[flang-commits] [flang] 6b92a3b - [flang][OpenMP] Detect BLOCK construct through lookahead (#150617)

[via flang-commits]

Author: Krzysztof Parzyszek
Date: 2025-07-25T08:42:23-05:00
New Revision: 6b92a3bc21cdcdeb161a06524a9860551831c67e

URL: https://github.com/llvm/llvm-project/commit/6b92a3bc21cdcdeb161a06524a9860551831c67e
DIFF: https://github.com/llvm/llvm-project/commit/6b92a3bc21cdcdeb161a06524a9860551831c67e.diff

LOG: [flang][OpenMP] Detect BLOCK construct through lookahead (#150617)

Avoid parsing the entire ExecutionPartConstruct in either the strictly-
or the loosely-structured block parser only to discard it when it's not
BLOCK (or is BLOCK) respectively. Doing so was not incorrct, but in
pathological cases (like Fujitsu 0981_0034) the recursive parsing can
take a very long time.

Instead, detect the presence of BLOCK first (via a simple lookahead),
and fail immediately if necessary.

Added: 
    

Modified: 
    flang/lib/Parser/openmp-parsers.cpp

Removed: 
    


################################################################################
diff  --git a/flang/lib/Parser/openmp-parsers.cpp b/flang/lib/Parser/openmp-parsers.cpp
index 25a692deba3cc..1c626148a03ae 100644
--- a/flang/lib/Parser/openmp-parsers.cpp
+++ b/flang/lib/Parser/openmp-parsers.cpp
@@ -1222,11 +1222,14 @@ struct StrictlyStructuredBlockParser {
   using resultType = Block;
 
   std::optional<resultType> Parse(ParseState &state) const {
-    if (auto epc{Parser<ExecutionPartConstruct>{}.Parse(state)}) {
-      if (IsFortranBlockConstruct(*epc)) {
-        Block block;
-        block.emplace_back(std::move(*epc));
-        return std::move(block);
+    // Detect BLOCK construct without parsing the entire thing.
+    if (lookAhead(skipStuffBeforeStatement >> "BLOCK"_tok).Parse(state)) {
+      if (auto epc{Parser<ExecutionPartConstruct>{}.Parse(state)}) {
+        if (IsFortranBlockConstruct(*epc)) {
+          Block block;
+          block.emplace_back(std::move(*epc));
+          return std::move(block);
+        }
       }
     }
     return std::nullopt;
@@ -1237,6 +1240,10 @@ struct LooselyStructuredBlockParser {
   using resultType = Block;
 
   std::optional<resultType> Parse(ParseState &state) const {
+    // Detect BLOCK construct without parsing the entire thing.
+    if (lookAhead(skipStuffBeforeStatement >> "BLOCK"_tok).Parse(state)) {
+      return std::nullopt;
+    }
     Block body;
     if (auto epc{attempt(Parser<ExecutionPartConstruct>{}).Parse(state)}) {
       if (!IsFortranBlockConstruct(*epc)) {