[FileCheck] Support comment directives

Sometimes you want to disable a FileCheck directive without removing
it entirely, or you want to write comments that mention a directive by
name.  The `COM:` directive makes it easy to do this.  For example,
you might have:

```
; X32: pinsrd_1:
; X32:    pinsrd $1, 4(%esp), %xmm0

; COM: FIXME: X64 isn't working correctly yet for this part of codegen, but
; COM: X64 will have something similar to X32:
; COM:
; COM:   X64: pinsrd_1:
; COM:   X64:    pinsrd $1, %edi, %xmm0
```

Without this patch, you need to use some combination of rewording and
directive syntax mangling to prevent FileCheck from recognizing the
commented occurrences of `X32:` and `X64:` above as directives.
Moreover, FileCheck diagnostics have been proposed that might complain
about the occurrences of `X64` that don't have the trailing `:`
because they look like directive typos:

  <http://lists.llvm.org/pipermail/llvm-dev/2020-April/140610.html>

I think dodging all these problems can prove tedious for test authors,
and directive syntax mangling already makes the purpose of existing
test code unclear.  `COM:` can avoid all these problems.

This patch also updates the small set of existing tests that define
`COM` as a check prefix:

- clang/test/CodeGen/default-address-space.c
- clang/test/CodeGenOpenCL/addr-space-struct-arg.cl
- clang/test/Driver/hip-device-libs.hip
- llvm/test/Assembler/drop-debug-info-nonzero-alloca.ll

I think lit should support `COM:` as well.  Perhaps `clang -verify`
should too.

Reviewed By: jhenderson, thopre

Differential Revision: https://reviews.llvm.org/D79276

15 files changed, 320 insertions, 31 deletions

diff --git a/llvm/docs/CommandGuide/FileCheck.rst b/llvm/docs/CommandGuide/FileCheck.rst
index 55fc3f0..7d9a69e 100644
--- a/llvm/docs/CommandGuide/FileCheck.rst
+++ b/llvm/docs/CommandGuide/FileCheck.rst
@@ -39,15 +39,31 @@ and from the command line.
  match.  By default, these patterns are prefixed with "``CHECK:``".
  If you'd like to use a different prefix (e.g. because the same input
  file is checking multiple different tool or options), the
- :option:`--check-prefix` argument allows you to specify one or more
- prefixes to match. Multiple prefixes are useful for tests which might
- change for different run options, but most lines remain the same.
+ :option:`--check-prefix` argument allows you to specify (without the trailing
+ "``:``") one or more prefixes to match. Multiple prefixes are useful for tests
+ which might change for different run options, but most lines remain the same.
+
+ FileCheck does not permit duplicate prefixes, even if one is a check prefix
+ and one is a comment prefix (see :option:`--comment-prefixes` below).
 
 .. option:: --check-prefixes prefix1,prefix2,...
 
  An alias of :option:`--check-prefix` that allows multiple prefixes to be
  specified as a comma separated list.
 
+.. option:: --comment-prefixes prefix1,prefix2,...
+
+ By default, FileCheck ignores any occurrence in ``match-filename`` of any check
+ prefix if it is preceded on the same line by "``COM:``" or "``RUN:``". See the
+ section `The "COM:" directive`_ for usage details.
+
+ These default comment prefixes can be overridden by
+ :option:`--comment-prefixes` if they are not appropriate for your testing
+ environment. However, doing so is not recommended in LLVM's LIT-based test
+ suites, which should be easier to maintain if they all follow a consistent
+ comment style. In that case, consider proposing a change to the default
+ comment prefixes instead.
+
 .. option:: --input-file filename
 
   File to check (defaults to stdin).
@@ -236,6 +252,57 @@ circumstances, for example, testing different architectural variants with
 In this case, we're testing that we get the expected code generation with
 both 32-bit and 64-bit code generation.
 
