Add a Configure test to see if NV 0.0 is stored as all bits zero, and

#define NV_ZERO_IS_ALLBITS_ZERO if so.  This is always true on VMS:

Subject: Re: Configure check for whether 0.0 is stored as all bits zero
From: "Craig A. Berry" <craigberry@mac.com>
Message-Id: <p06210214beda5bf31c4c@[172.16.52.1]>
Date: Sat, 18 Jun 2005 18:23:59 -0500

p4raw-id: //depot/perl@24898

4 files changed, 134 insertions, 0 deletions

diff --git a/Configure b/Configure
index 1bd54cb5e4..d9bd77a861 100755
--- a/Configure
+++ b/Configure
@@ -1002,6 +1002,7 @@ perl5=''
 perladmin=''
 perlpath=''
 d_nv_preserves_uv=''
+d_nv_zero_is_allbits_zero=''
 i16size=''
 i16type=''
 i32size=''
@@ -14833,6 +14834,128 @@ esac
 
 $rm -f try.* try
 
+$echo "Checking whether NV 0.0 is all bits zero in memory..." >&4
+: volatile so that the compiler has to store it out to memory.
+if test X"$d_volatile" = X"$define"; then
+	volatile=volatile
+fi
+$cat <<EOP >try.c
+#include <stdio.h>
+#$i_stdlib I_STDLIB
+#ifdef I_STDLIB
+#include <stdlib.h>
+#endif
+#$i_string I_STRING
+#ifdef I_STRING
+#  include <string.h>
+#else
+#  include <strings.h>
+#endif
+#include <sys/types.h>
+#include <signal.h>
+#ifdef SIGFPE
+$volatile int bletched = 0;
+$signal_t blech(s) int s; { bletched = 1; }
+#endif
+
+int checkit($nvtype d, char *where) {
+    unsigned char *p = (char *)&d;
+    unsigned char *end = p + sizeof(d);
+    int fail = 0;
+
+    while (p < end)
+	fail += *p++;
+
+    if (!fail)
+        return 0;
+
+    p = (char *)&d;
+    printf("No - %s: 0x", where);
+    while (p < end)
+	printf ("%02X", *p++);
+    printf("\n");
+    return 1;
+}
+
+int main(int argc, char **argv) {
+    $nvtype d = 0.0;
+    int fail = 0;
+    fail += checkit(d, "0.0");
+
+    /* The compiler shouldn't be assuming that bletched is 0  */
+    d = bletched;
+
+    fail += checkit(d, "bleched");
+
+#ifdef SIGFPE
+    signal(SIGFPE, blech);
+#endif
+
+    /* Paranoia - the compiler should have no way of knowing that ANSI says
+       that argv[argc] will always be NULL.  Actually, if it did assume this it
+       would be buggy, as this is C and main() can be called from elsewhere in
+       the program.  */
+    d = argv[argc] ? 1 : 0;
+
+    if (d) {
+	printf("Odd argv[argc]=%p, d=%g\n", argv[argc], d);
+    }
+
+    fail += checkit(d, "ternary");
+
+    memset(&d, sizeof(d), argv[argc] ? 1 : 0);
+
+    if (d != 0.0) {
+	printf("No - memset doesn't give 0.0\n");
+	/* This might just blow up:  */
+	printf("(gives %g)\n", d);
+	return 1;
+    }
+    
+#ifdef SIGFPE
+    if (bletched) {
+	printf("No - something bleched\n");
+	return 1;
+    }
+#endif
+    if (fail) {
+      printf("No - %d fail(s)\n", fail);
+      return 1;
+    }
+    printf("Yes\n");
+    return 0;
+}
+EOP
+set try
+
+d_nv_zero_is_allbits_zero="$undef"
+if eval $compile; then
+    xxx="`$run ./try`"
+    case "$?" in
+	0)
+	    case "$xxx" in
+		Yes)  cat >&4 <<EOM
+0.0 is represented as all bits zero in memory
+EOM
+		    d_nv_zero_is_allbits_zero="$define"
+		    ;;
+		*)  cat >&4 <<EOM
+0.0 is not represented as all bits zero in memory
+EOM
+		    d_nv_zero_is_allbits_zero="$undef"
+		    ;;
+	    esac
+	    ;;
+	*)  cat >&4 <<EOM
+0.0 is not represented as all bits zero in memory
+EOM
+	    d_nv_zero_is_allbits_zero="$undef"
+	    ;;
+    esac
+fi
+
+$rm -f try.* try
+
 
