Do not set the global bUseLongDouble flag based purely on sizeof(long double)

being more than 8.  That is a necessary but not sufficient condition to use
the long double routines.  Instead, do a run-time test to verify that long
double really does provide extra precision.

FossilOrigin-Name: aa999d490b743f455f2fb6c41009098330380f0bc2e9b1c28fbc82141a9138ce

1 files changed, 32 insertions, 2 deletions

diff --git a/src/main.c b/src/main.c
index d69ebf75b..a43afab16 100644
--- a/src/main.c
+++ b/src/main.c
@@ -160,6 +160,32 @@ char *sqlite3_temp_directory = 0;
 char *sqlite3_data_directory = 0;
 
 /*
+** Determine whether or not high-precision (long double) floating point
+** math works correctly on CPU currently running.
+*/
+static SQLITE_NOINLINE int hasHighPrecisionDouble(int rc){
+  if( sizeof(LONGDOUBLE_TYPE)<=8 ){
+    /* If the size of "long double" is not more than 8, then
+    ** high-precision math is not possible. */
+    return 0;
+  }else{
+    /* Just because sizeof(long double)>8 does not mean that the underlying
+    ** hardware actually supports high-precision floating point.  For example,
+    ** clearing the 0x100 bit in the floating-point control word on Intel
+    ** processors will make long double work like double, even though long
+    ** double takes up more space.  The only way to determine if long double
+    ** actually works is to run an experiment. */
+    LONGDOUBLE_TYPE a, b, c;
+    rc++;
+    a = 1.0+rc*0.1;
+    b = 1.0e+18+rc*25.0;
+    c = a+b;
+    return b!=c;
+  }
+}
+
+
+/*
 ** Initialize SQLite. 
 **
 ** This routine must be called to initialize the memory allocation,
@@ -354,6 +380,10 @@ int sqlite3_initialize(void){
   }
 #endif
 
+  /* Experimentally determine if high-precision floating point is
+  ** available. */
+  sqlite3Config.bUseLongDouble = hasHighPrecisionDouble(rc);
+
   return rc;
 }
 
@@ -4554,11 +4584,11 @@ int sqlite3_test_control(int op, ...){
     **   X<0     Make no changes to the bUseLongDouble.  Just report value.
     **   X==0    Disable bUseLongDouble
     **   X==1    Enable bUseLongDouble
-    **   X==2    Set bUseLongDouble to its default value for this platform
+    **   X>=2    Set bUseLongDouble to its default value for this platform
     */
     case SQLITE_TESTCTRL_USELONGDOUBLE: {
       int b = va_arg(ap, int);
-      if( b==2 ) b = sizeof(LONGDOUBLE_TYPE)>8;
+      if( b>=2 ) b = hasHighPrecisionDouble(b);
       if( b>=0 ) sqlite3Config.bUseLongDouble = b>0;
       rc = sqlite3Config.bUseLongDouble!=0;
       break;