+The "COM:" directive
+~~~~~~~~~~~~~~~~~~~~
+
+Sometimes you want to disable a FileCheck directive without removing it
+entirely, or you want to write comments that mention a directive by name. The
+"``COM:``" directive makes it easy to do this. For example, you might have:
+
+.. code-block:: llvm
+
+   ; X32: pinsrd_1:
+   ; X32:    pinsrd $1, 4(%esp), %xmm0
+
+   ; COM: FIXME: X64 isn't working correctly yet for this part of codegen, but
+   ; COM: X64 will have something similar to X32:
+   ; COM:
+   ; COM:   X64: pinsrd_1:
+   ; COM:   X64:    pinsrd $1, %edi, %xmm0
+
+Without "``COM:``", you would need to use some combination of rewording and
+directive syntax mangling to prevent FileCheck from recognizing the commented
+occurrences of "``X32:``" and "``X64:``" above as directives. Moreover,
+FileCheck diagnostics have been proposed that might complain about the above
+occurrences of "``X64``" that don't have the trailing "``:``" because they look
+like directive typos. Dodging all these problems can be tedious for a test
+author, and directive syntax mangling can make the purpose of test code unclear.
+"``COM:``" avoids all these problems.
+
+A few important usage notes:
+
+* "``COM:``" within another directive's pattern does *not* comment out the
+  remainder of the pattern. For example:
+
+  .. code-block:: llvm
+
+     ; X32: pinsrd $1, 4(%esp), %xmm0 COM: This is part of the X32 pattern!
+
+  If you need to temporarily comment out part of a directive's pattern, move it
+  to another line. The reason is that FileCheck parses "``COM:``" in the same
+  manner as any other directive: only the first directive on the line is
+  recognized as a directive.
+
+* For the sake of LIT, FileCheck treats "``RUN:``" just like "``COM:``". If this
+  is not suitable for your test environment, see :option:`--comment-prefixes`.
+
+* FileCheck does not recognize "``COM``", "``RUN``", or any user-defined comment
+  prefix as a comment directive if it's combined with one of the usual check
+  directive suffixes, such as "``-NEXT:``" or "``-NOT:``", discussed below.
+  FileCheck treats such a combination as plain text instead. If it needs to act
+  as a comment directive for your test environment, define it as such with
+  :option:`--comment-prefixes`.
+
 The "CHECK-NEXT:" directive
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/llvm/include/llvm/Support/FileCheck.h b/llvm/include/llvm/Support/FileCheck.h
index 0d1f92b..2f0e641 100644
--- a/llvm/include/llvm/Support/FileCheck.h
+++ b/llvm/include/llvm/Support/FileCheck.h
@@ -25,6 +25,7 @@ namespace llvm {
 /// Contains info about various FileCheck options.
 struct FileCheckRequest {
   std::vector<StringRef> CheckPrefixes;
+  std::vector<StringRef> CommentPrefixes;
   bool NoCanonicalizeWhiteSpace = false;
   std::vector<StringRef> ImplicitCheckNot;
   std::vector<StringRef> GlobalDefines;
@@ -53,6 +54,7 @@ enum FileCheckKind {
   CheckDAG,
   CheckLabel,
   CheckEmpty,
+  CheckComment,
 
   /// Indicates the pattern only matches the end of file. This is used for
   /// trailing CHECK-NOTs.
diff --git a/llvm/lib/Support/FileCheck.cpp b/llvm/lib/Support/FileCheck.cpp
index 369b6df..9cdb14c 100644
--- a/llvm/lib/Support/FileCheck.cpp
+++ b/llvm/lib/Support/FileCheck.cpp
@@ -1110,6 +1110,8 @@ std::string Check::FileCheckType::getDescription(StringRef Prefix) const {
     return Prefix.str() + "-LABEL";
   case Check::CheckEmpty:
     return Prefix.str() + "-EMPTY";
+  case Check::CheckComment:
+    return std::string(Prefix);
   case Check::CheckEOF:
     return "implicit EOF";
   case Check::CheckBadNot:
@@ -1121,13 +1123,24 @@ std::string Check::FileCheckType::getDescription(StringRef Prefix) const {
 }
 
 static std::pair<Check::FileCheckType, StringRef>
-FindCheckType(StringRef Buffer, StringRef Prefix) {
+FindCheckType(const FileCheckRequest &Req, StringRef Buffer, StringRef Prefix) {
   if (Buffer.size() <= Prefix.size())
     return {Check::CheckNone, StringRef()};
 
   char NextChar = Buffer[Prefix.size()];
 
