Summary:
Perf numbers for this stack are given in terms of before-stack and after-stack, but the changes are split up for ease of review, and also to show that this migration CAN happen per-component and is 100% opt-in. Most existing C++ components do not /need/ to change at all.
# Problem Statement
During certain renders (select critical scenarios in specific products), UIManagerBinding::createNode time takes over 50% of JS thread CPU time. This could be higher or lower depending on the specific product and interaction, but overall createNode takes a lot of CPU time. The question is: can we improve this? What is the minimal overhead needed?
The vast, vast majority of time is taken up by prop parsing (specifically, converting JS values across the JSI into concrete values on the C++ props structs). Other methods like appendChild, etc, do not take up a significant amount of time; so we conclude that createNode is special, and the JSI itself, or calling into C++, is not the problem. Props parsing is the perf problem.
Can we improve it? (Spoiler: yes)
# How does props parsing work today?
Today, props parsing works as follows:
1. The ConcreteComponentDescriptor will construct a RawPropsParser (one per component /type/, per application: so one for View, one for Image, one for Text... etc)
2. Once per component type per application, ConcreteComponentDescriptor will call "prepare" on the RawPropsParser with an empty, default-constructed ConcreteProps struct. This ConcreteProps struct will cause RawProps.at(field) for every single field.
3. Based on the RawProps::at calls in part 2, RawPropsParser constructs a Map from props string names (width, height, position, etc) to a position within a "value index" array.
4. The above is what happens before any actual props are parsed; and the RawPropsParser is now ready to parse actual Props.
5. When props are actually being parsed from a JSI dictionary, we now have two phases:
1. The RawPropsParser `preparse`s the RawProps, by iterating over the JSI map and filling in two additional data structures: a linear list of RawValues, and a mapping from the ValueIndex array (`keyIndexToValueIndex_`; see step 3) to a value's position in the values list (`value_` in RawPropsParser/RawProps);
2. The ConcretePropT constructor is called, which is the same as in step 2/3, which calls `fieldValue = rawProps.at("fieldName")` repeatedly.
3. For each `at` call, the RawProps will look up a prop name in the Map constructed in step 3, and either return an empty value, or map the key name to the `keyIndexToValueIndex_` array, which maps to a value in `values_`, which is then returned and further parsed.
So, a few things that become clear with the current architecture:
1. Complexity is a property of the number of /possible/ props that /can/ be parsed, not what is actually used in product code. This violates the "only pay for what you use" principal. If you have `<View opacity={0.5} />`, the ViewProps constructor will request ~170 properties, not 1!
2. There's a lot of pre-parsing which isn't free
3. The levels of indirection aren't free, and make cache misses more likely and pipelining is more challenging
4. The levels of indirection also require more memory - minor, but not free
# How can we improve it?
The goal is to improve props parsing with minimal or zero impact on backwards-compability. We should be able to migrate over components when it's clear there's a performance issue, without requiring everything gets migrated over at once. This both (1) helps us prove out the new architecture, (2) derisks the project, (3) gives us time, internally and externally, to perfect the APIs and gradually migrate everything over before deleting the old infrastructure code entirely.
Thus, the goal is to do something that introduces a zero-cost abstraction. This isn't entirely possible in practice, and in fact this method slightly regresses components that do not use the new architecture /at all/, while dramatically improving migrated components and causing the impact of the /old/ architecture to be minimal.
# Solution
1. We still keep the existing system in place entirely.
2. After Props are constructed (see ConcreteComponentDescriptor changes) we iterate over all the /values/ set from JS, and call PropsT::setProp. Incidentally, this allows us to easily reuse the same prop for multiple values for "free", which was expensive in the old system.
3. It's worth noting that this makes a Props struct "less immutable" than it was before, and essentially now we have a "builder pattern" for Props. (If we really wanted to, we could still require a single constructor for Props, and then actually use an intermediate PropsBuilder to accumulate values - but I don't think this overhead would be worth for the conceptual "immutability" win, and instead a "Construct/Set/Seal" model works fine, and we still have all the same guarantees of immutability after the parsing phase)
# Implementation Details
# How to properly construct a single Prop value
Minor detail: parsing a single prop is a 3-step process. We imagine two scenarios: (1) Creating a new ShadowNode/Props A from nothing/void, so the previous Props value is just the default constructor. (2) Cloning a ShadowNode A->B and therefore Props A must be copied to Props B before parsing.
We will denote this as a clone from A->B, where A may or may not be a previous node or a default-constructed Props node; and imagine in particular that we're setting the "opacity" value for PropsB.
We must first (1) copy a value over from the previous version of the Props struct, so B.opacity = A.opacity; (2) Determine if opacity has been set from JS. If so, and there is a value, B.opacity = parse(JSValue). (3) If JS has passed in a value for the prop, BUT the value is `null`, it means that JS is resetting or deleting the prop, so we must set it BACK to the default. In this case we set PropsB.opacity = DefaultConstructedProps.opacity.
We must take care in general to ensure that the correct behavior is achieved here, which should help to explain some of the code below.
