From 49424a7b11152296af87d87dd03a0eaf7c41fbc9 Mon Sep 17 00:00:00 2001
From: Nathan Froyd <froydnj@mozilla.com>
Date: Tue, 28 Aug 2012 12:55:32 -0400
Subject: [PATCH] Bug 748444 - eliminate relocations from histogram data;
 r=taras

---
 toolkit/components/telemetry/Telemetry.cpp    | 27 +++++++--
 .../telemetry/gen-histogram-data.py           | 55 +++++++++++++++++--
 2 files changed, 71 insertions(+), 11 deletions(-)

diff --git a/toolkit/components/telemetry/Telemetry.cpp b/toolkit/components/telemetry/Telemetry.cpp
index 35a4420f0eca..8829849ab1a9 100644
--- a/toolkit/components/telemetry/Telemetry.cpp
+++ b/toolkit/components/telemetry/Telemetry.cpp
@@ -187,17 +187,32 @@ StatisticsRecorder gStatisticsRecorder;
 
 // Hardcoded probes
 struct TelemetryHistogram {
-  const char *id;
   uint32_t min;
   uint32_t max;
   uint32_t bucketCount;
   uint32_t histogramType;
-  const char *comment;
+  uint16_t id_offset;
+  uint16_t comment_offset;
+
+  const char *id() const;
+  const char *comment() const;
 };
 
 #include "TelemetryHistogramData.inc"
 bool gCorruptHistograms[Telemetry::HistogramCount];
 
+const char *
+TelemetryHistogram::id() const
+{
+  return &gHistogramStringTable[this->id_offset];
+}
+
+const char *
+TelemetryHistogram::comment() const
+{
+  return &gHistogramStringTable[this->comment_offset];
+}
+
 bool
 TelemetryHistogramType(Histogram *h, uint32_t *result)
 {
@@ -268,7 +283,7 @@ GetHistogramByEnumId(Telemetry::ID id, Histogram **ret)
   }
 
   const TelemetryHistogram &p = gHistograms[id];
-  nsresult rv = HistogramGet(p.id, p.min, p.max, p.bucketCount, p.histogramType, &h);
+  nsresult rv = HistogramGet(p.id(), p.min, p.max, p.bucketCount, p.histogramType, &h);
   if (NS_FAILED(rv))
     return rv;
 
@@ -585,7 +600,7 @@ TelemetryImpl::GetHistogramEnumId(const char *name, Telemetry::ID *id)
   TelemetryImpl::HistogramMapType *map = &sTelemetry->mHistogramMap;
   if (!map->Count()) {
     for (uint32_t i = 0; i < Telemetry::HistogramCount; i++) {
-      CharPtrEntryType *entry = map->PutEntry(gHistograms[i].id);
+      CharPtrEntryType *entry = map->PutEntry(gHistograms[i].id());
       if (NS_UNLIKELY(!entry)) {
         map->Clear();
         return NS_ERROR_OUT_OF_MEMORY;
@@ -1153,10 +1168,10 @@ TelemetryImpl::GetRegisteredHistograms(JSContext *cx, jsval *ret)
   JS::AutoObjectRooter root(cx, info);
 
   for (size_t i = 0; i < count; ++i) {
-    JSString *comment = JS_InternString(cx, gHistograms[i].comment);
+    JSString *comment = JS_InternString(cx, gHistograms[i].comment());
     
     if (!(comment
-          && JS_DefineProperty(cx, info, gHistograms[i].id,
+          && JS_DefineProperty(cx, info, gHistograms[i].id(),
                                STRING_TO_JSVAL(comment), NULL, NULL,
                                JSPROP_ENUMERATE))) {
       return NS_ERROR_FAILURE;
diff --git a/toolkit/components/telemetry/gen-histogram-data.py b/toolkit/components/telemetry/gen-histogram-data.py
index db144b836375..d96a84d209b9 100644
--- a/toolkit/components/telemetry/gen-histogram-data.py
+++ b/toolkit/components/telemetry/gen-histogram-data.py
@@ -16,22 +16,67 @@ banner = """/* This file is auto-generated, see gen-histogram-data.py.  */
 
 # Write out the gHistograms array.
 
-def print_array_entry(histogram):
+class StringTable:
+    def __init__(self):
+        self.current_index = 0;
+        self.table = {}
+
+    def c_strlen(self, string):
+        return len(string) + 1
+
+    def stringIndex(self, string):
+        if string in self.table:
+            return self.table[string]
+        else:
+            result = self.current_index
+            self.table[string] = result
+            self.current_index += self.c_strlen(string)
+            return result
+
+    def writeDefinition(self, f, name):
+        entries = self.table.items()
+        entries.sort(key=lambda x:x[1])
+        # Avoid null-in-string warnings with GCC and potentially
+        # overlong string constants; write everything out the long way.
+        def explodeToCharArray(string):
+            def toCChar(s):
+                if s == "'":
+                    return "'\\''"
+                else:
+                    return "'%s'" % s
+            return ", ".join(map(toCChar, string))
+        f.write("const char %s[] = {\n" % name)
+        for (string, offset) in entries[:-1]:
+            f.write("  /* %5d */ %s, '\\0',\n"
+                    % (offset, explodeToCharArray(string)))
+        f.write("  /* %5d */ %s, '\\0' };\n\n"
+                % (entries[-1][1], explodeToCharArray(entries[-1][0])))
+
+def print_array_entry(histogram, name_index, desc_index):
     cpp_guard = histogram.cpp_guard()
     if cpp_guard:
         print "#if defined(%s)" % cpp_guard
-    print "  { \"%s\", %s, %s, %s, %s, \"%s\" }," \
-        % (histogram.name(), histogram.low(), histogram.high(),
+    print "  { %s, %s, %s, %s, %d, %d }," \
+        % (histogram.low(), histogram.high(),
            histogram.n_buckets(), histogram.nsITelemetry_kind(),
-           histogram.description())
+           name_index, desc_index)
     if cpp_guard:
         print "#endif"
 
 def write_histogram_table(histograms):
+    table = StringTable()
+
     print "const TelemetryHistogram gHistograms[] = {"
     for histogram in histograms:
-        print_array_entry(histogram)
+        name_index = table.stringIndex(histogram.name())
+        desc_index = table.stringIndex(histogram.description())
+        print_array_entry(histogram, name_index, desc_index)
     print "};"
+
+    strtab_name = "gHistogramStringTable"
+    table.writeDefinition(sys.stdout, strtab_name)
+    static_assert("sizeof(%s) <= UINT16_MAX" % strtab_name,
+                  "index overflow")
 
 # Write out static asserts for histogram data.  We'd prefer to perform
 # these checks in this script itself, but since several histograms