[webkit-dev] FontPlatformData, FontCache and HashMap

n179911 n179911 at gmail.com
Sat Feb 20 21:02:01 PST 2010

I have a related question about SimpleFontData and FontPlatformData.

When does Webkit create a FontPlatformData and SimpleFontData?
I too put debug printfs in these classes constructors. And I see
Webkit creates different FontPlatformData and SimpleFontData for same
font family and same font size.

Thank you for any tip in helping me understanding.

On Thu, Feb 18, 2010 at 12:28 PM, Stephan Assmus <superstippi at gmx.de> wrote:
> On 2010-02-18 at 18:43:08 [+0100], Stephan Assmus <superstippi at gmx.de> wrote:
>> Hi all,
>> currently, I am investigating some weirdness with regards to
>> FontPlatformData and the FontCache which I see in my debugging output on
>> the Haiku port. I am not even 100% sure that what I see is a weirdness, but
>> at least I don't understand it and I was wondering if anyone could help me
>> shed some light into this.
>> So I understand these things:
>> 1) SimpleFontData has a member m_platformData of type FontPlatformData,
>> which is always instantiated by passing it a const reference of another
>> FontPlatformData.
>> 2) The FontCache maintains SimplaFontData instances in a HashMap, and looks
>> these up and compares them by using FontPlatformData objects as keys into
>> the map.
>> 3) FontCache has a static "empty" FontPlatformData instance in
>> FontDataCacheKeyTraits:
>>     static const FontPlatformData& emptyValue()
>>     {
>>         DEFINE_STATIC_LOCAL(FontPlatformData, key, (0.f, false, false));
>>         return key;
>>     }
>> Don't yet know how this comes into play, but possibly it's of relevance.
>> My analysis is a bit awkward, since I cannot use a source level debugger on
>> Haiku, and I won't bore you with the embarrasing details of why. So I added
>> printf() statements in all my constructors of FontPlatformData, and printed
>> among other things the memory address of the FontPlatformData instance
>> (this). Also, I am printing "this" in a number of other places, operator=()
>> for example. The output I am getting is a little weird, since it shows that
>> operator=() is called on objects, for which no constructor has been called.
>> Basically, I've narrowed it down to SimpleFontData*
>> FontCache::getCachedFontData(const FontPlatformData* platformData) in
>> FontCache.cpp. In this method, a new SimpleFontData instance is to be
>> created and inserted into the HashMap. I will show you the output from the
>> program start. It printed which FontPlatformData instances have been
>> created up to "Kill Thread" (I let it drop into the debugger from the first
>> invokation of FontPlatformData::operator=()):
>> The first font is created:
>> getCachedFontPlatformData(0)
>> createFontPlatformData(const FontDescription&)
>>   0x18018660->FontPlatformData(const FontDescription(16.0, Myriad Pro))
>>     0x18019bf0->FontPlatformDataPrivate()
>>   created result: 0x18018660
>> getCachedFontPlatformData() - done: 0x18018660
>> Next, getCachedFontData() is called to insert a new SimpleFontData for this
>> FontDescription:
>> getCachedFontData(0x18018660)
>> Next comes the output from line 289 of FontCache.cpp:
>> pair<SimpleFontData*, unsigned> newValue(new SimpleFontData(*platformData),
>> 1);
>> A new SimpleFontData object is created with it's m_fontData member at
>> address 0x2f4c6a8:
>>   0x2f4c6a8->FontPlatformData(const FontPlatformData& 0x18018660)
>>     0x18019bf0->addRef()
>>   new: 0x2f4c680
>> Next comes the call to HashMap::set(), which results in this output:
>>   0x18018970->FontPlatformData(0.0, 0, 0)
>> This is probably the key created from HashMap::inlineAdd(). And next,
>> HashMap::set() invokes operator=() on the key (line 250) which gives this
>> output:
>>   0x2ec8594 ((nil), 0.0, 0, 0) ->operator=(0x18018660, 0x18019bf0)
>> nil is a pointer to FontPlatformDataPrivate, which is ok. But 0x2ec8594
>> should be the pointer to the FontPlatformData instance created in
>> inlineAdd().
>> How can I be doing something wrong there? Nothing happens in my constructor
>> for FontPlatformData() when the empty key values are used. Maybe the usage
>> of the HashMap is broken in FontCache, since it would seem that such a
>> low-level utility class as HashMap would not likely contain any bugs.
> Ok, I've finally figured it out. FontCache.cpp defines the following for use
> in the HashMap:
> struct FontDataCacheKeyTraits : WTF::GenericHashTraits<FontPlatformData> {
>    static const bool emptyValueIsZero = true;
>    static const bool needsDestruction = true;
>    static const FontPlatformData& emptyValue()
>    {
>        DEFINE_STATIC_LOCAL(FontPlatformData, key, (0.f, false, false));
>        return key;
>    }
>    static void constructDeletedValue(FontPlatformData& slot)
>    {
>        new (&slot) FontPlatformData(HashTableDeletedValue);
>    }
>    static bool isDeletedValue(const FontPlatformData& value)
>    {
>        return value.isHashTableDeletedValue();
>    }
> };
> The "emptyValueIsZero" means the HashTable implementation will not call
> constructors for new slots. IMHO, this is a dangerous assumption, since the
> porters may not realize this and that this actually works is completely
> implementation specific.
> Best regards,
> -Stephan
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