Strange conversion to int64

[Tony Bernardin]

Hey, I'm having a little trouble understanding the conversion gcc is
doing in a statement with the following types:

int64 = int32 * uint32

This is running on a 64bit machine with gcc (GCC) 4.4.2 20091027 (Red
Hat 4.4.2-7)

Following simple code snippet:

  1 #include <stdint.h>
  2 #include <iostream>
  3
  4 using namespace std;
  5
  6 int main(int argc, char* argv[])
  7 {
  8     int32_t  neg  = -1;
  9     uint32_t mult = 64;
 10
 11     int64_t res  = neg * mult;
 12     int32_t res2 = neg * mult;
 13
 14     cout << res << endl << res2 << endl;
 15
 16     uint64_t mult64 = 64;
 17
 18     res  = neg * mult64;
 19     res2 = neg * mult64;
 20
 21     cout << res << endl << res2 << endl;
 22
 23     return 0;
 24 }

with the output

4294967232
-64
-64
-64

When compiled it gives an understandable warning

main.cpp: In function ‘int main(int, char**)’:
main.cpp:19: warning: conversion to ‘int32_t’ from ‘uint64_t’ may
alter its value

still that operation will generate the output I expect.

Is there some strange casting rule that I'm not following properly? In
particular I find lines 11 and 18 interesting as the only change
between the type of 'mult' uint32 vs uint64.

cheers,
Tony