Make gen-as-const.py handle '--' consistently with awk script.

It was reported in
<https://sourceware.org/ml/libc-alpha/2018-12/msg00045.html> that
gen-as-const.py fails to generate test code in the case where a .sym
file has no symbols in it, so resulting in a test failing to link for
Hurd.

The relevant difference from the old awk script is that the old script
treated '--' lines as indicating that the text to do at the start of
the test (or file used to compute constants) should be output at that
point if not already output, as well as treating lines with actual
entries for constants like that.  This patch changes gen-as-const.py
accordingly, making it the sole responsibility of the code parsing
.sym files to determine when such text should be output and ensuring
it's always output at some point even if there are no symbols and no
'--' lines, since not outputting it means the test fails to link.
Handling '--' like that also avoids any problems that would arise if
the first entry for a symbol were inside #ifdef (since the text in
question must not be output inside #ifdef).

Tested for x86_64, and with build-many-glibcs.py for i686-gnu.  Note
that there are still compilation test failures for i686-gnu
(linknamespace tests, possibly arising from recent posix_spawn-related
changes).

	* scripts/gen-as-const.py (compute_c_consts): Take an argument
	'START' to indicate that start text should be output.
	(gen_test): Likewise.
	(main): Generate 'START' for first symbol or '--' line, or at end
	of input if not previously generated.

2 files changed, 43 insertions, 29 deletions

diff --git a/ChangeLog b/ChangeLog
index a534c4a5f4..a7adb8b3c9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2018-12-03  Joseph Myers  <joseph@codesourcery.com>
+
+	* scripts/gen-as-const.py (compute_c_consts): Take an argument
+	'START' to indicate that start text should be output.
+	(gen_test): Likewise.
+	(main): Generate 'START' for first symbol or '--' line, or at end
+	of input if not previously generated.
+
 2018-12-03  Rafael Ávila de Espíndola  <rafael@espindo.la>
 
 	[BZ #19767]
diff --git a/scripts/gen-as-const.py b/scripts/gen-as-const.py
index cabf401ed1..eb85ef1aa0 100644
--- a/scripts/gen-as-const.py
+++ b/scripts/gen-as-const.py
@@ -34,28 +34,28 @@ def compute_c_consts(sym_data, cc):
     """Compute the values of some C constants.
 
     The first argument is a list whose elements are either strings
-    (preprocessor directives) or pairs of strings (a name and a C
+    (preprocessor directives, or the special string 'START' to
+    indicate this function should insert its initial boilerplate text
+    in the output there) or pairs of strings (a name and a C
     expression for the corresponding value).  Preprocessor directives
     in the middle of the list may be used to select which constants
     end up being evaluated using which expressions.
 
     """
     out_lines = []
-    started = False
     for arg in sym_data:
         if isinstance(arg, str):
-            out_lines.append(arg)
+            if arg == 'START':
+                out_lines.append('void\ndummy (void)\n{')
+            else:
+                out_lines.append(arg)
             continue
         name = arg[0]
         value = arg[1]
-        if not started:
-            out_lines.append('void\ndummy (void)\n{')
-            started = True
         out_lines.append('asm ("@@@name@@@%s@@@value@@@%%0@@@end@@@" '
                          ': : \"i\" ((long int) (%s)));'
                          % (name, value))
-    if started:
-        out_lines.append('}')
+    out_lines.append('}')
     out_lines.append('')
     out_text = '\n'.join(out_lines)
     with tempfile.TemporaryDirectory() as temp_dir:
@@ -89,32 +89,32 @@ def gen_test(sym_data):
 
     """
     out_lines = []
-    started = False
     for arg in sym_data:
         if isinstance(arg, str):
-            out_lines.append(arg)
+            if arg == 'START':
+                out_lines.append('#include <stdint.h>\n'
+                                 '#include <stdio.h>\n'
+                                 '#include <bits/wordsize.h>\n'
+                                 '#if __WORDSIZE == 64\n'
+                                 'typedef uint64_t c_t;\n'
+                                 '# define U(n) UINT64_C (n)\n'
+                                 '#else\n'
+                                 'typedef uint32_t c_t;\n'
+                                 '# define U(n) UINT32_C (n)\n'
+                                 '#endif\n'
+                                 'static int\n'
+                                 'do_test (void)\n'
+                                 '{\n'
+                                 # Compilation test only, using static
+                                 # assertions.
+                                 '  return 0;\n'
+                                 '}\n'
+                                 '#include <support/test-driver.c>')
+            else:
+                out_lines.append(arg)
             continue
         name = arg[0]
         value = arg[1]
-        if not started:
-            out_lines.append('#include <stdint.h>\n'
-                             '#include <stdio.h>\n'
-                             '#include <bits/wordsize.h>\n'
-                             '#if __WORDSIZE == 64\n'
-                             'typedef uint64_t c_t;\n'
-                             '# define U(n) UINT64_C (n)\n'
-                             '#else\n'
-                             'typedef uint32_t c_t;\n'
-                             '# define U(n) UINT32_C (n)\n'
-                             '#endif\n'
-                             'static int\n'
-                             'do_test (void)\n'
-                             '{\n'
-                             # Compilation test only, using static assertions.
-                             '  return 0;\n'
-                             '}\n'
-                             '#include <support/test-driver.c>')
-            started = True
         out_lines.append('_Static_assert (U (asconst_%s) == (c_t) (%s), '
                          '"value of %s");'
                          % (name, value, name))
@@ -134,6 +134,7 @@ def main():
     args = parser.parse_args()
     sym_data = []
     with open(args.sym_file, 'r') as sym_file:
+        started = False
         for line in sym_file:
             line = line.strip()
             if line == '':
@@ -143,12 +144,17 @@ def main():
                 sym_data.append(line)
                 continue
             words = line.split(maxsplit=1)
+            if not started:
+                sym_data.append('START')
+                started = True
             # Separator.
             if words[0] == '--':
                 continue
             name = words[0]
             value = words[1] if len(words) > 1 else words[0]
             sym_data.append((name, value))
+        if not started:
+            sym_data.append('START')
     if args.test:
         print(gen_test(sym_data))
     else: