[flang-commits] [flang] [flang] Better recovery from errors in a loop control (PR #117025)

[Peter Klausler via flang-commits]

https://github.com/klausler created https://github.com/llvm/llvm-project/pull/117025

When there's an error in a DO statement loop control, error recovery isn't great.  A bare "DO" is a value statement, so a failure to parse its loop control doesn't fail on the whole statement.  Its partial parse ends after the keyword, and as some other statement parsers can get further into the input before failing, errors in the loop control can lead to confusing error messages about bad pointer assignment statements and others.  So just check that a bare "DO" is followed by the end of the statement.

>From 3348886cd4b241a2ce4a6f70687740637ca5c01a Mon Sep 17 00:00:00 2001
From: Peter Klausler <pklausler at nvidia.com>
Date: Wed, 20 Nov 2024 10:36:02 -0800
Subject: [PATCH] [flang] Better recovery from errors in a loop control

When there's an error in a DO statement loop control, error recovery
isn't great.  A bare "DO" is a value statement, so a failure to parse
its loop control doesn't fail on the whole statement.  Its partial
parse ends after the keyword, and as some other statement parsers can
get further into the input before failing, errors in the loop control
can lead to confusing error messages about bad pointer assignment statements
and others.  So just check that a bare "DO" is followed by the end of the
statement.
---
 flang/lib/Parser/executable-parsers.cpp | 15 +++++++++++----
 flang/lib/Parser/type-parsers.h         |  1 -
 flang/test/Parser/recovery07.f90        |  6 ++++++
 3 files changed, 17 insertions(+), 5 deletions(-)
 create mode 100644 flang/test/Parser/recovery07.f90

diff --git a/flang/lib/Parser/executable-parsers.cpp b/flang/lib/Parser/executable-parsers.cpp
index 730165613d91db..00ab03d6f86b55 100644
--- a/flang/lib/Parser/executable-parsers.cpp
+++ b/flang/lib/Parser/executable-parsers.cpp
@@ -9,7 +9,6 @@
 // Per-type parsers for executable statements
 
 #include "basic-parsers.h"
-#include "debug-parser.h"
 #include "expr-parsers.h"
 #include "misc-parsers.h"
 #include "stmt-parser.h"
@@ -282,18 +281,26 @@ TYPE_CONTEXT_PARSER("loop control"_en_US,
                 "CONCURRENT" >> concurrentHeader,
                 many(Parser<LocalitySpec>{})))))
 
+// "DO" is a valid statement, so the loop control is optional; but for
+// better recovery from errors in the loop control, don't parse a
+// DO statement with a bad loop control as a DO statement that has
+// no loop control and is followed by garbage.
+static constexpr auto loopControlOrEndOfStmt{
+    construct<std::optional<LoopControl>>(Parser<LoopControl>{}) ||
+    lookAhead(":\n"_ch) >> construct<std::optional<LoopControl>>()};
+
 // R1121 label-do-stmt -> [do-construct-name :] DO label [loop-control]
 // A label-do-stmt with a do-construct-name is parsed as a nonlabel-do-stmt
 // with an optional label.
 TYPE_CONTEXT_PARSER("label DO statement"_en_US,
-    construct<LabelDoStmt>("DO" >> label, maybe(loopControl)))
+    construct<LabelDoStmt>("DO" >> label, loopControlOrEndOfStmt))
 
 // R1122 nonlabel-do-stmt -> [do-construct-name :] DO [loop-control]
 TYPE_CONTEXT_PARSER("nonlabel DO statement"_en_US,
     construct<NonLabelDoStmt>(
-        name / ":", "DO" >> maybe(label), maybe(loopControl)) ||
+        name / ":", "DO" >> maybe(label), loopControlOrEndOfStmt) ||
         construct<NonLabelDoStmt>(construct<std::optional<Name>>(),
-            construct<std::optional<Label>>(), "DO" >> maybe(loopControl)))
+            construct<std::optional<Label>>(), "DO" >> loopControlOrEndOfStmt))
 
 // R1132 end-do-stmt -> END DO [do-construct-name]
 TYPE_CONTEXT_PARSER("END DO statement"_en_US,
diff --git a/flang/lib/Parser/type-parsers.h b/flang/lib/Parser/type-parsers.h
index adbf6d23cbd99a..f800398a22de9a 100644
--- a/flang/lib/Parser/type-parsers.h
+++ b/flang/lib/Parser/type-parsers.h
@@ -102,7 +102,6 @@ constexpr Parser<ForallAssignmentStmt> forallAssignmentStmt; // R1053
 constexpr Parser<ForallStmt> forallStmt; // R1055
 constexpr Parser<Selector> selector; // R1105
 constexpr Parser<EndSelectStmt> endSelectStmt; // R1143 & R1151 & R1155
-constexpr Parser<LoopControl> loopControl; // R1123
 constexpr Parser<ConcurrentHeader> concurrentHeader; // R1125
 constexpr Parser<IoUnit> ioUnit; // R1201, R1203
 constexpr Parser<FileUnitNumber> fileUnitNumber; // R1202
diff --git a/flang/test/Parser/recovery07.f90 b/flang/test/Parser/recovery07.f90
new file mode 100644
index 00000000000000..3a66779b595819
--- /dev/null
+++ b/flang/test/Parser/recovery07.f90
@@ -0,0 +1,6 @@
+! RUN: not %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+! CHECK: error: expected ':'
+! CHECK: in the context: loop control
+do concurrent(I = 1, N)
+end do
+end