[FileCheck] Add CHECK-EMPTY directive for checking for blank lines

Prior to this change, there was no clean way of getting FileCheck to
check that a line is completely empty. The expected way of using
"CHECK: {{^$}}" does not work because the '^' matches the end of the
previous match (this behaviour may be desirable in certain instances).
For the same reason, "CHECK-NEXT: {{^$}}" will fail when the previous
match was at the end of the line, as the pattern will match there.
Using the recommended [[:space:]] to match an explicit new line could
also match a space, and thus is not always desired. Literal '\n'
matches also do not work. A workaround was suggested in the review, but
it is a little clunky.

This change adds a new directive that behaves the same as CHECK-NEXT,
except that it only matches against empty lines (nothing, not even
whitespace, is allowed). As with CHECK-NEXT, it will fail if more than
one newline occurs before the next blank line. Example usage:
; test.txt
foo

bar
; CHECK: foo
; CHECK-EMPTY:
; CHECK-NEXT: bar

Differential Revision: https://reviews.llvm.org/D28896

Reviewed by: probinson

llvm-svn: 335613

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/docs/CommandGuide/FileCheck.rst b/llvm/docs/CommandGuide/FileCheck.rst
index 9078f65e01c..e169ea5c132 100644
--- a/llvm/docs/CommandGuide/FileCheck.rst
+++ b/llvm/docs/CommandGuide/FileCheck.rst
@@ -241,6 +241,25 @@ For example, the following works like you'd expect:
 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
+
+   foo
+
+   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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~