   StringRef Rest = Buffer.drop_front(Prefix.size() + 1);
+
+  // Check for comment.
+  if (Req.CommentPrefixes.end() != std::find(Req.CommentPrefixes.begin(),
+                                             Req.CommentPrefixes.end(),
+                                             Prefix)) {
+    if (NextChar == ':')
+      return {Check::CheckComment, Rest};
+    // Ignore a comment prefix if it has a suffix like "-NOT".
+    return {Check::CheckNone, StringRef()};
+  }
+
   // Verify that the : is present after the prefix.
   if (NextChar == ':')
     return {Check::CheckPlain, Rest};
@@ -1206,8 +1219,9 @@ static size_t SkipWord(StringRef Str, size_t Loc) {
 /// If no valid prefix is found, the state of Buffer, LineNumber, and CheckTy
 /// is unspecified.
 static std::pair<StringRef, StringRef>
-FindFirstMatchingPrefix(Regex &PrefixRE, StringRef &Buffer,
-                        unsigned &LineNumber, Check::FileCheckType &CheckTy) {
+FindFirstMatchingPrefix(const FileCheckRequest &Req, Regex &PrefixRE,
+                        StringRef &Buffer, unsigned &LineNumber,
+                        Check::FileCheckType &CheckTy) {
   SmallVector<StringRef, 2> Matches;
 
   while (!Buffer.empty()) {
@@ -1235,7 +1249,7 @@ FindFirstMatchingPrefix(Regex &PrefixRE, StringRef &Buffer,
     if (Skipped.empty() || !IsPartOfWord(Skipped.back())) {
       // Now extract the type.
       StringRef AfterSuffix;
-      std::tie(CheckTy, AfterSuffix) = FindCheckType(Buffer, Prefix);
+      std::tie(CheckTy, AfterSuffix) = FindCheckType(Req, Buffer, Prefix);
 
       // If we've found a valid check type for this prefix, we're done.
       if (CheckTy != Check::CheckNone)
@@ -1316,7 +1330,7 @@ bool FileCheck::readCheckFile(
   // found.
   unsigned LineNumber = 1;
 
-  bool FoundUsedPrefix = false;
+  bool FoundUsedCheckPrefix = false;
   while (1) {
     Check::FileCheckType CheckTy;
 
@@ -1324,10 +1338,11 @@ bool FileCheck::readCheckFile(
     StringRef UsedPrefix;
     StringRef AfterSuffix;
     std::tie(UsedPrefix, AfterSuffix) =
-        FindFirstMatchingPrefix(PrefixRE, Buffer, LineNumber, CheckTy);
+        FindFirstMatchingPrefix(Req, PrefixRE, Buffer, LineNumber, CheckTy);
     if (UsedPrefix.empty())
       break;
-    FoundUsedPrefix = true;
+    if (CheckTy != Check::CheckComment)
+      FoundUsedCheckPrefix = true;
 
     assert(UsedPrefix.data() == Buffer.data() &&
            "Failed to move Buffer's start forward, or pointed prefix outside "
@@ -1370,9 +1385,17 @@ bool FileCheck::readCheckFile(
     // Remember the location of the start of the pattern, for diagnostics.
     SMLoc PatternLoc = SMLoc::getFromPointer(Buffer.data());
 
+    // Extract the pattern from the buffer.
+    StringRef PatternBuffer = Buffer.substr(0, EOL);
+    Buffer = Buffer.substr(EOL);
+
+    // If this is a comment, we're done.
+    if (CheckTy == Check::CheckComment)
+      continue;
+
     // Parse the pattern.
     Pattern P(CheckTy, PatternContext.get(), LineNumber);
-    if (P.parsePattern(Buffer.substr(0, EOL), UsedPrefix, SM, Req))
+    if (P.parsePattern(PatternBuffer, UsedPrefix, SM, Req))
       return true;
 
     // Verify that CHECK-LABEL lines do not define or use variables
@@ -1384,8 +1407,6 @@ bool FileCheck::readCheckFile(
       return true;
     }
 
-    Buffer = Buffer.substr(EOL);
-
     // Verify that CHECK-NEXT/SAME/EMPTY lines have at least one CHECK line before them.
     if ((CheckTy == Check::CheckNext || CheckTy == Check::CheckSame ||
          CheckTy == Check::CheckEmpty) &&
@@ -1414,7 +1435,7 @@ bool FileCheck::readCheckFile(
 
   // When there are no used prefixes we report an error except in the case that
   // no prefix is specified explicitly but -implicit-check-not is specified.
-  if (!FoundUsedPrefix &&
+  if (!FoundUsedCheckPrefix &&
       (ImplicitNegativeChecks.empty() || !Req.IsDefaultCheckPrefix)) {
     errs() << "error: no check strings found with prefix"
            << (Req.CheckPrefixes.size() > 1 ? "es " : " ");
@@ -1874,50 +1895,76 @@ size_t FileCheckString::CheckDag(const SourceMgr &SM, StringRef Buffer,
   return StartPos;
 }
 
-static bool ValidatePrefixes(StringSet<> &UniquePrefixes,
+static bool ValidatePrefixes(StringRef Kind, StringSet<> &UniquePrefixes,
                              ArrayRef<StringRef> SuppliedPrefixes) {
   for (StringRef Prefix : SuppliedPrefixes) {
     if (Prefix.empty()) {
-      errs() << "error: supplied check prefix must not be the empty string\n";
+      errs() << "error: supplied " << Kind << " prefix must not be the empty "
+             << "string\n";
       return false;
     }
     static const Regex Validator("^[a-zA-Z0-9_-]*$");
     if (!Validator.match(Prefix)) {
-      errs() << "error: supplied check prefix must start with a letter and "
-             << "contain only alphanumeric characters, hyphens, and "
+      errs() << "error: supplied " << Kind << " prefix must start with a "
+             << "letter and contain only alphanumeric characters, hyphens, and "
              << "underscores: '" << Prefix << "'\n";
       return false;
     }
     if (!UniquePrefixes.insert(Prefix).second) {
-      errs() << "error: supplied check prefix must be unique among check "
-             << "prefixes: '" << Prefix << "'\n";
+      errs() << "error: supplied " << Kind << " prefix must be unique among "
+             << "check and comment prefixes: '" << Prefix << "'\n";
       return false;
     }
   }
   return true;
 }
 
+static const char *DefaultCheckPrefixes[] = {"CHECK"};
+static const char *DefaultCommentPrefixes[] = {"COM", "RUN"};
+
 bool FileCheck::ValidateCheckPrefixes() {
   StringSet<> UniquePrefixes;
-  if (!ValidatePrefixes(UniquePrefixes, Req.CheckPrefixes))
+  // Add default prefixes to catch user-supplied duplicates of them below.
+  if (Req.CheckPrefixes.empty()) {
+    for (const char *Prefix : DefaultCheckPrefixes)
+      UniquePrefixes.insert(Prefix);
+  }
+  if (Req.CommentPrefixes.empty()) {
+    for (const char *Prefix : DefaultCommentPrefixes)
+      UniquePrefixes.insert(Prefix);
+  }
+  // Do not validate the default prefixes, or diagnostics about duplicates might
+  // incorrectly indicate that they were supplied by the user.
+  if (!ValidatePrefixes("check", UniquePrefixes, Req.CheckPrefixes))
+    return false;
+  if (!ValidatePrefixes("comment", UniquePrefixes, Req.CommentPrefixes))
     return false;
   return true;
 }
 
 Regex FileCheck::buildCheckPrefixRegex() {
   if (Req.CheckPrefixes.empty()) {
-    Req.CheckPrefixes.push_back("CHECK");
+    for (const char *Prefix : DefaultCheckPrefixes)
+      Req.CheckPrefixes.push_back(Prefix);
     Req.IsDefaultCheckPrefix = true;
   }
+  if (Req.CommentPrefixes.empty()) {
+    for (const char *Prefix : DefaultCommentPrefixes)
+      Req.CommentPrefixes.push_back(Prefix);
+  }
 
-  // We already validated the contents of CheckPrefixes so just concatenate
-  // them as alternatives.
+  // We already validated the contents of CheckPrefixes and CommentPrefixes so
+  // just concatenate them as alternatives.
   SmallString<32> PrefixRegexStr;
   for (size_t I = 0, E = Req.CheckPrefixes.size(); I != E; ++I) {
     if (I != 0)
       PrefixRegexStr.push_back('|');
     PrefixRegexStr.append(Req.CheckPrefixes[I]);
   }
+  for (StringRef Prefix : Req.CommentPrefixes) {
+    PrefixRegexStr.push_back('|');
+    PrefixRegexStr.append(Prefix);
+  }
 
