[flang] Fix parsing time explosion

[klausler]

When parsing a deeply-nested expression like
A1(A2(A3(A4(A5(A6(...A99(i)...))))))
the parser can get into an exponential state due to the need to consider the possibility that each "An(...)" might be the beginning of a reference to a procedure component ("An(...)%PROC(...)") so that alternative has to be attempted first before proceeding to try parsing "An(...)" as a function reference or as an array element designator. The parser for a structure component, which is used by the procedure designator parser, was not protected with the usual failure memoization technique, leading to exponentially bad behavior parsing a deeply-nested expression. Fix by exploiting the instrumented() parser combinator so that failed structure component parsers aren't repeated.

Fixes #76477.

[llvmbot]

@llvm/pr-subscribers-flang-parser

Author: Peter Klausler (klausler)

Changes

When parsing a deeply-nested expression like
A1(A2(A3(A4(A5(A6(...A99(i)...))))))
the parser can get into an exponential state due to the need to consider the possibility that each "An(...)" might be the beginning of a reference to a procedure component ("An(...)%PROC(...)") so that alternative has to be attempted first before proceeding to try parsing "An(...)" as a function reference or as an array element designator. The parser for a structure component, which is used by the procedure designator parser, was not protected with the usual failure memoization technique, leading to exponentially bad behavior parsing a deeply-nested expression. Fix by exploiting the instrumented() parser combinator so that failed structure component parsers aren't repeated.

Fixes #76477.

---

Full diff: https://github.com/llvm/llvm-project/pull/76533.diff

1 Files Affected:

• (modified) flang/lib/Parser/Fortran-parsers.cpp (+3-2)

diff --git a/flang/lib/Parser/Fortran-parsers.cpp b/flang/lib/Parser/Fortran-parsers.cpp
index c070bc1de37352..0dd95d69d3c662 100644
--- a/flang/lib/Parser/Fortran-parsers.cpp
+++ b/flang/lib/Parser/Fortran-parsers.cpp
@@ -1151,8 +1151,9 @@ TYPE_PARSER(construct<PartRef>(name,
 
 // R913 structure-component -> data-ref
 // The final part-ref in the data-ref is not allowed to have subscripts.
-TYPE_PARSER(construct<StructureComponent>(
-    construct<DataRef>(some(Parser<PartRef>{} / percentOrDot)), name))
+TYPE_CONTEXT_PARSER("component"_en_US,
+    construct<StructureComponent>(
+        construct<DataRef>(some(Parser<PartRef>{} / percentOrDot)), name))
 
 // R919 subscript -> scalar-int-expr
 constexpr auto subscript{scalarIntExpr};

[vzakhari]

LGTM