[PATCH 2/3] autoconf: prefer an unrolled loop for trivial AC_CHECK_HEADERS

An unrolled loop avoids the cost of spawning sed in AS_TR_SH and
AS_TR_CPP.  Prefer it if there is nothing in the second and third
argument of AC_CHECK_HEADERS and the first argument is a literal.
Modify AC_CHECK_HEADERS_ONCE to avoid the variable indirection too.

* lib/autoconf/headers.m4 (AC_CHECK_HEADERS): Unroll loop if safe.
(_AC_CHECK_HEADERS): Move basic implementation here.
(_AC_CHECK_HEADER_ONCE): Expand AC_CHECK_HEADERS here...
(_AC_HEADERS_EXPANSION): ... and not here, so remove.
(AC_CHECK_INCLUDES_DEFAULT): Remove unnecessary arguments after the first.

Signed-off-by: Paolo Bonzini <[email protected]>
---
 lib/autoconf/headers.m4 | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/lib/autoconf/headers.m4 b/lib/autoconf/headers.m4
index 72262c1..e855025 100644
--- a/lib/autoconf/headers.m4
+++ b/lib/autoconf/headers.m4
@@ -182,22 +182,25 @@ m4_define([AH_CHECK_HEADERS],
 # header.  Either ACTION may include `break' to stop the search.
 AC_DEFUN([AC_CHECK_HEADERS],
 [m4_map_args_w([$1], [_AH_CHECK_HEADER(], [)])]dnl
-[AS_FOR([AC_header], [ac_header], [$1],
-[AC_CHECK_HEADER(AC_header,
-		 [AC_DEFINE_UNQUOTED(AS_TR_CPP([HAVE_]AC_header)) $2],
-		 [$3], [$4])dnl])
+[m4_if([$2$3]AS_LITERAL_IF([$1], [yes], [no]), []yes,
+       [m4_map_args_w([$1], [_$0(], [, [], [], [$4])])],
+       [AS_FOR([AC_header], [ac_header], [$1], [_$0(AC_header, [$2], [$3], [$4])])])
 ])# AC_CHECK_HEADERS
 
+m4_define([_AC_CHECK_HEADERS],
+[AC_CHECK_HEADER([$1],
+		 [AC_DEFINE_UNQUOTED(AS_TR_CPP([HAVE_]$1)) $2],
+		 [$3], [$4])
+])
+
 
 # _AC_CHECK_HEADER_ONCE(HEADER-FILE)
 # ----------------------------------
 # Check for a single HEADER-FILE once.
 m4_define([_AC_CHECK_HEADER_ONCE],
-[_AH_CHECK_HEADER([$1])AC_DEFUN([_AC_Header_]m4_translit([[$1]],
-    [./-], [___]),
-  [m4_divert_text([INIT_PREPARE], [AS_VAR_APPEND([ac_header_list], [" $1"])])
-_AC_HEADERS_EXPANSION])AC_REQUIRE([_AC_Header_]m4_translit([[$1]],
-    [./-], [___]))])
+[AC_DEFUN([_AC_Header_]m4_translit([[$1]], [./-], [___]),
+          [AC_CHECK_HEADERS([$1], [], [], [$ac_includes_default])])
+AC_REQUIRE([_AC_Header_]m4_translit([[$1]], [./-], [___]))])
 
 
 # AC_CHECK_HEADERS_ONCE(HEADER-FILE...)
@@ -213,12 +216,6 @@ AC_DEFUN([AC_CHECK_HEADERS_ONCE],
 AC_DEFUN([_AC_CHECK_HEADERS_ONCE],
   [m4_map_args_w([$1], [_AC_CHECK_HEADER_ONCE(], [)])])
 
-m4_define([_AC_HEADERS_EXPANSION],
-  [m4_divert_text([DEFAULTS], [ac_header_list=])]dnl
-  [AC_CHECK_HEADERS([$ac_header_list], [], [], [$ac_includes_default])]dnl
-  [m4_define([_AC_HEADERS_EXPANSION], [])])
-
-
 
 
 ## --------------------- ##
@@ -261,8 +258,7 @@ ac_includes_default="\
 #endif"
 ])]dnl
 [_AC_CHECK_HEADERS_ONCE(
-  [sys/types.h sys/stat.h strings.h inttypes.h stdint.h unistd.h],
-  [], [], [$ac_includes_default])]dnl
+  [sys/types.h sys/stat.h strings.h inttypes.h stdint.h unistd.h])]dnl
 dnl For backward compatibility, provide unconditional AC_DEFINEs of
 dnl HAVE_STDLIB_H, HAVE_STRING_H, and STDC_HEADERS.
 [AC_DEFINE([HAVE_STDLIB_H], [1],
-- 
2.7.4