[llvm] [MC] eatToEndOfStatement: eat trailing EOL with Lex() (PR #218590)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/218590

eatToEndOfStatement() advances the lexer with Lexer.Lex(), which neither
filters comment tokens nor pops the include stack at Eof. It therefore
leaves the token after the newline unfiltered, and leads to a spurious
error:

```
.extern foo
/* comment */
nop

error: unexpected token at start of statement
```

Fix with Lex(). Loosely take and clean up tests from #218456

>From d1bc61bf7489ab773de5bd22b0d9f02ea30eee17 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Mon, 24 Aug 2026 22:27:09 -0700
Subject: [PATCH] [MC] eatToEndOfStatement: eat trailing EOL with Lex()

eatToEndOfStatement() advances the lexer with Lexer.Lex(), which neither
filters comment tokens nor pops the include stack at Eof. It therefore
leaves the token after the newline unfiltered, and leads to a spurious
error:

```
.extern foo
/* comment */
nop

error: unexpected token at start of statement
```

Fix with Lex(). Loosely take and clean up tests from #218456
---
 llvm/lib/MC/MCParser/AsmParser.cpp     |  4 +-
 llvm/test/MC/AsmParser/block-comment.s | 65 ++++++++++++++++++++++++++
 2 files changed, 67 insertions(+), 2 deletions(-)
 create mode 100644 llvm/test/MC/AsmParser/block-comment.s

diff --git a/llvm/lib/MC/MCParser/AsmParser.cpp b/llvm/lib/MC/MCParser/AsmParser.cpp
index c4c5b1da9aa91..320a3d896e9d0 100644
--- a/llvm/lib/MC/MCParser/AsmParser.cpp
+++ b/llvm/lib/MC/MCParser/AsmParser.cpp
@@ -1090,9 +1090,9 @@ void AsmParser::eatToEndOfStatement() {
   while (Lexer.isNot(AsmToken::EndOfStatement) && Lexer.isNot(AsmToken::Eof))
     Lexer.Lex();
 
-  // Eat EOL.
+  // Eat EOL and skip the comments that follow.
   if (Lexer.is(AsmToken::EndOfStatement))
-    Lexer.Lex();
+    Lex();
 }
 
 StringRef AsmParser::parseStringToEndOfStatement() {
diff --git a/llvm/test/MC/AsmParser/block-comment.s b/llvm/test/MC/AsmParser/block-comment.s
new file mode 100644
index 0000000000000..7f57936c9d6ee
--- /dev/null
+++ b/llvm/test/MC/AsmParser/block-comment.s
@@ -0,0 +1,65 @@
+## A block comment after a statement discarded by eatToEndOfStatement() is not the start of the next statement.
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -triple=x86_64 -preserve-comments a.s | FileCheck %s --match-full-lines --strict-whitespace
+# RUN: llvm-mc -triple=x86_64 include.s | FileCheck %s --check-prefix=INC --match-full-lines --strict-whitespace
+# RUN: not llvm-mc -triple=x86_64 err.s -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR --implicit-check-not=error:
+
+#      CHECK:	# comment{{ }}
+# CHECK-NEXT:	nop
+#CHECK-EMPTY:
+# CHECK-NEXT:	# space+comment{{ }}
+# CHECK-NEXT:	nop
+#CHECK-EMPTY:
+# CHECK-NEXT:	# spanning
+# CHECK-NEXT:	#  two lines{{ }}
+# CHECK-NEXT:	nop
+#CHECK-EMPTY:
+# CHECK-NEXT:	nop	# then an instruction{{ }}
+#CHECK-EMPTY:
+# CHECK-NEXT:	nop	# line comment
+#CHECK-EMPTY:
+# CHECK-NEXT:	# at end of file{{ }}
+
+#       INC:	retq
+# INC-EMPTY:
+#       INC:	nop
+
+#--- a.s
+.extern a
+/* comment */
+nop
+
+.extern a1
+  /* space+comment */
+nop
+
+.extern b
+/* spanning
+  two lines */
+nop
+
+.extern c
+/* then an instruction */ nop
+
+.extern d # line comment
+nop
+
+.extern e
+/* at end of file */
+
+#--- include.s
+.include "included.s"
+nop
+
+#--- included.s
+ret
+.extern b
+/* comment */
+.extern a
+#--- err.s
+.byte 1 2
+/* comment-only line */
+nop
+
+#      ERR:err.s:1:9: error: unexpected token
+# ERR-NEXT:.byte 1 2