just for this.
     js::GeckoProfilerThread geckoProfiler_;
 
   public:
@@ -1093,9 +1093,9 @@ GetContextZone(const JSContext* cx)
 }
 
 inline ProfilingStack*
-GetContextProfilingStack(JSContext* cx)
+GetContextProfilingStackIfEnabled(JSContext* cx)
 {
-    return JS::RootingContext::get(cx)->geckoProfiler().getProfilingStack();
+    return JS::RootingContext::get(cx)->geckoProfiler().getProfilingStackIfEnabled();
 }
 
 /**

js/src/vm/GeckoProfiler.cpp

@@ -30,6 +30,7 @@ using mozilla::DebugOnly;
 
 GeckoProfilerThread::GeckoProfilerThread()
   : profilingStack_(nullptr)
+  , profilingStackIfEnabled_(nullptr)
 {
 }
 
@@ -44,9 +45,10 @@ GeckoProfilerRuntime::GeckoProfilerRuntime(JSRuntime* rt)
 }
 
 void
-GeckoProfilerThread::setProfilingStack(ProfilingStack* profilingStack)
+GeckoProfilerThread::setProfilingStack(ProfilingStack* profilingStack, bool enabled)
 {
     profilingStack_ = profilingStack;
+    profilingStackIfEnabled_ = enabled ? profilingStack : nullptr;
 }
 
 void
@@ -472,12 +474,14 @@ ProfilingStackFrame::setPC(jsbytecode* pc)
 JS_FRIEND_API(void)
 js::SetContextProfilingStack(JSContext* cx, ProfilingStack* profilingStack)
 {
-    cx->geckoProfiler().setProfilingStack(profilingStack);
+    cx->geckoProfiler().setProfilingStack(profilingStack,
+        cx->runtime()->geckoProfiler().enabled());
 }
 
 JS_FRIEND_API(void)
 js::EnableContextProfilingStack(JSContext* cx, bool enabled)
 {
+    cx->geckoProfiler().enable(enabled);
     cx->runtime()->geckoProfiler().enable(enabled);
 }
 

tools/profiler/public/GeckoProfiler.h

@@ -728,20 +728,16 @@ class MOZ_RAII AutoProfilerLabel
     Push(sProfilingStack.get(), aLabel, aDynamicString, aLine, aCategory);
   }
 
-  // This is the AUTO_PROFILER_LABEL_FAST variant. It's guarded on
-  // profiler_is_active() and retrieves the ProfilingStack from the JSContext.
+  // This is the AUTO_PROFILER_LABEL_FAST variant. It retrieves the ProfilingStack
+  // from the JSContext and does nothing if the profiler is inactive.
   AutoProfilerLabel(JSContext* aJSContext,
                     const char* aLabel, const char* aDynamicString,
                     uint32_t aLine, js::ProfilingStackFrame::Category aCategory
                     MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
   {
     MOZ_GUARD_OBJECT_NOTIFIER_INIT;
-    if (profiler_is_active()) {
-      Push(js::GetContextProfilingStack(aJSContext),
-           aLabel, aDynamicString, aLine, aCategory);
-    } else {
-      mProfilingStack = nullptr;
-    }
+    Push(js::GetContextProfilingStackIfEnabled(aJSContext),
+         aLabel, aDynamicString, aLine, aCategory);
   }
 
   void Push(ProfilingStack* aProfilingStack,