llvm/docs/CommandGuide/FileCheck.rst

FileCheck - Flexible pattern matching file verifier
===================================================

.. program:: FileCheck

SYNOPSIS
--------

:program:`FileCheck` *match-filename* [*--check-prefix=XXX*] [*--strict-whitespace*]

DESCRIPTION
-----------

:program:`FileCheck` reads two files (one from standard input, and one
specified on the command line) and uses one to verify the other.  This
behavior is particularly useful for the testsuite, which wants to verify that
the output of some tool (e.g. :program:`llc`) contains the expected information
(for example, a movsd from esp or whatever is interesting).  This is similar to
using :program:`grep`, but it is optimized for matching multiple different
inputs in one file in a specific order.

The ``match-filename`` file specifies the file that contains the patterns to
match.  The file to verify is read from standard input unless the
:option:`--input-file` option is used.

OPTIONS
-------

Options are parsed from the environment variable ``FILECHECK_OPTS``
and from the command line.

.. option:: -help

 Print a summary of command line options.

.. option:: --check-prefix prefix

 FileCheck searches the contents of ``match-filename`` for patterns to
 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-prefixes prefix1,prefix2,...

 An alias of :option:`--check-prefix` that allows multiple prefixes to be
 specified as a comma separated list.

.. option:: --input-file filename

  File to check (defaults to stdin).

.. option:: --match-full-lines

 By default, FileCheck allows matches of anywhere on a line. This
 option will require all positive matches to cover an entire
 line. Leading and trailing whitespace is ignored, unless
 :option:`--strict-whitespace` is also specified. (Note: negative
 matches from ``CHECK-NOT`` are not affected by this option!)

 Passing this option is equivalent to inserting ``{{^ *}}`` or
 ``{{^}}`` before, and ``{{ *$}}`` or ``{{$}}`` after every positive
 check pattern.

.. option:: --strict-whitespace

 By default, FileCheck canonicalizes input horizontal whitespace (spaces and
 tabs) which causes it to ignore these differences (a space will match a tab).
 The :option:`--strict-whitespace` argument disables this behavior. End-of-line
 sequences are canonicalized to UNIX-style ``\n`` in all modes.

.. option:: --ignore-case

  By default, FileCheck uses case-sensitive matching. This option causes
  FileCheck to use case-insensitive matching.

.. option:: --implicit-check-not check-pattern

  Adds implicit negative checks for the specified patterns between positive
  checks. The option allows writing stricter tests without stuffing them with
  ``CHECK-NOT``\ s.

  For example, "``--implicit-check-not warning:``" can be useful when testing
  diagnostic messages from tools that don't have an option similar to ``clang
  -verify``. With this option FileCheck will verify that input does not contain
  warnings not covered by any ``CHECK:`` patterns.

.. option:: --dump-input <mode>

  Dump input to stderr, adding annotations representing currently enabled
  diagnostics.  Do this either 'always', on 'fail', or 'never'.  Specify 'help'
  to explain the dump format and quit.

.. option:: --dump-input-on-failure

  When the check fails, dump all of the original input.  This option is
  deprecated in favor of `--dump-input=fail`.

.. option:: --enable-var-scope

  Enables scope for regex variables.

  Variables with names that start with ``$`` are considered global and
  remain set throughout the file.

  All other variables get undefined after each encountered ``CHECK-LABEL``.

.. option:: -D<VAR=VALUE>

  Sets a filecheck pattern variable ``VAR`` with value ``VALUE`` that can be
  used in ``CHECK:`` lines.

.. option:: -D#<NUMVAR>=<NUMERIC EXPRESSION>

  Sets a filecheck numeric variable ``NUMVAR`` to the result of evaluating
  ``<NUMERIC EXPRESSION>`` that can be used in ``CHECK:`` lines. See section
  ``FileCheck Numeric Variables and Expressions`` for details on supported
  numeric expressions.

.. option:: -version

 Show the version number of this program.

.. option:: -v

  Print good directive pattern matches.  However, if ``-input-dump=fail`` or
  ``-input-dump=always``, add those matches as input annotations instead.

.. option:: -vv

  Print information helpful in diagnosing internal FileCheck issues, such as
  discarded overlapping ``CHECK-DAG:`` matches, implicit EOF pattern matches,
  and ``CHECK-NOT:`` patterns that do not have matches.  Implies ``-v``.
  However, if ``-input-dump=fail`` or ``-input-dump=always``, just add that
  information as input annotations instead.

