fix UB in constant folding of double -> signed integer conversion

tccgen.c

@@ -3249,7 +3249,10 @@ error:
                 vtop->c.i = (vtop->c.ld != 0);
             } else {
                 if(sf)
-                    vtop->c.i = vtop->c.ld;
+                    /* the range of [int64_t] is enough to hold the integer part of any float value.
+                       Meanwhile, converting negative double to unsigned integer is UB.
+                       So first convert to [int64_t] here. */
+                    vtop->c.i = (int64_t)vtop->c.ld;
                 else if (sbt_bt == VT_LLONG || (PTR_SIZE == 8 && sbt == VT_PTR))
                     ;
                 else if (sbt & VT_UNSIGNED)

tests/tests2/134_double_to_signed.c

@@ -0,0 +1,10 @@
+#include <stdio.h>
+int main() {
+  printf("%d\n", (int)-1.0);
+  double d = -1.0;
+  printf("%d\n", (int)d);
+
+  printf("%d\n", (int)-2147483648.0);
+  d = -2147483648.0;
+  printf("%d\n", (int)d);
+}

tests/tests2/134_double_to_signed.expect

@@ -0,0 +1,4 @@
+-1
+-1
+-2147483648
+-2147483648