[flang] Fix prescanner bug w/ empty macros in line continuation

[klausler]

When processing free form source line continuation, the prescanner treats empty keyword macros as if they were spaces or tabs. After skipping over them, however, there's code that only works if the skipped characters ended with an actual space or tab. If the last skipped item was an empty keyword macro's name, the last character of that name would end up being the first character of the continuation line. Fix.

[llvmbot]

@llvm/pr-subscribers-flang-parser

Author: Peter Klausler (klausler)

Changes

When processing free form source line continuation, the prescanner treats empty keyword macros as if they were spaces or tabs. After skipping over them, however, there's code that only works if the skipped characters ended with an actual space or tab. If the last skipped item was an empty keyword macro's name, the last character of that name would end up being the first character of the continuation line. Fix.

---

Full diff: https://github.com/llvm/llvm-project/pull/141274.diff

2 Files Affected:

• (modified) flang/lib/Parser/prescan.cpp (+1-1)
• (added) flang/test/Preprocessing/bug890.F90 (+6)

diff --git a/flang/lib/Parser/prescan.cpp b/flang/lib/Parser/prescan.cpp
index 3bc2ea0b37508..9aef0c9981e3c 100644
--- a/flang/lib/Parser/prescan.cpp
+++ b/flang/lib/Parser/prescan.cpp
@@ -1473,7 +1473,7 @@ const char *Prescanner::FreeFormContinuationLine(bool ampersand) {
             GetProvenanceRange(p, p + 1),
             "Character literal continuation line should have been preceded by '&'"_port_en_US);
       }
-    } else if (p > lineStart) {
+    } else if (p > lineStart && IsSpaceOrTab(p - 1)) {
       --p;
     } else {
       insertASpace_ = true;
diff --git a/flang/test/Preprocessing/bug890.F90 b/flang/test/Preprocessing/bug890.F90
new file mode 100644
index 0000000000000..0ce2d8c3f1569
--- /dev/null
+++ b/flang/test/Preprocessing/bug890.F90
@@ -0,0 +1,6 @@
+! RUN: %flang -E %s 2>&1 | FileCheck %s
+!CHECK: subroutine sub()
+#define empty
+subroutine sub ( &
+  empty)
+end