   return Regex(PrefixRegexStr);
 }
diff --git a/llvm/test/Assembler/drop-debug-info-nonzero-alloca.ll b/llvm/test/Assembler/drop-debug-info-nonzero-alloca.ll
index 739edba..3e2aefb 100644
--- a/llvm/test/Assembler/drop-debug-info-nonzero-alloca.ll
+++ b/llvm/test/Assembler/drop-debug-info-nonzero-alloca.ll
@@ -1,7 +1,7 @@
-; RUN: llvm-as < %s -o %t.bc -data-layout=A5 2>&1 | FileCheck -check-prefixes=COM,AS %s
-; RUN: llvm-dis < %t.bc | FileCheck -check-prefixes=COM,DIS %s
-; RUN: opt < %s -S -data-layout=A5 2>&1 | FileCheck -check-prefixes=COM,AS %s
-; RUN: opt < %t.bc -S | FileCheck -check-prefixes=COM,DIS %s
+; RUN: llvm-as < %s -o %t.bc -data-layout=A5 2>&1 | FileCheck -check-prefixes=ALL,AS %s
+; RUN: llvm-dis < %t.bc | FileCheck -check-prefixes=ALL,DIS %s
+; RUN: opt < %s -S -data-layout=A5 2>&1 | FileCheck -check-prefixes=ALL,AS %s
+; RUN: opt < %t.bc -S | FileCheck -check-prefixes=ALL,DIS %s
 