.. option:: --allow-deprecated-dag-overlap

  Enable overlapping among matches in a group of consecutive ``CHECK-DAG:``
  directives.  This option is deprecated and is only provided for convenience
  as old tests are migrated to the new non-overlapping ``CHECK-DAG:``
  implementation.

.. option:: --color

  Use colors in output (autodetected by default).

EXIT STATUS
-----------

If :program:`FileCheck` verifies that the file matches the expected contents,
it exits with 0.  Otherwise, if not, or if an error occurs, it will exit with a
non-zero value.

TUTORIAL
--------

FileCheck is typically used from LLVM regression tests, being invoked on the RUN
line of the test.  A simple example of using FileCheck from a RUN line looks
like this:

.. code-block:: llvm

   ; RUN: llvm-as < %s | llc -march=x86-64 | FileCheck %s

This syntax says to pipe the current file ("``%s``") into ``llvm-as``, pipe
that into ``llc``, then pipe the output of ``llc`` into ``FileCheck``.  This
means that FileCheck will be verifying its standard input (the llc output)
against the filename argument specified (the original ``.ll`` file specified by
"``%s``").  To see how this works, let's look at the rest of the ``.ll`` file
(after the RUN line):

.. code-block:: llvm

   define void @sub1(i32* %p, i32 %v) {
   entry:
   ; CHECK: sub1:
   ; CHECK: subl
           %0 = tail call i32 @llvm.atomic.load.sub.i32.p0i32(i32* %p, i32 %v)
           ret void
   }

   define void @inc4(i64* %p) {
   entry:
   ; CHECK: inc4:
   ; CHECK: incq
           %0 = tail call i64 @llvm.atomic.load.add.i64.p0i64(i64* %p, i64 1)
           ret void
   }

Here you can see some "``CHECK:``" lines specified in comments.  Now you can
see how the file is piped into ``llvm-as``, then ``llc``, and the machine code
output is what we are verifying.  FileCheck checks the machine code output to
verify that it matches what the "``CHECK:``" lines specify.

The syntax of the "``CHECK:``" lines is very simple: they are fixed strings that
must occur in order.  FileCheck defaults to ignoring horizontal whitespace
differences (e.g. a space is allowed to match a tab) but otherwise, the contents
of the "``CHECK:``" line is required to match some thing in the test file exactly.

One nice thing about FileCheck (compared to grep) is that it allows merging
test cases together into logical groups.  For example, because the test above
is checking for the "``sub1:``" and "``inc4:``" labels, it will not match
unless there is a "``subl``" in between those labels.  If it existed somewhere
else in the file, that would not count: "``grep subl``" matches if "``subl``"
exists anywhere in the file.

The FileCheck -check-prefix option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The FileCheck `-check-prefix` option allows multiple test
configurations to be driven from one `.ll` file.  This is useful in many
circumstances, for example, testing different architectural variants with
:program:`llc`.  Here's a simple example:

.. code-block:: llvm

   ; RUN: llvm-as < %s | llc -mtriple=i686-apple-darwin9 -mattr=sse41 \
   ; RUN:              | FileCheck %s -check-prefix=X32
   ; RUN: llvm-as < %s | llc -mtriple=x86_64-apple-darwin9 -mattr=sse41 \
   ; RUN:              | FileCheck %s -check-prefix=X64

   define <4 x i32> @pinsrd_1(i32 %s, <4 x i32> %tmp) nounwind {
           %tmp1 = insertelement <4 x i32>; %tmp, i32 %s, i32 1
           ret <4 x i32> %tmp1
   ; X32: pinsrd_1:
   ; X32:    pinsrd $1, 4(%esp), %xmm0

   ; X64: pinsrd_1:
   ; X64:    pinsrd $1, %edi, %xmm0
   }

In this case, we're testing that we get the expected code generation with
both 32-bit and 64-bit code generation.

The "CHECK-NEXT:" directive
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes you want to match lines and would like to verify that matches
happen on exactly consecutive lines with no other lines in between them.  In
this case, you can use "``CHECK:``" and "``CHECK-NEXT:``" directives to specify
this.  If you specified a custom check prefix, just use "``<PREFIX>-NEXT:``".
For example, something like this works as you'd expect:

.. code-block:: llvm

   define void @t2(<2 x double>* %r, <2 x double>* %A, double %B) {
 	%tmp3 = load <2 x double>* %A, align 16
 	%tmp7 = insertelement <2 x double> undef, double %B, i32 0
 	%tmp9 = shufflevector <2 x double> %tmp3,
                               <2 x double> %tmp7,
                               <2 x i32> < i32 0, i32 2 >
 	store <2 x double> %tmp9, <2 x double>* %r, align 16
 	ret void

   ; CHECK:          t2:
   ; CHECK: 	        movl	8(%esp), %eax
   ; CHECK-NEXT: 	movapd	(%eax), %xmm0
   ; CHECK-NEXT: 	movhpd	12(%esp), %xmm0
   ; CHECK-NEXT: 	movl	4(%esp), %eax
   ; CHECK-NEXT: 	movapd	%xmm0, (%eax)
   ; CHECK-NEXT: 	ret
   }

"``CHECK-NEXT:``" directives reject the input unless there is exactly one
newline between it and the previous directive.  A "``CHECK-NEXT:``" cannot be
the first directive in a file.

The "CHECK-SAME:" directive
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes you want to match lines and would like to verify that matches happen
on the same line as the previous match.  In this case, you can use "``CHECK:``"
and "``CHECK-SAME:``" directives to specify this.  If you specified a custom
check prefix, just use "``<PREFIX>-SAME:``".

"``CHECK-SAME:``" is particularly powerful in conjunction with "``CHECK-NOT:``"
(described below).

For example, the following works like you'd expect:

.. code-block:: llvm

   !0 = !DILocation(line: 5, scope: !1, inlinedAt: !2)

   ; CHECK:       !DILocation(line: 5,
   ; CHECK-NOT:               column:
   ; CHECK-SAME:              scope: ![[SCOPE:[0-9]+]]

"``CHECK-SAME:``" directives reject the input if there are any newlines between
it and the previous directive.  A "``CHECK-SAME:``" cannot be the first
directive in a file.

The "CHECK-EMPTY:" directive
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you need to check that the next line has nothing on it, not even whitespace,
you can use the "``CHECK-EMPTY:``" directive.

.. code-block:: llvm

   declare void @foo()

   declare void @bar()
   ; CHECK: foo
   ; CHECK-EMPTY:
   ; CHECK-NEXT: bar

Just like "``CHECK-NEXT:``" the directive will fail if there is more than one
newline before it finds the next blank line, and it cannot be the first
directive in a file.

The "CHECK-NOT:" directive
~~~~~~~~~~~~~~~~~~~~~~~~~~

The "``CHECK-NOT:``" directive is used to verify that a string doesn't occur
between two matches (or before the first match, or after the last match).  For
example, to verify that a load is removed by a transformation, a test like this
can be used:

.. code-block:: llvm

   define i8 @coerce_offset0(i32 %V, i32* %P) {
     store i32 %V, i32* %P

     %P2 = bitcast i32* %P to i8*
     %P3 = getelementptr i8* %P2, i32 2

     %A = load i8* %P3
     ret i8 %A
   ; CHECK: @coerce_offset0
   ; CHECK-NOT: load
   ; CHECK: ret i8
   }

The "CHECK-COUNT:" directive
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you need to match multiple lines with the same pattern over and over again
you can repeat a plain ``CHECK:`` as many times as needed. If that looks too
boring you can instead use a counted check "``CHECK-COUNT-<num>:``", where
``<num>`` is a positive decimal number. It will match the pattern exactly
``<num>`` times, no more and no less. If you specified a custom check prefix,
just use "``<PREFIX>-COUNT-<num>:``" for the same effect.
Here is a simple example:

.. code-block:: text

   Loop at depth 1
   Loop at depth 1
   Loop at depth 1
   Loop at depth 1
     Loop at depth 2
       Loop at depth 3

   ; CHECK-COUNT-6: Loop at depth {{[0-9]+}}
   ; CHECK-NOT:     Loop at depth {{[0-9]+}}

The "CHECK-DAG:" directive
~~~~~~~~~~~~~~~~~~~~~~~~~~

If it's necessary to match strings that don't occur in a strictly sequential
order, "``CHECK-DAG:``" could be used to verify them between two matches (or
before the first match, or after the last match). For example, clang emits
vtable globals in reverse order. Using ``CHECK-DAG:``, we can keep the checks
in the natural order:

