Fix -Os log1p, log1pf build (bug 21314).

As reported in bug 21314, building log1p and log1pf fails with -Os
because of a spurious -Wmaybe-uninitialized warning (reported there
for GCC 5 for MIPS, I see it also with GCC 7 for x86_64).  This patch,
based on the patches in the bug, fixes this using the DIAG_* macros.

Tested for x86_64 with -Os that this eliminates those warnings and so
allows the build to progress further.

2018-02-01  Carlos O'Donell  <carlos@redhat.com>
	    Ramin Seyed-Moussavi  <lordrasmus@gmail.com>
	    Joseph Myers  <joseph@codesourcery.com>

	[BZ #21314]
	* sysdeps/ieee754/dbl-64/s_log1p.c: Include <libc-diag.h>.
	(__log1p): Disable -Wmaybe-uninitialized for -Os around
	computation using c.
	* sysdeps/ieee754/flt-32/s_log1pf.c: Include <libc-diag.h>.
	(__log1pf): Disable -Wmaybe-uninitialized for -Os around
	computation using c.

1 files changed, 11 insertions, 1 deletions

diff --git a/sysdeps/ieee754/dbl-64/s_log1p.c b/sysdeps/ieee754/dbl-64/s_log1p.c
index 340f6377f7..b7cf5ce321 100644
--- a/sysdeps/ieee754/dbl-64/s_log1p.c
+++ b/sysdeps/ieee754/dbl-64/s_log1p.c
@@ -81,6 +81,7 @@
 #include <float.h>
 #include <math.h>
 #include <math_private.h>
+#include <libc-diag.h>
 
 static const double
   ln2_hi = 6.93147180369123816490e-01,  /* 3fe62e42 fee00000 */
@@ -191,5 +192,14 @@ __log1p (double x)
   if (k == 0)
     return f - (hfsq - s * (hfsq + R));
   else
-    return k * ln2_hi - ((hfsq - (s * (hfsq + R) + (k * ln2_lo + c))) - f);
+    {
+      /* With GCC 7 when compiling with -Os the compiler warns that c
+	 might be used uninitialized.  This can't be true because k
+	 must be 0 for c to be uninitialized and we handled that
+	 computation earlier without using c.  */
+      DIAG_PUSH_NEEDS_COMMENT;
+      DIAG_IGNORE_Os_NEEDS_COMMENT (7, "-Wmaybe-uninitialized");
+      return k * ln2_hi - ((hfsq - (s * (hfsq + R) + (k * ln2_lo + c))) - f);
+      DIAG_POP_NEEDS_COMMENT;
+    }
 }