fix F0convert() on edge cases

The F0convert() function used to implement the %.0f format specifier
more cheaply went wrong on some edge cases.  Its rounding went wrong
when the exponent is such that fractional values are not representable,
making the "+= 0.5" invoke floating point rounding.  Fix that by only
invoking that rounding logic for values that start out fractional.
That fixes the output part of [perl #47602].  It also failed to emit the
sign for negative zero.  Fix that by making it not apply to zero values.

2 files changed, 14 insertions, 4 deletions

diff --git a/sv.c b/sv.c
index 2c13b3b333..6ad33c0bfc 100644
--- a/sv.c
+++ b/sv.c
@@ -11057,12 +11057,15 @@ S_F0convert(NV nv, char *const endbuf, STRLEN *const len)
     assert(!Perl_isinfnan(nv));
     if (neg)
 	nv = -nv;
-    if (nv < UV_MAX) {
+    if (nv != 0.0 && nv < UV_MAX) {
 	char *p = endbuf;
-	nv += 0.5;
 	uv = (UV)nv;
-	if (uv & 1 && uv == nv)
-	    uv--;			/* Round to even */
+	if (uv != nv) {
+	    nv += 0.5;
+	    uv = (UV)nv;
+	    if (uv & 1 && uv == nv)
+		uv--;			/* Round to even */
+	}
 	do {
 	    const unsigned dig = uv % 10;
 	    *--p = '0' + dig;
diff --git a/t/op/sprintf2.t b/t/op/sprintf2.t
index bf092032bb..5fb75974cb 100644
--- a/t/op/sprintf2.t
+++ b/t/op/sprintf2.t
@@ -1123,5 +1123,12 @@ like sprintf("%p", 0+'NaN'), qr/^[0-9a-f]+$/, "%p and NaN";
         "\\x80 in format";
 }
 
+foreach(
+    0.0, -0.0,
+    4503599627370501, -4503599627370501,
+    4503599627370503, -4503599627370503,
+) {
+    is sprintf("%.0f", $_), sprintf("%-.0f", $_), "special-case %.0f on $_";
+}
 
 done_testing();