[PATCH] build: with --enable-gcc-warnings avoid unwarranted failure w/gcc-4.6.1

[Jim Meyering]

Building coreutils on fedora rawhide (using --enable-gcc-warnings),
I was annoyed one too many times by failure to compile due to this
bogus warning from "gcc version 4.6.1 20110824 (Red Hat 4.6.1-8) (GCC)":

    test.c:95:1: error: function might be candidate for attribute 'pure' if it \
      is known to return normally [-Werror=suggest-attribute=pure]
    cc1: all warnings being treated as errors

I filed a bug about this months ago, and it's been fixed in 4.7 for
some time, but since rawhide is still using 4.6.1, ...

I want to --enable-gcc-warnings and have been using gcc-built-from-trunk
on my F15 desktop for precisely this reason, but refuse to build gcc from
sources everywhere just to get by this problem.


>From 419c9067271982df2299d4db378e9eac6ea13889 Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Mon, 12 Sep 2011 15:04:32 +0200
Subject: [PATCH] build: with --enable-gcc-warnings avoid unwarranted failure
 w/gcc-4.6.1

* configure.ac (WERROR_CFLAGS): With --enable-gcc-warnings, use
-Wsuggest-attribute=pure only only with gcc 4.7 or newer.
---
 configure.ac |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/configure.ac b/configure.ac
index 291b19e..2a9ed81 100644
--- a/configure.ac
+++ b/configure.ac
@@ -102,9 +102,18 @@ if test "$gl_gcc_warnings" = yes; then
   gl_WARN_ADD([-Wno-sign-compare])     # Too many warnings for now
   gl_WARN_ADD([-Wno-unused-parameter]) # Too many warnings for now
   gl_WARN_ADD([-Wsuggest-attribute=const])
-  gl_WARN_ADD([-Wsuggest-attribute=pure])
   gl_WARN_ADD([-Wsuggest-attribute=noreturn])

+  # Enable this warning only with gcc-4.7 and newer.  With 4.6.1 20110824,
+  # it suggests test.c's advance function may be pure, even though it
+  # increments a global variable.  Oops.
+  # Normally we'd write code to test for the precise failure, but that
+  # requires a relatively large input to make gcc exhibit the failure.
+  case `$CC -v` in
+    *'gcc version '4.[0-6]*) ;;
+    *'gcc version '[4-9]*) gl_WARN_ADD([-Wsuggest-attribute=pure]) ;;
+  esac
+
   # In spite of excluding -Wlogical-op above, it is enabled, as of
   # gcc 4.5.0 20090517, and it provokes warnings in cat.c, dd.c, truncate.c
   gl_WARN_ADD([-Wno-logical-op])
--
1.7.7.rc0.362.g5a14