## String comparisons vs hash comparisons
In the previous system, a RawPropsKey is three `const char*` strings, concatenated together repeatedly /at runtime/. In practice, the ONLY reason we have the prefix, name, suffix Key structure is for the templated prop parsing in ViewProps and YogaStyableProps - that's it. It's not used anywhere else. Further, the key {"margin", "Left", "Width"} is identical to the key {"marginLeftWidth", null, null} and we don't do anything fancy with matching prefixes before comparing the whole string, or similar. Before comparison, keys are concatenated into a single string and then we use `strcmp`. The performance of this isn't terrible, but it's nonzero overhead.
I think we can do better and it's sufficient to compare hashed string values; even better, we can construct most of these /at compile time/ using constexpr, and using `switch` statements guarantee no hash collisions within a single Props struct (it's possible there's a collision between Props.cpp and ViewProps.cpp, for example, since they're different switch statements). We may eventually want to be more robust against has collisions; I personally don't find the risk to be too great, hash collisions with these keys are exceedingly unlikely (or maybe I just like to live dangerously). Thus, at runtime, each setProp requires computing a single hash for the value coming from JS, and then int comparisons with a bunch of pre-compiled values.
If we want to be really paranoid, we could be robust to hash collisions by doing `switch COMPILED_HASH("opacity"): if (strcmp(strFromJs, "opacity") == 0)`. I'm happy to do this if there's enough concern.
## Macros
Yuck! I'm using lots of C preprocessor macros. In general I found this way, way easier in reducing code and (essentially) doing codegen for me vs templated code for the switch cases and hashing prop names at compile-time. Maybe there's a better way.
Changelog: [Added][Fabric] New API for efficient props construction
Reviewed By: javache
Differential Revision: D37050215
fbshipit-source-id: d2dcd351a93b9715cfeb5197eb0d6f9194ec6eb9
Summary:
These dynamic_casts aren't really giving us much (they have never fired once in dev! and don't run in prod anyway). They also prevent us from disabling RTTI. So, let's get rid of them.
Changelog: [Internal]
Reviewed By: philIip
Differential Revision: D31634895
fbshipit-source-id: 4a9b259837127feb324f64fa3e9e23eb1cc481a6
Summary:
changelog: [internal]
LayoutAnimations only animates changes inside View and Paragraph nodes. This diff extends it to any node that's ViewKind.
Reviewed By: JoshuaGross
Differential Revision: D30603138
fbshipit-source-id: 63ca1e5df420149c4ba66151e97fea419fdfe631
Summary:
This diff is part of a bigger effort to remove the RTTI flags.
To do so we need to remove occurrences of `dynamic_cast` and other functions that rely on runtime
type informations.
Changelog:
[Internal][Changed] - Removed extra asserts relying on dynamic_cast
Reviewed By: JoshuaGross
Differential Revision: D30483554
fbshipit-source-id: 92b31281841a92c7b43e918938248431265dd654
Summary:
See previous diffs for context. This updates all of the relevant props structs.
Changelog: [Internal]
Reviewed By: mdvacca
Differential Revision: D29855426
fbshipit-source-id: 30177c3380ef82ecf8f2a4321f128cfbe8a576e0
Summary:
Changelog: [internal]
`ShadowNode::Unshared` is preferred over `UnsharedShadowNode`. This diff removes last uses of the alias.
Differential Revision: D27407197
fbshipit-source-id: aa1440f80dcab523d61c186f2d3ce052f314e52c
Summary:
Changelog: [internal]
This method is not designed to be subclassed. Let's make it non-virtual.
Reviewed By: JoshuaGross
Differential Revision: D27464824
fbshipit-source-id: b3186d1c4cf03226005f9a3caa9c002011798faa
Summary:
See react_native_assert.{h,cpp}. Because of the BUCK+Android issue where NDEBUG is always defined, we use react_native_assert instead of assert to enable xplat asserts in debug/dev mode.
This migrates most of the codebase, but probably not 100%. The goal is to increase assertion coverage on Android, not to get to 100% (yet).
Changelog: [Internal]
Reviewed By: RSNara
Differential Revision: D26562866
fbshipit-source-id: a7bf2055b973e1d3650ed8d68a6d02d556604af9
Summary:
In Android, only changed prop values are sent to the mounting layer via folly::dynamic maps. In the LayoutAnimation system, before this, we only sent that
map at the /end/ of the animation for any non-interpolated values (for example, image source is not interpolated so it was not updated until the end of the animation).
However, what we probably expect is that all non-interpolated values change immediately, and interpolated values smoothly transition. This diff makes that change on Android
by using the final RawProps as the /initial/ value that interpolations are stacked on.
Changelog: [Internal]
Reviewed By: mdvacca
Differential Revision: D25727483
fbshipit-source-id: e692d37b9965fedcdf429a81d60b7cb7f0c6abe1
Summary:
This diff removes the inner folder of react/renderer/core, moving all its files into react/renderer/core
This is necessary to simplify the compilation of Fabric in OSS
More details: https://fb.quip.com/amaRA631DX3K
changelog: [internal] Internal
Reviewed By: fkgozali, JoshuaGross
Differential Revision: D22875854
fbshipit-source-id: e2d969c3ec67eab1bbdc9288e5a4285c740fa944
Pieter De Baets
Joshua Gross
Samuel Susla
Andres Suarez
Nicola Corti
David Vacca