Partial backport of:

commit 7e1d42400c1b8f03316fe14176133c8853cd3bbe
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Fri Nov 30 15:20:41 2018 +0000

    Replace gen-as-const.awk by gen-as-const.py.
    
    This patch replaces gen-as-const.awk, and some fragments of the
    Makefile code that used it, by a Python script.  The point is not such
    much that awk is problematic for this particular script, as that I'd
    like to build up a general Python infrastructure for extracting
    information from C headers, for use in writing tests of such headers.
    Thus, although this patch does not set up such infrastructure, the
    compute_c_consts function in gen-as-const.py might be moved to a
    separate Python module in a subsequent patch as a starting point for
    such infrastructure.
    
    The general idea of the code is the same as in the awk version, but no
    attempt is made to make the output files textually identical.  When
    generating a header, a dict of constant names and values is generated
    internally then defines are printed in sorted order (rather than the
    order in the .sym file, which would have been used before).  When
    generating a test that the values computed match those from a normal
    header inclusion, the test code is made into a compilation test using
    _Static_assert, where previously the comparisons were done only when
    the test was executed.  One fragment of test generation (converting
    the previously generated header to use asconst_* prefixes on its macro
    names) is still in awk code in the makefiles; only the .sym processing
    and subsequent execution of the compiler to extract constants have
    moved to the Python script.
    
    Tested for x86_64, and with build-many-glibcs.py.
    
            * scripts/gen-as-const.py: New file.
            * scripts/gen-as-const.awk: Remove.
            * Makerules ($(common-objpfx)%.h $(common-objpfx)%.h.d): Use
            gen-as-const.py.
            ($(objpfx)test-as-const-%.c): Likewise.

In the downstream version, scripts/gen-as-const.awk is not removed and
still used in Makerules.

diff --git a/scripts/gen-as-const.py b/scripts/gen-as-const.py
new file mode 100644
index 0000000000000000..b7a5744bb192dd67
--- /dev/null
+++ b/scripts/gen-as-const.py
@@ -0,0 +1,159 @@
+#!/usr/bin/python3
+# Produce headers of assembly constants from C expressions.
+# Copyright (C) 2018 Free Software Foundation, Inc.
+# This file is part of the GNU C Library.
+#
+# The GNU C Library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# The GNU C Library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with the GNU C Library; if not, see
+# <http://www.gnu.org/licenses/>.
+
+# The input to this script looks like:
+#       #cpp-directive ...
+#       NAME1
+#       NAME2 expression ...
+# A line giving just a name implies an expression consisting of just that name.
+
+import argparse
+import os.path
+import re
+import subprocess
+import tempfile
+
+
+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
+    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)
+            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_text = '\n'.join(out_lines)
+    with tempfile.TemporaryDirectory() as temp_dir:
+        c_file_name = os.path.join(temp_dir, 'test.c')
+        s_file_name = os.path.join(temp_dir, 'test.s')
+        with open(c_file_name, 'w') as c_file:
+            c_file.write(out_text)
+        # Compilation has to be from stdin to avoid the temporary file
+        # name being written into the generated dependencies.
+        cmd = ('%s -S -o %s -x c - < %s' % (cc, s_file_name, c_file_name))
+        subprocess.check_call(cmd, shell=True)
+        consts = {}
+        with open(s_file_name, 'r') as s_file:
+            for line in s_file:
+                match = re.search('@@@name@@@([^@]*)'
+                                  '@@@value@@@[^0-9Xxa-fA-F-]*'
+                                  '([0-9Xxa-fA-F-]+).*@@@end@@@', line)
+                if match:
+                    if (match.group(1) in consts
+                        and match.group(2) != consts[match.group(1)]):
+                        raise ValueError('duplicate constant %s'
+                                         % match.group(1))
+                    consts[match.group(1)] = match.group(2)
+        return consts
+
+
+def gen_test(sym_data):
+    """Generate a test for the values of some C constants.
+
+    The first argument is as for compute_c_consts.
+
+    """
+    out_lines = []
+    started = False
+    for arg in sym_data:
+        if isinstance(arg, str):
+            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))
+    return '\n'.join(out_lines)
+
+
+def main():
+    """The main entry point."""
+    parser = argparse.ArgumentParser(
+        description='Produce headers of assembly constants.')
+    parser.add_argument('--cc', metavar='CC',
+                        help='C compiler (including options) to use')
+    parser.add_argument('--test', action='store_true',
+                        help='Generate test case instead of header')
+    parser.add_argument('sym_file',
+                        help='.sym file to process')
+    args = parser.parse_args()
+    sym_data = []
+    with open(args.sym_file, 'r') as sym_file:
+        for line in sym_file:
+            line = line.strip()
+            if line == '':
+                continue
+            # Pass preprocessor directives through.
+            if line.startswith('#'):
+                sym_data.append(line)
+                continue
+            words = line.split(maxsplit=1)
+            # Separator.
+            if words[0] == '--':
+                continue
+            name = words[0]
+            value = words[1] if len(words) > 1 else words[0]
+            sym_data.append((name, value))
+    if args.test:
+        print(gen_test(sym_data))
+    else:
+        consts = compute_c_consts(sym_data, args.cc)
+        print('\n'.join('#define %s %s' % c for c in sorted(consts.items())))
+
+if __name__ == '__main__':
+    main()
