item_func.cc:

  fix for bug#8461

  BUG 8461 - TRUNCATE returns incorrect result if 2nd argument is negative
  Reason: Both TRUNCATE/ROUND converts INTEGERS to DOUBLE and back to INTEGERS
  Changed the integer routine to work on integers only.
  This bug affects 4.1, 5.0 and 5.1
  Fixing in 4.1 will need to change the routine to handle different types individually.
  5.0 did had different routines for different types already just the INTEGER routine was bad.

mysql-test/r/func_math.result

@@ -203,3 +203,18 @@ NULL
 Warnings:
 Error	1365	Division by 0
 set sql_mode='';
+select round(111,-10);
+round(111,-10)
+0
+select round(-5000111000111000155,-1);
+round(-5000111000111000155,-1)
+-5000111000111000160
+select round(15000111000111000155,-1);
+round(15000111000111000155,-1)
+15000111000111000160
+select truncate(-5000111000111000155,-1);
+truncate(-5000111000111000155,-1)
+-5000111000111000150
+select truncate(15000111000111000155,-1);
+truncate(15000111000111000155,-1)
+15000111000111000150

mysql-test/t/func_math.test

@@ -141,3 +141,17 @@ select log(2,-1);
 select log(-2,1);
 set sql_mode='';
 
+#
+# Bug #8461 truncate() and round() return false results 2nd argument negative.
+# 
+# round(a,-b) log_10(b) > a
+select round(111,-10);
+# round on bigint 
+select round(-5000111000111000155,-1);
+# round on unsigned bigint
+select round(15000111000111000155,-1);
+# truncate on bigint 
+select truncate(-5000111000111000155,-1);
+# truncate on unsigned bigint
+select truncate(15000111000111000155,-1);
+

sql/item_func.cc

@@ -1863,28 +1863,30 @@ longlong Item_func_round::int_op()
     return value; // integer have not digits after point
 
   abs_dec= -dec;
-  double tmp;
-  /*
-    tmp2 is here to avoid return the value with 80 bit precision
-    This will fix that the test round(0.1,1) = round(0.1,1) is true
-  */
-  volatile double tmp2;
+  longlong tmp;
   
-  tmp= (abs_dec < array_elements(log_10) ?
-        log_10[abs_dec] : pow(10.0, (double) abs_dec));
+  if(abs_dec >= array_elements(log_10_int))
+    return 0;
+  
+  tmp= log_10_int[abs_dec];
   
   if (truncate)
   {
     if (unsigned_flag)
-      tmp2= floor(ulonglong2double(value)/tmp)*tmp;
-    else if (value >= 0)
-      tmp2= floor(((double)value)/tmp)*tmp;
+      value= (ulonglong(value)/tmp)*tmp;
     else
-      tmp2= ceil(((double)value)/tmp)*tmp;
+      value= (value/tmp)*tmp;
   }
   else
-    tmp2= rint(((double)value)/tmp)*tmp;
-  return (longlong)tmp2;
+  {
+    if (unsigned_flag)
+      value= ((ulonglong(value)+(tmp>>1))/tmp)*tmp;
+    else if ( value >= 0)
+      value= ((value+(tmp>>1))/tmp)*tmp;
+    else
+      value= ((value-(tmp>>1))/tmp)*tmp;
+  }
+  return value;
 }