 define void @foo() {
 entry:
@@ -12,7 +12,7 @@ entry:
       metadata i8* undef,
       metadata !DILocalVariable(scope: !1),
       metadata !DIExpression())
-; COM-NOT: Allocation instruction pointer not in the stack address space!
+; ALL-NOT: Allocation instruction pointer not in the stack address space!
 ; AS: llvm.dbg.value intrinsic requires a !dbg attachment
 ; AS: warning: ignoring invalid debug info in <stdin>
 ret void
diff --git a/llvm/test/FileCheck/comment/after-words.txt b/llvm/test/FileCheck/comment/after-words.txt
new file mode 100644
index 0000000..46eeb65
--- /dev/null
+++ b/llvm/test/FileCheck/comment/after-words.txt
@@ -0,0 +1,16 @@
+# Comment prefixes are not recognized at ends of words.
+
+RUN: echo 'foo'                 >  %t.in
+RUN: echo 'bar'                 >> %t.in
+RUN: echo 'foo'                 >> %t.in
+RUN: echo 'bar'                 >> %t.in
+RUN: echo 'FOO-COM: CHECK: foo' >  %t.chk
+RUN: echo 'RUN_COM: CHECK: bar' >> %t.chk
+RUN: echo 'RUN3COM: CHECK: foo' >> %t.chk
+RUN: echo ' COMRUN: CHECK: bar' >> %t.chk
+RUN: %ProtectFileCheckOutput FileCheck -vv %t.chk < %t.in 2>&1 | FileCheck %s
+
+CHECK: .chk:1:17: remark: CHECK: expected string found in input
+CHECK: .chk:2:17: remark: CHECK: expected string found in input
+CHECK: .chk:3:17: remark: CHECK: expected string found in input
+CHECK: .chk:4:17: remark: CHECK: expected string found in input
diff --git a/llvm/test/FileCheck/comment/bad-comment-prefix.txt b/llvm/test/FileCheck/comment/bad-comment-prefix.txt
new file mode 100644
index 0000000..58a8873
--- /dev/null
+++ b/llvm/test/FileCheck/comment/bad-comment-prefix.txt
@@ -0,0 +1,48 @@
+# Bad comment prefixes are diagnosed.
+
+# Check empty comment prefix.
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                      -comment-prefixes= | \
+RUN:   FileCheck -check-prefix=PREFIX-EMPTY %s
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                      -comment-prefixes=,FOO | \
+RUN:   FileCheck -check-prefix=PREFIX-EMPTY %s
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                      -comment-prefixes=FOO, | \
+RUN:   FileCheck -check-prefix=PREFIX-EMPTY %s
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                      -comment-prefixes=FOO,,BAR | \
+RUN:   FileCheck -check-prefix=PREFIX-EMPTY %s
+PREFIX-EMPTY: error: supplied comment prefix must not be the empty string
+
+# Check invalid characters in comment prefix.
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                       -comment-prefixes=. | \
+RUN:   FileCheck -check-prefix=PREFIX-BAD-CHAR1 %s
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                       -comment-prefixes='foo ' | \
+RUN:   FileCheck -check-prefix=PREFIX-BAD-CHAR2 %s
+PREFIX-BAD-CHAR1: error: supplied comment prefix must start with a letter and contain only alphanumeric characters, hyphens, and underscores: '.'
+PREFIX-BAD-CHAR2: error: supplied comment prefix must start with a letter and contain only alphanumeric characters, hyphens, and underscores: 'foo '
+
+# Check duplicate comment prefixes.
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                      -comment-prefixes=FOO,BAR,BAZ,BAR | \
+RUN:   FileCheck -check-prefix=COMMENT-PREFIX-DUP %s
+COMMENT-PREFIX-DUP: error: supplied comment prefix must be unique among check and comment prefixes: 'BAR'
+
+# Check user-supplied check prefix that duplicates a default comment prefix.
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                      -check-prefixes=FOO,COM | \
+RUN:   FileCheck -check-prefix=CHECK-PREFIX-DUP-COM %s
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                      -check-prefixes=RUN,FOO | \
+RUN:   FileCheck -check-prefix=CHECK-PREFIX-DUP-RUN_ %s
+CHECK-PREFIX-DUP-COM: error: supplied check prefix must be unique among check and comment prefixes: 'COM'
+CHECK-PREFIX-DUP-RUN_: error: supplied check prefix must be unique among check and comment prefixes: 'RUN'
+
+# Check user-supplied comment prefix that duplicates default check prefixes.
+RUN: %ProtectFileCheckOutput not FileCheck /dev/null < /dev/null 2>&1 \
+RUN:                                      -comment-prefixes=CHECK | \
+RUN:   FileCheck -check-prefix=COMMENT-PREFIX-DUP-CHECK %s
+COMMENT-PREFIX-DUP-CHECK: error: supplied comment prefix must be unique among check and comment prefixes: 'CHECK'
diff --git a/llvm/test/FileCheck/comment/blank-comments.txt b/llvm/test/FileCheck/comment/blank-comments.txt
new file mode 100644
index 0000000..b035ddd
--- /dev/null
+++ b/llvm/test/FileCheck/comment/blank-comments.txt
@@ -0,0 +1,9 @@
+# Comment directives with blank contents are fine.
+
+RUN: echo 'foo'        >  %t.in
+RUN: echo 'COM:'       >  %t.chk
+RUN: echo 'CHECK: foo' >> %t.chk
+RUN: echo ' COM: '     >> %t.chk
+RUN: %ProtectFileCheckOutput FileCheck -vv %t.chk < %t.in 2>&1 | FileCheck %s
+
+CHECK: .chk:2:8: remark: CHECK: expected string found in input
diff --git a/llvm/test/FileCheck/comment/suffixes.txt b/llvm/test/FileCheck/comment/suffixes.txt
new file mode 100644
index 0000000..47805b4
--- /dev/null
+++ b/llvm/test/FileCheck/comment/suffixes.txt
@@ -0,0 +1,22 @@
+# Comment prefixes plus check directive suffixes are not comment directives
+# and are treated as plain text.
+
+RUN: echo foo                    >  %t.in