.. code-block:: c++

    // RUN: %clang_cc1 %s -emit-llvm -o - | FileCheck %s

    struct Foo { virtual void method(); };
    Foo f;  // emit vtable
    // CHECK-DAG: @_ZTV3Foo =

    struct Bar { virtual void method(); };
    Bar b;
    // CHECK-DAG: @_ZTV3Bar =

``CHECK-NOT:`` directives could be mixed with ``CHECK-DAG:`` directives to
exclude strings between the surrounding ``CHECK-DAG:`` directives. As a result,
the surrounding ``CHECK-DAG:`` directives cannot be reordered, i.e. all
occurrences matching ``CHECK-DAG:`` before ``CHECK-NOT:`` must not fall behind
occurrences matching ``CHECK-DAG:`` after ``CHECK-NOT:``. For example,

.. code-block:: llvm

   ; CHECK-DAG: BEFORE
   ; CHECK-NOT: NOT
   ; CHECK-DAG: AFTER

This case will reject input strings where ``BEFORE`` occurs after ``AFTER``.

With captured variables, ``CHECK-DAG:`` is able to match valid topological
orderings of a DAG with edges from the definition of a variable to its use.
It's useful, e.g., when your test cases need to match different output
sequences from the instruction scheduler. For example,

.. code-block:: llvm

   ; CHECK-DAG: add [[REG1:r[0-9]+]], r1, r2
   ; CHECK-DAG: add [[REG2:r[0-9]+]], r3, r4
   ; CHECK:     mul r5, [[REG1]], [[REG2]]

In this case, any order of that two ``add`` instructions will be allowed.

If you are defining `and` using variables in the same ``CHECK-DAG:`` block,
be aware that the definition rule can match `after` its use.

So, for instance, the code below will pass:

.. code-block:: text

  ; CHECK-DAG: vmov.32 [[REG2:d[0-9]+]][0]
  ; CHECK-DAG: vmov.32 [[REG2]][1]
  vmov.32 d0[1]
  vmov.32 d0[0]

While this other code, will not:

.. code-block:: text

  ; CHECK-DAG: vmov.32 [[REG2:d[0-9]+]][0]
  ; CHECK-DAG: vmov.32 [[REG2]][1]
  vmov.32 d1[1]
  vmov.32 d0[0]

While this can be very useful, it's also dangerous, because in the case of
register sequence, you must have a strong order (read before write, copy before
use, etc). If the definition your test is looking for doesn't match (because
of a bug in the compiler), it may match further away from the use, and mask
real bugs away.

In those cases, to enforce the order, use a non-DAG directive between DAG-blocks.

A ``CHECK-DAG:`` directive skips matches that overlap the matches of any
preceding ``CHECK-DAG:`` directives in the same ``CHECK-DAG:`` block.  Not only
is this non-overlapping behavior consistent with other directives, but it's
also necessary to handle sets of non-unique strings or patterns.  For example,
the following directives look for unordered log entries for two tasks in a
parallel program, such as the OpenMP runtime:

.. code-block:: text

    // CHECK-DAG: [[THREAD_ID:[0-9]+]]: task_begin
    // CHECK-DAG: [[THREAD_ID]]: task_end
    //
    // CHECK-DAG: [[THREAD_ID:[0-9]+]]: task_begin
    // CHECK-DAG: [[THREAD_ID]]: task_end

The second pair of directives is guaranteed not to match the same log entries
as the first pair even though the patterns are identical and even if the text
of the log entries is identical because the thread ID manages to be reused.

The "CHECK-LABEL:" directive
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes in a file containing multiple tests divided into logical blocks, one
or more ``CHECK:`` directives may inadvertently succeed by matching lines in a
later block. While an error will usually eventually be generated, the check
flagged as causing the error may not actually bear any relationship to the
actual source of the problem.

In order to produce better error messages in these cases, the "``CHECK-LABEL:``"
directive can be used. It is treated identically to a normal ``CHECK``
directive except that FileCheck makes an additional assumption that a line
matched by the directive cannot also be matched by any other check present in
``match-filename``; this is intended to be used for lines containing labels or
other unique identifiers. Conceptually, the presence of ``CHECK-LABEL`` divides
the input stream into separate blocks, each of which is processed independently,
preventing a ``CHECK:`` directive in one block matching a line in another block.
If ``--enable-var-scope`` is in effect, all local variables are cleared at the
beginning of the block.

