From 08afe3dc6c9d88ea9c55231948c87f11470cb1f8 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Wed, 6 Jan 2021 19:59:10 +0100
Subject: [PATCH] logb: Fix test failure on glibc/powerpc.

* doc/posix-functions/logb.texi: Update platform info.
* m4/logb.m4 (gl_FUNC_LOGB_WORKS): Test against bug with negative
subnormal numbers.
---
 ChangeLog                     |  7 +++++++
 doc/posix-functions/logb.texi |  2 +-
 m4/logb.m4                    | 32 ++++++++++++++++++++++++--------
 3 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index b68667bcf3..0831c85e1c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2021-01-06  Bruno Haible  <bruno@clisp.org>
+
+	logb: Fix test failure on glibc/powerpc.
+	* doc/posix-functions/logb.texi: Update platform info.
+	* m4/logb.m4 (gl_FUNC_LOGB_WORKS): Test against bug with negative
+	subnormal numbers.
+
 2021-01-06  Paul Eggert  <eggert@cs.ucla.edu>
 
 	libc-config: simplify clang __has_* macros
diff --git a/doc/posix-functions/logb.texi b/doc/posix-functions/logb.texi
index 799628da06..ea9b3f5f75 100644
--- a/doc/posix-functions/logb.texi
+++ b/doc/posix-functions/logb.texi
@@ -16,7 +16,7 @@ This function is missing a declaration on some platforms:
 Cygwin 1.5.x.
 @item
 This function produces wrong results for subnormal numbers on some platforms:
-glibc 2.11/ppc, glibc 2.7/sparc, glibc 2.7/hppa, Solaris 11.4, Cygwin 1.5.x.
+glibc 2.17/ppc, glibc 2.7/sparc, glibc 2.7/hppa, Solaris 11.4, Cygwin 1.5.x.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/m4/logb.m4 b/m4/logb.m4
index 68e2c854d6..cd803c1002 100644
--- a/m4/logb.m4
+++ b/m4/logb.m4
@@ -1,4 +1,4 @@
-# logb.m4 serial 8
+# logb.m4 serial 9
 dnl Copyright (C) 2010-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -87,6 +87,8 @@ dnl Test whether logb() works.
 dnl On glibc 2.11/ppc, glibc 2.7/sparc, glibc 2.7/hppa, Solaris 10/SPARC,
 dnl Solaris 11.4/x86_64, Cygwin 1.5.x, the return value for subnormal
 dnl (denormalized) arguments is too large.
+dnl On glibc 2.17/ppc likewise but only for negative subnormal (denormalized)
+dnl arguments.
 AC_DEFUN([gl_FUNC_LOGB_WORKS],
 [
   AC_REQUIRE([AC_PROG_CC])
@@ -105,13 +107,27 @@ double logb (double);
 volatile double x;
 int main ()
 {
-  int i;
-  for (i = 1, x = 1.0; i >= DBL_MIN_EXP; i--, x *= 0.5)
-    ;
-  /* Here i = DBL_MIN_EXP - 1. Either x = 2^(i-1) is subnormal or x = 0.0.  */
-  if (x > 0.0 && !(logb (x) == (double)(i - 1)))
-    return 1;
-  return 0;
+  int result = 0;
+  /* This test fails on 2.11/ppc, glibc 2.7/sparc, glibc 2.7/hppa,
+     Solaris 10/SPARC, Solaris 11.4/x86_64, Cygwin 1.5.x.  */
+  {
+    int i;
+    for (i = 1, x = 1.0; i >= DBL_MIN_EXP; i--, x *= 0.5)
+      ;
+    /* Here i = DBL_MIN_EXP - 1. Either x = 2^(i-1) is subnormal or x = 0.0.  */
+    if (x > 0.0 && !(logb (x) == (double)(i - 1)))
+      result |= 1;
+  }
+  /* This test fails on glibc 2.17/ppc.  */
+  {
+    int i;
+    for (i = 1, x = -1.0; i >= DBL_MIN_EXP; i--, x *= 0.5)
+      ;
+    /* Here i = DBL_MIN_EXP - 1. Either x = -2^(i-1) is subnormal or x = -0.0.  */
+    if (x < 0.0 && !(logb (x) == (double)(i - 1)))
+      result |= 2;
+  }
+  return result;
 }
 ]])],
         [gl_cv_func_logb_works=yes],
-- 
2.11.4.GIT