[flang] Handle duplicate backslashes in preprocessor.

[olee7]

Fixes:
112604

Fixes the issue where preprocessor will escape the backslashes but the compiler will not re-combine them.

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[llvmbot]

@llvm/pr-subscribers-flang-parser

Author: Lake (olee7)

Changes

Fixes:
112604

Fixes the issue where preprocessor will escape the backslashes but the compiler will not re-combine them.

---

Full diff: https://github.com/llvm/llvm-project/pull/115822.diff

2 Files Affected:

• (modified) flang/lib/Parser/prescan.cpp (+1-1)
• (added) flang/test/Parser/duplicate-backslashes.f90 (+23)

diff --git a/flang/lib/Parser/prescan.cpp b/flang/lib/Parser/prescan.cpp
index 1d2f1e97668792..8c991678047ecd 100644
--- a/flang/lib/Parser/prescan.cpp
+++ b/flang/lib/Parser/prescan.cpp
@@ -843,7 +843,7 @@ void Prescanner::QuotedCharacterLiteral(
     if (*at_ == '\\') {
       if (escapesEnabled) {
         isEscaped = !isEscaped;
-      } else {
+      } else if (!preprocessingOnly_) {
         // The parser always processes escape sequences, so don't confuse it
         // when escapes are disabled.
         insert('\\');
diff --git a/flang/test/Parser/duplicate-backslashes.f90 b/flang/test/Parser/duplicate-backslashes.f90
new file mode 100644
index 00000000000000..896a5e864bc93d
--- /dev/null
+++ b/flang/test/Parser/duplicate-backslashes.f90
@@ -0,0 +1,23 @@
+! RUN: %flang -cpp -E %s -o %t.f90
+! RUN: %flang %t.f90 -o %t
+! RUN: %t | FileCheck %s
+
+program main
+  implicit none
+
+  ! Test single backslash
+  write(*, '(A)') "\"   ! Expected single backslash in output
+  ! CHECK: \
+  ! CHECK-NOT: \\
+
+  ! Test double backslash
+  write(*, '(A)') "\\"   ! Expected double backslashes in output
+  ! CHECK: \\
+  ! CHECK-NOT: \\\\
+
+  ! Test quadruple backslash
+  write(*, '(A)') "\\\\"   ! Expected quadruple backslashes in output
+  ! CHECK: \\\\
+  ! CHECK-NOT: \\\\\\\\
+
+end program main