On x64 hardware, round-trip uint64_t→double→uint64_t conversions

fail for values greater than UINT64_MAX-2047.  This caused the SQLite
text-to-float converter routine to give incorrect results for values
between '1.8446744073709550592eNNN' and '1.8446744073709551609eNNN' for any
exponent NNN.  This problem was introduced by check-in [761d8fd18b0ee868]
and first appeared in version 3.47.0 and was reported by
[forum:/forumpost/569a7209179a7f5e|forum post 569a7209179a7f5e].  Fixed
by this check-in.

FossilOrigin-Name: 81342fa6dd03fffbe7d4d699ff049dcef4d30344578bb6f91cb58a4e5a4f6036

1 files changed, 10 insertions, 4 deletions

diff --git a/src/util.c b/src/util.c
index 52c2a744c..31b05666a 100644
--- a/src/util.c
+++ b/src/util.c
@@ -643,7 +643,7 @@ do_atof_calc:
   e = (e*esign) + d;
 
   /* Try to adjust the exponent to make it smaller */
-  while( e>0 && s<(LARGEST_UINT64/10) ){
+  while( e>0 && s<((LARGEST_UINT64-0x7ff)/10) ){
     s *= 10;
     e--;
   }
@@ -653,11 +653,17 @@ do_atof_calc:
   }
 
   rr[0] = (double)s;
-  s2 = (u64)rr[0];
+  if( s<(LARGEST_UINT64-0x7ff) ){
+    s2 = (u64)rr[0];
 #if defined(_MSC_VER) && _MSC_VER<1700
-  if( s2==0x8000000000000000LL ){ s2 = 2*(u64)(0.5*rr[0]); }
+    if( s2==0x8000000000000000LL ){ s2 = 2*(u64)(0.5*rr[0]); }
 #endif
-  rr[1] = s>=s2 ? (double)(s - s2) : -(double)(s2 - s);
+    rr[1] = s>=s2 ? (double)(s - s2) : -(double)(s2 - s);
+  }else{
+    s2 = s;
+    rr[1] = 0.0;
+  }
+
   if( e>0 ){
     while( e>=100  ){
       e -= 100;