Bug 221766 – Reduce string copies when converting from NSString/CFStringRef to WTF::String

• Component: New Bugs
• Status: RESOLVED FIXED

[Alex Christensen]

Reduce string copies when converting from NSString/CFStringRef to WTF::String

[Alex Christensen]

Created attachment 420020 [details]
Patch

[Geoffrey Garen]

Comment on attachment 420020 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=420020&action=review

r=me -- but please revert the buffer casting behavior before landing (or post a new patch)

> Source/WTF/wtf/text/cf/StringCF.cpp:52
> +    buffer.resize(size * sizeof(UChar) / sizeof(LChar));
> +
> +    StringBuffer<UChar>& ucharBuffer = *reinterpret_cast<StringBuffer<UChar>*>(&buffer);

I don't think we should do this part. It's not type-safe or strict-aliasing-safe. Also, in order for the resize() to be an optimization, malloc would need to over-allocate the original buffer by 2X. In bmalloc, that will never happen.

So, I recommend just using a new StringBuffer. Or, if I've missed something and there's a reason to believe that resize() will be faster, then I recommend adding a releaseRaw() that gives you the malloc buffer, along with an adopting constructor. That's type-safe and strict-aliasing-safe.

(Side note: I think the argument to resize() should have been just "size".)

[Alex Christensen]

Created attachment 420025 [details]
Patch

[Geoffrey Garen]

> Also, in order for the resize() to be an optimization,
> malloc would need to over-allocate the original buffer by 2X. In bmalloc,
> that will never happen.

Well, I guess it could be an optimization if the allocation were large and the adjacent memory were free? Anyway, in that case, you'd only save the function call to malloc(); the data copy would still have to restart from scratch. So, I'm not sure it would be a win.

[Alex Christensen]

If resize doesn't find adjacent memory, wouldn't it need to copy the bytes that we are going to throw away?  I think two malloc calls is actually most efficient.
I added a scope to free the first before allocating the second

[Geoffrey Garen]

Comment on attachment 420025 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=420025&action=review

> Source/WTF/wtf/text/cf/StringCF.cpp:47
> +        StringBuffer<LChar> buffer(size);
>          CFIndex usedBufLen;
> -        CFIndex convertedsize = CFStringGetBytes(str, CFRangeMake(0, size), kCFStringEncodingISOLatin1, 0, false, lcharBuffer.data(), size, &usedBufLen);
> -        if ((convertedsize == size) && (usedBufLen == size)) {
> -            m_impl = StringImpl::create(lcharBuffer.data(), size);
> +        CFIndex convertedSize = CFStringGetBytes(str, CFRangeMake(0, size), kCFStringEncodingISOLatin1, 0, false, buffer.characters(), size, &usedBufLen);
> +        if (convertedSize == size && usedBufLen == size) {

If we ever regret this behavior, I suppose we can preflight it with a CFStringGetFastestEncoding() check?

[Geoffrey Garen]

(In reply to Alex Christensen from comment #5)
> If resize doesn't find adjacent memory, wouldn't it need to copy the bytes
> that we are going to throw away?

Yes, good point.

[EWS]

commit-queue failed to commit attachment 420025 [details] to WebKit repository. To retry, please set cq+ flag again.

[EWS]

Committed r272752: <https://commits.webkit.org/r272752>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 420025 [details].

[Radar WebKit Bug Importer]

<rdar://problem/74250930>

[Darin Adler]

Comment on attachment 420025 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=420025&action=review

> Source/WTF/wtf/text/cf/StringCF.cpp:48
> +            m_impl = StringImpl::adopt(WTFMove(buffer));

This makes a 2-block StringImpl (BufferOwned), but the old code made a 1-block StringImpl (BufferInternal). Doesn’t that mean we are using more memory now?

> Source/WTF/wtf/text/cf/StringCF.cpp:55
> +    m_impl = StringImpl::adopt(WTFMove(ucharBuffer));

Ditto.

[Geoffrey Garen]

> This makes a 2-block StringImpl (BufferOwned), but the old code made a
> 1-block StringImpl (BufferInternal). Doesn’t that mean we are using more
> memory now?

bmalloc small allocations are precise, so I don't think BufferOwned intrinsically uses more memory. But maybe having two objects instead of one increases risk of fragmentation.