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

[Krzysztof Parzyszek via flang-commits]

https://github.com/kparzysz created https://github.com/llvm/llvm-project/pull/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.

>From 11e2a91da6cd77d526ece0b640033360b720652d Mon Sep 17 00:00:00 2001
From: Krzysztof Parzyszek <Krzysztof.Parzyszek at amd.com>
Date: Fri, 25 Jul 2025 08:17:55 -0500
Subject: [PATCH] [flang][OpenMP] Detect BLOCK construct through lookahead

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.
---
 flang/lib/Parser/openmp-parsers.cpp | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

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)) {