 : check for off64_t
 echo " "
@@ -21012,6 +21135,7 @@ d_mymalloc='$d_mymalloc'
 d_nice='$d_nice'
 d_nl_langinfo='$d_nl_langinfo'
 d_nv_preserves_uv='$d_nv_preserves_uv'
+d_nv_zero_is_allbits_zero='$d_nv_zero_is_allbits_zero'
 d_off64_t='$d_off64_t'
 d_old_pthread_create_joinable='$d_old_pthread_create_joinable'
 d_oldpthreads='$d_oldpthreads'
diff --git a/Porting/Glossary b/Porting/Glossary
index 99b8e34e7a..ac31fa561e 100644
--- a/Porting/Glossary
+++ b/Porting/Glossary
@@ -1383,6 +1383,10 @@ d_nv_preserves_uv (perlxv.U):
 	This variable indicates whether a variable of type nvtype
 	can preserve all the bits a variable of type uvtype.
 
+d_nv_zero_is_allbits_zero (perlxv.U):
+	This variable indicates whether a variable of type nvtype
+	stores 0.0 in memory as all bits zero.
+
 d_off64_t (d_off64_t.U):
 	This symbol will be defined if the C compiler supports off64_t.
 
diff --git a/config_h.SH b/config_h.SH
index 422bd6cce2..7ff827f8d6 100644
--- a/config_h.SH
+++ b/config_h.SH
@@ -3235,6 +3235,10 @@ sed <<!GROK!THIS! >$CONFIG_H -e 's!^#undef\(.*/\)\*!/\*#define\1 \*!' -e 's!^#un
  *	This symbol contains the number of bits a variable of type NVTYPE
  *	can preserve of a variable of type UVTYPE.
  */
+/* NV_ZERO_IS_ALLBITS_ZERO
+ *	This symbol, if defined, indicates that a variable of type NVTYPE
+ *	stores 0.0 in memory as all bits zero.
+ */
 #define	IVTYPE		$ivtype		/**/
 #define	UVTYPE		$uvtype		/**/
 #define	I8TYPE		$i8type		/**/
@@ -3263,6 +3267,7 @@ sed <<!GROK!THIS! >$CONFIG_H -e 's!^#undef\(.*/\)\*!/\*#define\1 \*!' -e 's!^#un
 #define	NVSIZE		$nvsize		/**/
 #$d_nv_preserves_uv	NV_PRESERVES_UV
 #define	NV_PRESERVES_UV_BITS	$nv_preserves_uv_bits
+#$d_nv_zero_is_allbits_zero	NV_ZERO_IS_ALLBITS_ZERO
 #if UVSIZE == 8
 #   ifdef BYTEORDER
 #       if BYTEORDER == 0x1234
diff --git a/configure.com b/configure.com
index 8be30fccb1..0e4cbb0002 100644
--- a/configure.com
+++ b/configure.com
@@ -5587,6 +5587,7 @@ $ WC "d_nice='define'"
 $ WC "d_nl_langinfo='" + d_nl_langinfo + "'"
 $ WC "d_nv_preserves_uv='" + d_nv_preserves_uv + "'"
 $ WC "nv_preserves_uv_bits='" + nv_preserves_uv_bits + "'"
+$ WC "d_nv_zero_is_allbits_zero='define'"
 $ WC "d_off64_t='" + d_off64_t + "'"
 $ WC "d_old_pthread_create_joinable='" + d_old_pthread_create_joinable + "'"
 $ WC "d_oldarchlib='define'"