For example,

.. code-block:: llvm

  define %struct.C* @C_ctor_base(%struct.C* %this, i32 %x) {
  entry:
  ; CHECK-LABEL: C_ctor_base:
  ; CHECK: mov [[SAVETHIS:r[0-9]+]], r0
  ; CHECK: bl A_ctor_base
  ; CHECK: mov r0, [[SAVETHIS]]
    %0 = bitcast %struct.C* %this to %struct.A*
    %call = tail call %struct.A* @A_ctor_base(%struct.A* %0)
    %1 = bitcast %struct.C* %this to %struct.B*
    %call2 = tail call %struct.B* @B_ctor_base(%struct.B* %1, i32 %x)
    ret %struct.C* %this
  }

  define %struct.D* @D_ctor_base(%struct.D* %this, i32 %x) {
  entry:
  ; CHECK-LABEL: D_ctor_base:

The use of ``CHECK-LABEL:`` directives in this case ensures that the three
``CHECK:`` directives only accept lines corresponding to the body of the
``@C_ctor_base`` function, even if the patterns match lines found later in
the file. Furthermore, if one of these three ``CHECK:`` directives fail,
FileCheck will recover by continuing to the next block, allowing multiple test
failures to be detected in a single invocation.

There is no requirement that ``CHECK-LABEL:`` directives contain strings that
correspond to actual syntactic labels in a source or output language: they must
simply uniquely match a single line in the file being verified.

``CHECK-LABEL:`` directives cannot contain variable definitions or uses.

FileCheck Regex Matching Syntax
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

All FileCheck directives take a pattern to match.
For most uses of FileCheck, fixed string matching is perfectly sufficient.  For
some things, a more flexible form of matching is desired.  To support this,
FileCheck allows you to specify regular expressions in matching strings,
surrounded by double braces: ``{{yourregex}}``. FileCheck implements a POSIX
regular expression matcher; it supports Extended POSIX regular expressions
(ERE). Because we want to use fixed string matching for a majority of what we
do, FileCheck has been designed to support mixing and matching fixed string
matching with regular expressions.  This allows you to write things like this:

.. code-block:: llvm

   ; CHECK: movhpd	{{[0-9]+}}(%esp), {{%xmm[0-7]}}

In this case, any offset from the ESP register will be allowed, and any xmm
register will be allowed.

Because regular expressions are enclosed with double braces, they are
visually distinct, and you don't need to use escape characters within the double
braces like you would in C.  In the rare case that you want to match double
braces explicitly from the input, you can use something ugly like
``{{[}][}]}}`` as your pattern.  Or if you are using the repetition count
syntax, for example ``[[:xdigit:]]{8}`` to match exactly 8 hex digits, you
would need to add parentheses like this ``{{([[:xdigit:]]{8})}}`` to avoid
confusion with FileCheck's closing double-brace.

FileCheck String Substitution Blocks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is often useful to match a pattern and then verify that it occurs again
later in the file.  For codegen tests, this can be useful to allow any
register, but verify that that register is used consistently later.  To do
this, :program:`FileCheck` supports string substitution blocks that allow
string variables to be defined and substituted into patterns.  Here is a simple
example:

.. code-block:: llvm

   ; CHECK: test5:
   ; CHECK:    notw	[[REGISTER:%[a-z]+]]
   ; CHECK:    andw	{{.*}}[[REGISTER]]

The first check line matches a regex ``%[a-z]+`` and captures it into the
string variable ``REGISTER``.  The second line verifies that whatever is in
``REGISTER`` occurs later in the file after an "``andw``". :program:`FileCheck`
string substitution blocks are always contained in ``[[ ]]`` pairs, and string
variable names can be formed with the regex ``[a-zA-Z_][a-zA-Z0-9_]*``.  If a
colon follows the name, then it is a definition of the variable; otherwise, it
is a substitution.

:program:`FileCheck` variables can be defined multiple times, and substitutions
always get the latest value.  Variables can also be substituted later on the
same line they were defined on. For example:

.. code-block:: llvm

    ; CHECK: op [[REG:r[0-9]+]], [[REG]]

Can be useful if you want the operands of ``op`` to be the same register,
and don't care exactly which register it is.

