[SR-14225] [AutoDiff] Control flow crash, dominated active value has no tangent space

[dan-zheng]

Previous ID	SR-14225
Radar	None
Original Reporter	@dan-zheng
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, AutoDiff
Assignee	None
Priority	Medium

md5: 3dd687f0115babbacf97b20cbcb90eba

Issue Description:

@differentiable
func TF_966(_ x: Float, _ bool: Bool) -> Float {
  let tuple = (x, 1)
  let result = bool ? tuple : tuple
  return result.0
}

$ swift -Xllvm -debug-only=differentiation tf-966.swift
PROPAGATING ADJOINT VALUE FOR ACTIVE VALUE
(%16, **%17**) = destructure_tuple %8 : $(Float, Int) // user: %18
$Int
Assertion failed: (hasVal), function getValue, file /Users/danielzheng/swift-bart/llvm/include/llvm/ADT/Optional.h, line 173.
Stack dump:
0.  Program arguments: /Users/danielzheng/swift-bart/build/Xcode-ReleaseAssert+swift-DebugAssert/swift-macosx-x86_64/Debug/bin/swift -frontend -interpret tf-966.swift -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -color-diagnostics -Xllvm -debug-only=differentiation -module-name main
1.  Swift version 5.1.1-dev (Swift f655d9e91a)
2.  While running pass #​17 SILModuleTransform "Differentiation".
3.  While processing `[differentiable source 0 wrt 0]` attribute on SIL function "@$s4main6TF_966yS2f_SbtF".
 for 'TF_966(_:_:)' (at tf-966.swift:2:1)
0  swift                    0x000000010f317525 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x000000010f316508 llvm::sys::RunSignalHandlers() + 248
2  swift                    0x000000010f317b28 SignalHandler(int) + 264
3  libsystem_platform.dylib 0x00007fff728e4b5d _sigtramp + 29
4  libsystem_platform.dylib 0x0000000000013e38 _sigtramp + 2373120760
5  libsystem_c.dylib        0x00007fff7279e6a6 abort + 127
6  libsystem_c.dylib        0x00007fff7276720d basename_r + 0
7  swift                    0x000000010be20c65 llvm::optional_detail::OptionalStorage<swift::VectorSpace, true>::getValue() & + 69
8  swift                    0x000000010be20c15 llvm::Optional<swift::VectorSpace>::getPointer() + 21
9  swift                    0x000000010be07f95 llvm::Optional<swift::VectorSpace>::operator->() + 21
10 swift                    0x000000010a649362 (anonymous namespace)::PullbackEmitter::getRemappedTangentType(swift::SILType) + 98
11 swift                    0x000000010a629ea9 (anonymous namespace)::PullbackEmitter::run() + 3529

Pullback generation crashes while propagating adjoint values for active values because dominated active values may have no tangent space.
Perhaps we can modify activity analysis to avoid this issue. Otherwise, we can skip non-differentiable values while tracking dominated active values.

[jkshtj]

Throws an error on 05/24 toolchain.

/Users/kshitij/workspace/scratch/test.swift:3:2: error: function is not differentiable
1 | import _Differentiation
2 |
3 | @differentiable(reverse)
  |  `- error: function is not differentiable
4 | func TF_966(_ x: Float, _ bool: Bool) -> Float {
  |      `- note: when differentiating this function definition
5 |   let tuple = (x, 1)
6 |   let result = bool ? tuple : tuple
  |                       `- note: expression is not differentiable
7 |   return result.0
8 | }