+RUN: echo bar                    >> %t.in
+RUN: echo 'COM-NEXT: CHECK: foo' >  %t.chk
+RUN: echo 'RUN-NOT: CHECK: bar'  >> %t.chk
+
+RUN: %ProtectFileCheckOutput FileCheck -vv %t.chk < %t.in 2>&1 | \
+RUN:   FileCheck -check-prefix=CHECK1 %s
+
+CHECK1: .chk:1:18: remark: CHECK: expected string found in input
+CHECK1: .chk:2:17: remark: CHECK: expected string found in input
+
+# But we can define them as comment prefixes.
+
+RUN: %ProtectFileCheckOutput \
+RUN: FileCheck -vv -comment-prefixes=COM,RUN,RUN-NOT %t.chk < %t.in 2>&1 | \
+RUN:   FileCheck -check-prefix=CHECK2 %s
+
+CHECK2: .chk:1:18: remark: CHECK: expected string found in input
+CHECK2-NOT: .chk:2
diff --git a/llvm/test/FileCheck/comment/suppresses-checks.txt b/llvm/test/FileCheck/comment/suppresses-checks.txt
new file mode 100644
index 0000000..98f0181
--- /dev/null
+++ b/llvm/test/FileCheck/comment/suppresses-checks.txt
@@ -0,0 +1,33 @@
+# Comment directives successfully comment out check directives.
+
+# Check all default comment prefixes.
+# Check that a comment directive at the beginning/end of the file is handled.
+# Check that the preceding/following line's check directive is not affected.
+RUN: echo 'foo'                    >  %t-1.in
+RUN: echo 'COM: CHECK: bar'        >  %t-1.chk
+RUN: echo 'CHECK: foo'             >> %t-1.chk
+RUN: echo 'RUN: echo "CHECK: baz"' >> %t-1.chk
+RUN: %ProtectFileCheckOutput FileCheck -vv %t-1.chk < %t-1.in 2>&1 | \
+RUN:   FileCheck -DCHECK_LINE=2 %s
+
+# Check the case of one user-specified comment prefix.
+# Check that a comment directive not at the beginning of a line is handled.
+RUN: echo 'foo'                                      >  %t-2.in
+RUN: echo 'CHECK: foo'                               >  %t-2.chk
+RUN: echo 'letters then space MY-PREFIX: CHECK: bar' >> %t-2.chk
+RUN: %ProtectFileCheckOutput \
+RUN: FileCheck -vv %t-2.chk -comment-prefixes=MY-PREFIX < %t-2.in 2>&1 | \
+RUN:   FileCheck -DCHECK_LINE=1 %s
+
+# Check the case of multiple user-specified comment prefixes.
+RUN: echo 'foo'               >  %t-3.in
+RUN: echo 'Bar_2: CHECK: Bar' >  %t-3.chk
+RUN: echo 'CHECK: foo'        >> %t-3.chk
+RUN: echo 'Foo_1: CHECK: Foo' >> %t-3.chk
+RUN: echo 'Baz_3: CHECK: Baz' >> %t-3.chk
+RUN: %ProtectFileCheckOutput \
+RUN: FileCheck -vv %t-3.chk -comment-prefixes=Foo_1,Bar_2 \
+RUN:           -comment-prefixes=Baz_3 < %t-3.in 2>&1 | \
+RUN:   FileCheck -DCHECK_LINE=2 %s
+
+CHECK: .chk:[[CHECK_LINE]]:8: remark: CHECK: expected string found in input
diff --git a/llvm/test/FileCheck/comment/unused-check-prefixes.txt b/llvm/test/FileCheck/comment/unused-check-prefixes.txt
new file mode 100644
index 0000000..6c3f830
--- /dev/null
+++ b/llvm/test/FileCheck/comment/unused-check-prefixes.txt
@@ -0,0 +1,8 @@
+# Comment directives do not suppress diagnostics for unused check prefixes.
+
+RUN: echo 'foo'       >  %t.in
+RUN: echo 'COM: foo'  >  %t.chk
+RUN: echo 'RUN: echo' >> %t.chk
+RUN: %ProtectFileCheckOutput not FileCheck %t.chk < %t.in 2>&1 | FileCheck %s
+
+CHECK: error: no check strings found with prefix 'CHECK:'
diff --git a/llvm/test/FileCheck/comment/unused-comment-prefixes.txt b/llvm/test/FileCheck/comment/unused-comment-prefixes.txt
new file mode 100644
index 0000000..29212ec
--- /dev/null
+++ b/llvm/test/FileCheck/comment/unused-comment-prefixes.txt
@@ -0,0 +1,16 @@
+# Not using comment directives is always fine.
+
+RUN: echo 'foo'        > %t.in
+RUN: echo 'CHECK: foo' > %t.chk
+
+# Check the case of default comment prefixes.
+RUN: %ProtectFileCheckOutput \
+RUN: FileCheck -vv %t.chk < %t.in 2>&1 | FileCheck %s
+
+# Specifying non-default comment prefixes doesn't mean you have to use them.
+# For example, they might be applied to an entire test suite via
+# FILECHECK_OPTS or via a wrapper command or substitution.
+RUN: %ProtectFileCheckOutput \
+RUN: FileCheck -vv -comment-prefixes=FOO %t.chk < %t.in 2>&1 | FileCheck %s
+
+CHECK: .chk:1:8: remark: CHECK: expected string found in input
diff --git a/llvm/test/FileCheck/comment/within-checks.txt b/llvm/test/FileCheck/comment/within-checks.txt
new file mode 100644
index 0000000..b488edb
--- /dev/null
+++ b/llvm/test/FileCheck/comment/within-checks.txt
@@ -0,0 +1,8 @@
+# A comment directive is not recognized within a check directive's pattern and
+# thus does not comment out the remainder of the pattern.
+
+RUN: echo 'foo'                 > %t.in
+RUN: echo 'CHECK: foo COM: bar' > %t.chk
+RUN: %ProtectFileCheckOutput not FileCheck %t.chk < %t.in 2>&1 | FileCheck %s
+
+CHECK: .chk:1:8: error: CHECK: expected string not found in input
diff --git a/llvm/test/FileCheck/first-character-match.txt b/llvm/test/FileCheck/first-character-match.txt
index 4b09c21..3c8b153 100644
--- a/llvm/test/FileCheck/first-character-match.txt
+++ b/llvm/test/FileCheck/first-character-match.txt
@@ -1,2 +1,2 @@
-RUN: FileCheck -check-prefix=RUN -input-file %s %s
+RUN: FileCheck -check-prefix=RUN --comment-prefixes=COM -input-file %s %s
 // Prefix is at the first character in the file. The run line then matches itself.
diff --git a/llvm/test/FileCheck/validate-check-prefix.txt b/llvm/test/FileCheck/validate-check-prefix.txt
index 29f352c..af43afd 100644
--- a/llvm/test/FileCheck/validate-check-prefix.txt
+++ b/llvm/test/FileCheck/validate-check-prefix.txt
@@ -8,6 +8,6 @@ foobar
 
