Bug 1123237 - Part 6. A new API to get backtrace without allocating memory in profiler. r=mstange

Based on patch from Ting-Yuan Huang <laszio.bugzilla@gmail.com>

tools/profiler/core/platform.cpp

@@ -1027,6 +1027,33 @@ void mozilla_sampler_free_backtrace(ProfilerBacktrace* aBacktrace)
   delete aBacktrace;
 }
 
+// Fill the output buffer with the following pattern:
+// "Lable 1" "\0" "Label 2" "\0" ... "Label N" "\0" "\0"
+// TODO: use the unwinder instead of pseudo stack.
+void mozilla_sampler_get_backtrace_noalloc(char *output, size_t outputSize)
+{
+  MOZ_ASSERT(outputSize >= 2);
+  char *bound = output + outputSize - 2;
+  output[0] = output[1] = '\0';
+  PseudoStack *pseudoStack = tlsPseudoStack.get();
+  if (!pseudoStack) {
+    return;
+  }
+
+  volatile StackEntry *pseudoFrames = pseudoStack->mStack;
+  uint32_t pseudoCount = pseudoStack->stackSize();
+
+  for (uint32_t i = 0; i < pseudoCount; i++) {
+    size_t len = strlen(pseudoFrames[i].label());
+    if (output + len >= bound)
+      break;
+    strcpy(output, pseudoFrames[i].label());
+    output += len;
+    *output++ = '\0';
+    *output = '\0';
+  }
+}
+
 void mozilla_sampler_tracing(const char* aCategory, const char* aInfo,
                              TracingMetadata aMetaData)
 {

tools/profiler/public/GeckoProfiler.h

@@ -149,6 +149,7 @@ static inline void profiler_resume() {}
 
 // Immediately capture the current thread's call stack and return it
 static inline ProfilerBacktrace* profiler_get_backtrace() { return nullptr; }
+static inline void profiler_get_backtrace_noalloc(char *output, size_t outputSize) { return; }
 
 // Free a ProfilerBacktrace returned by profiler_get_backtrace()
 static inline void profiler_free_backtrace(ProfilerBacktrace* aBacktrace) {}

tools/profiler/public/GeckoProfilerFunc.h

@@ -48,6 +48,7 @@ void mozilla_sampler_resume();
 
 ProfilerBacktrace* mozilla_sampler_get_backtrace();
 void mozilla_sampler_free_backtrace(ProfilerBacktrace* aBacktrace);
+void mozilla_sampler_get_backtrace_noalloc(char *output, size_t outputSize);
 
 bool mozilla_sampler_is_active();
 

tools/profiler/public/GeckoProfilerImpl.h

@@ -121,6 +121,12 @@ void profiler_free_backtrace(ProfilerBacktrace* aBacktrace)
   mozilla_sampler_free_backtrace(aBacktrace);
 }
 
+static inline
+void profiler_get_backtrace_noalloc(char *output, size_t outputSize)
+{
+  return mozilla_sampler_get_backtrace_noalloc(output, outputSize);
+}
+
 static inline
 bool profiler_is_active()
 {