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[AutoDiff] NFC: prettify commutative diagrams. #31525

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Merged
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May 4, 2020
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[AutoDiff] NFC: prettify commutative diagrams. #31525

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31 changes: 16 additions & 15 deletions include/swift/SIL/TypeSubstCloner.h
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Original file line number Diff line number Diff line change
Expand Up @@ -322,28 +322,29 @@ class TypeSubstCloner : public SILClonerWithScopes<ImplClass> {
return;
}
// If the extractee is a derivative function, check whether the *remapped
// derivative function type* (BC) is equal to the *derivative remapped
// function type* (AD).
// derivative function type* (bc) is equal to the *derivative remapped
// function type* (ad).
//
// +----------------+ remap +-------------------------+
// | orig. fn type | -------(A)------> | remapped orig. fn type |
// +----------------+ +-------------------------+
// | |
// (B, SILGen) getAutoDiffDerivativeFunctionType (D, here)
// V V
// +----------------+ remap +-------------------------+
// | deriv. fn type | -------(C)------> | remapped deriv. fn type |
// +----------------+ +-------------------------+
// ┌────────────────┐ remap ┌─────────────────────────┐
// │ orig. fn type │ ───────(a)──────► │ remapped orig. fn type │
// └────────────────┘ └─────────────────────────┘
// │ │
// (b, SILGen) getAutoDiffDerivativeFunctionType (d, here)
// │ │
// ▼ ▼
// ┌────────────────┐ remap ┌─────────────────────────┐
// │ deriv. fn type │ ───────(c)──────► │ remapped deriv. fn type │
// └────────────────┘ └─────────────────────────┘
//
// (AD) does not always commute with (BC):
// - (AD) is the result of remapping, then computing the derivative type.
// (ad) does not always commute with (bc):
// - (ad) is the result of remapping, then computing the derivative type.
// This is the default cloning behavior, but may break invariants in the
// initial SIL generated by SILGen.
// - (BC) is the result of computing the derivative type (SILGen), then
// - (bc) is the result of computing the derivative type (SILGen), then
// remapping. This is the expected type, preserving invariants from
// earlier transforms.
//
// If (AD) is not equal to (BC), use (BC) as the explicit type.
// If (ad) is not equal to (bc), use (bc) as the explicit type.
SILType remappedOrigType = getOpType(dfei->getOperand()->getType());
auto remappedOrigFnType = remappedOrigType.castTo<SILFunctionType>();
auto derivativeRemappedFnType =
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31 changes: 16 additions & 15 deletions lib/SIL/IR/SILFunctionType.cpp
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Expand Up @@ -3088,30 +3088,31 @@ TypeConverter::getConstantInfo(TypeExpansionContext expansion,
// If the constant refers to a derivative function, get the SIL type of the
// original function and use it to compute the derivative SIL type.
//
// This is necessary because the "lowered AST derivative function type" (BC)
// This is necessary because the "lowered AST derivative function type" (bc)
// may differ from the "derivative type of the lowered original function type"
// (AD):
// (ad):
//
// +--------------------+ lowering +--------------------+
// | AST orig. fn type | -------(A)------> | SIL orig. fn type |
// +--------------------+ +--------------------+
// | |
// (B, Sema) getAutoDiffDerivativeFunctionType (D, here)
// V V
// +--------------------+ lowering +--------------------+
// | AST deriv. fn type | -------(C)------> | SIL deriv. fn type |
// +--------------------+ +--------------------+
// ┌────────────────────┐ lowering ┌────────────────────┐
// │ AST orig. fn type │ ───────(a)──────► │ SIL orig. fn type │
// └────────────────────┘ └────────────────────┘
// │ │
// (b, Sema) getAutoDiffDerivativeFunctionType (d, here)
// │ │
// ▼ ▼
// ┌────────────────────┐ lowering ┌────────────────────┐
// │ AST deriv. fn type │ ───────(c)──────► │ SIL deriv. fn type │
// └────────────────────┘ └────────────────────┘
//
// (AD) does not always commute with (BC):
// - (BC) is the result of computing the AST derivative type (Sema), then
// (ad) does not always commute with (bc):
// - (bc) is the result of computing the AST derivative type (Sema), then
// lowering it via SILGen. This is the default lowering behavior, but may
// break SIL typing invariants because expected lowered derivative types are
// computed from lowered original function types.
// - (AD) is the result of lowering the original function type, then computing
// - (ad) is the result of lowering the original function type, then computing
// its derivative type. This is the expected lowered derivative type,
// preserving SIL typing invariants.
//
// Always use (AD) to compute lowered derivative function types.
// Always use (ad) to compute lowered derivative function types.
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Nice autodiff pun: Always use (ad)

if (auto *derivativeId = constant.derivativeFunctionIdentifier) {
// Get lowered original function type.
auto origFnConstantInfo = getConstantInfo(
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