[SR-12175] Fix forward-mode class differentiation crash

[dan-zheng]

Previous ID	SR-12175
Radar	rdar://problem/59496016
Original Reporter	@dan-zheng
Type	Sub-task

Additional Detail from JIRA

Votes	0
Component/s
Labels	Sub-task
Assignee	None
Priority	Medium

md5: 93e14b077ba1c338e9439a8336b784bc

Parent-Task:

• [SR-12148] Make differentiation work with class types #52130

Issue Description:

class C: Differentiable {
  @differentiable
  var float: Float = 0.0
}

$ swift -enable-experimental-forward-mode-differentiation -Xllvm -debug-only=differentiation sr-12175.swift
...
[AD] Original bb0: To differentiate or not to differentiate?
[ ]   debug_value %0 : $C, let, name "self", argno 1  // id: %1
[x]   %2 = ref_element_addr %0 : $C, #C.float         // user: %3
[x]   %3 = begin_access [read] [dynamic] %2 : $*Float // users: %5, %4
[x]   %4 = load [trivial] %3 : $*Float                // user: %6
[x]   end_access %3 : $*Float                         // id: %5
[ ]   return %4 : $Float                              // id: %6
[AD] Adding tangent for %0 = argument of bb0 : $C                         // users: %2, %1
[AD] Assigned parameter %0 = argument of bb0 : $C.TangentVector
 as the tangent of original result %0 = argument of bb0 : $C                         // users: %2, %1
[AD] JVPEmitter visited:
[ORIG]  %2 = ref_element_addr %0 : $C, #C.float         // user: %3
[TAN] Emitted in differential:
[AD] JVPEmitter visited:
[ORIG]  %3 = begin_access [read] [dynamic] %2 : $*Float // users: %5, %4
Assertion failed: (!insertion.second && "tangent buffer should already exist"), function getTangentBuffer, file /Users/danielzheng/swift-dev/swift/lib/SILOptimizer/Utils/Differentiation/JVPEmitter.cpp, line 257.
Stack dump:
0.  Program arguments: /Users/danielzheng/swift-dev/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift -frontend -interpret sr-12175.swift -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -color-diagnostics -enable-experimental-forward-mode-differentiation -Xllvm -debug-only=differentiation -module-name main
1.  Swift version 5.2-dev (Swift 9a55217738)
2.  While running pass #​507 SILModuleTransform "Differentiation".
3.  While processing // differentiability witness for C.float.getter
sil_differentiability_witness hidden [parameters 0] [results 0] @$s4main1CC5floatSfvg : $@convention(method) (@guaranteed C) -> Float {
}

 on SIL function "@$s4main1CC5floatSfvg".
 for getter for float (at sr-12175.swift:3:7)
4.  While generating JVP and differential for SIL function "@$s4main1CC5floatSfvg".
 for getter for float (at sr-12175.swift:3:7)

[beccadax]

@swift-ci create

[BradLarson]

Class and forward-mode differentiation support still needs some work, but this particular crash no longer occurs, so I'm going to close this out.