 ; BAD_PREFIX: supplied check prefix must start with a letter and contain only alphanumeric characters, hyphens, and underscores: 'A!'
 
-; DUPLICATE_PREFIX: error: supplied check prefix must be unique among check prefixes: 'REPEAT'
+; DUPLICATE_PREFIX: error: supplied check prefix must be unique among check and comment prefixes: 'REPEAT'
 
 ; EMPTY_PREFIX: error: supplied check prefix must not be the empty string
diff --git a/llvm/utils/FileCheck/FileCheck.cpp b/llvm/utils/FileCheck/FileCheck.cpp
index 46821ec..3ee7c5a 100644
--- a/llvm/utils/FileCheck/FileCheck.cpp
+++ b/llvm/utils/FileCheck/FileCheck.cpp
@@ -44,6 +44,14 @@ static cl::alias CheckPrefixesAlias(
     cl::desc(
         "Alias for -check-prefix permitting multiple comma separated values"));
 
+static cl::list<std::string> CommentPrefixes(
+    "comment-prefixes", cl::CommaSeparated, cl::Hidden,
+    cl::desc("Comma-separated list of comment prefixes to use from check file\n"
+             "(defaults to 'COM,RUN'). Please avoid using this feature in\n"
+             "LLVM's LIT-based test suites, which should be easier to\n"
+             "maintain if they all follow a consistent comment style. This\n"
+             "feature is meant for non-LIT test suites using FileCheck."));
+
 static cl::opt<bool> NoCanonicalizeWhiteSpace(
     "strict-whitespace",
     cl::desc("Do not treat all horizontal whitespace as equivalent"));
@@ -279,6 +287,8 @@ std::string GetCheckTypeAbbreviation(Check::FileCheckType Ty) {
     return "label";
   case Check::CheckEmpty:
     return "empty";
+  case Check::CheckComment:
+    return "com";
   case Check::CheckEOF:
     return "eof";
   case Check::CheckBadNot:
@@ -565,6 +575,9 @@ int main(int argc, char **argv) {
   for (StringRef Prefix : CheckPrefixes)
     Req.CheckPrefixes.push_back(Prefix);
 
+  for (StringRef Prefix : CommentPrefixes)
+    Req.CommentPrefixes.push_back(Prefix);
+
   for (StringRef CheckNot : ImplicitCheckNot)
     Req.ImplicitCheckNot.push_back(CheckNot);