Minimum binary arithmetic reverse autodiff working. (#2514)

* Initial plumbing of backward autodiff in the frontend.

* More plumbing.

* Initial reverse autodiff working.

* Bug fixes.

* Misc.

* Remove redundant code.

* More clean up.

* Misc.

* Rebase and add backward diff test.

* Disable test.

* Clean up.

* Minor fix.

Co-authored-by: Yong He <yhe@nvidia.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/source/slang/slang-parser.cpp b/source/slang/slang-parser.cpp
index 78edd4deb..d3dc5964e 100644
--- a/source/slang/slang-parser.cpp
+++ b/source/slang/slang-parser.cpp
@@ -2109,6 +2109,26 @@ namespace Slang
         return parseForwardDifferentiate(parser);
     }
 
+        /// Parse an expression of the form __bwd_diff(fn) where fn is an 
+        /// identifier pointing to a function.
+    static Expr* parseBackwardDifferentiate(Parser* parser)
+    {
+        BackwardDifferentiateExpr* bwdDiffExpr = parser->astBuilder->create<BackwardDifferentiateExpr>();
+
+        parser->ReadToken(TokenType::LParent);
+
+        bwdDiffExpr->baseFunction = parser->ParseExpression();
+
+        parser->ReadToken(TokenType::RParent);
+
+        return bwdDiffExpr;
+    }
+
+    static NodeBase* parseBackwardDifferentiate(Parser* parser, void* /* unused */)
+    {
+        return parseBackwardDifferentiate(parser);
+    }
+
         /// Parse a `This` type expression
     static Expr* parseThisTypeExpr(Parser* parser)
     {
@@ -6646,7 +6666,8 @@ namespace Slang
         _makeParseExpr("none", parseNoneExpr),
         _makeParseExpr("try",     parseTryExpr),
         _makeParseExpr("__TaggedUnion", parseTaggedUnionType),
-        _makeParseExpr("__fwd_diff", parseForwardDifferentiate)
+        _makeParseExpr("__fwd_diff", parseForwardDifferentiate),
+        _makeParseExpr("__bwd_diff", parseBackwardDifferentiate)
     };
 
     ConstArrayView<SyntaxParseInfo> getSyntaxParseInfos()