If ``--enable-var-scope`` is in effect, variables with names that
start with ``$`` are considered to be global. All others variables are
local.  All local variables get undefined at the beginning of each
CHECK-LABEL block. Global variables are not affected by CHECK-LABEL.
This makes it easier to ensure that individual tests are not affected
by variables set in preceding tests.

FileCheck Numeric Substitution Blocks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

:program:`FileCheck` also supports numeric substitution blocks that allow
defining numeric variables and checking for numeric values that satisfy a
numeric expression constraint based on those variables via a numeric
substitution. This allows ``CHECK:`` directives to verify a numeric relation
between two numbers, such as the need for consecutive registers to be used.

The syntax to define a numeric variable is ``[[#<NUMVAR>:]]`` where
``<NUMVAR>`` is the name of the numeric variable to define to the matching
value.

For example:

.. code-block:: llvm

    ; CHECK: mov r[[#REG:]], 42

would match ``mov r5, 42`` and set ``REG`` to the value ``5``.

The syntax of a numeric substitution is ``[[#<expr>]]`` where ``<expr>`` is an
expression. An expression is recursively defined as:

* a numeric operand, or
* an expression followed by an operator and a numeric operand.

A numeric operand is a previously defined numeric variable, or an integer
literal. The supported operators are ``+`` and ``-``. Spaces are accepted
before, after and between any of these elements.

For example:

.. code-block:: llvm

    ; CHECK: load r[[#REG:]], [r0]
    ; CHECK: load r[[#REG+1]], [r1]

The above example would match the text:

.. code-block:: gas

    load r5, [r0]
    load r6, [r1]

but would not match the text:

.. code-block:: gas

    load r5, [r0]
    load r7, [r1]

due to ``7`` being unequal to ``5 + 1``.

The syntax also supports an empty expression, equivalent to writing {{[0-9]+}},
for cases where the input must contain a numeric value but the value itself
does not matter:

.. code-block:: gas

    ; CHECK-NOT: mov r0, r[[#]]

to check that a value is synthesized rather than moved around.

A numeric variable can also be defined to the result of a numeric expression,
in which case the numeric expression is checked and if verified the variable is
assigned to the value. The unified syntax for both defining numeric variables
and checking a numeric expression is thus ``[[#<NUMVAR>: <expr>]]`` with each
element as described previously.

The ``--enable-var-scope`` option has the same effect on numeric variables as
on string variables.

Important note: In its current implementation, an expression cannot use a
numeric variable defined earlier in the same CHECK directive.

FileCheck Pseudo Numeric Variables
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes there's a need to verify output that contains line numbers of the
match file, e.g. when testing compiler diagnostics.  This introduces a certain
fragility of the match file structure, as "``CHECK:``" lines contain absolute
line numbers in the same file, which have to be updated whenever line numbers
change due to text addition or deletion.

To support this case, FileCheck expressions understand the ``@LINE`` pseudo
numeric variable which evaluates to the line number of the CHECK pattern where
it is found.

This way match patterns can be put near the relevant test lines and include
relative line number references, for example:

.. code-block:: c++

   // CHECK: test.cpp:[[# @LINE + 4]]:6: error: expected ';' after top level declarator
   // CHECK-NEXT: {{^int a}}
   // CHECK-NEXT: {{^     \^}}
   // CHECK-NEXT: {{^     ;}}
   int a

To support legacy uses of ``@LINE`` as a special string variable,
:program:`FileCheck` also accepts the following uses of ``@LINE`` with string
substitution block syntax: ``[[@LINE]]``, ``[[@LINE+<offset>]]`` and
``[[@LINE-<offset>]]`` without any spaces inside the brackets and where
``offset`` is an integer.

Matching Newline Characters
~~~~~~~~~~~~~~~~~~~~~~~~~~~

To match newline characters in regular expressions the character class
``[[:space:]]`` can be used. For example, the following pattern:

.. code-block:: c++

   // CHECK: DW_AT_location [DW_FORM_sec_offset] ([[DLOC:0x[0-9a-f]+]]){{[[:space:]].*}}"intd"

matches output of the form (from llvm-dwarfdump):

.. code-block:: text

       DW_AT_location [DW_FORM_sec_offset]   (0x00000233)
       DW_AT_name [DW_FORM_strp]  ( .debug_str[0x000000c9] = "intd")

letting us set the :program:`FileCheck` variable ``DLOC`` to the desired value 
``0x00000233``, extracted from the line immediately preceding "``intd``".