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How to multiply 32 bit integers in c

Performance:

#define HIGH32(V64) ((uint32_t)((V64 >> 32)&0xffFFffFF))
#define LOW32(V64) ((uint32_t)(V64&0xffFFffFF))

uint32_t a = 0xffFFffFF;
uint32_t b = 0xffFFffFF;
uint64_t res = a * b;
printf("res = %08X %08X\n", HIGH32(res), LOW32(res));

Gives:

"res = 00000000 00000001"

But I expect: fffffffe00000001. What have I done wrong? The only purpose:

res = 0x0123456789ABCDEF;
printf("res = %08X %08X\n", HIGH32(res), LOW32(res));

gives

res = 01234567 89ABCDEF

Environment:

$gcc --version
gcc (GCC) 4.8.3
Copyright (C) 2013 Free Software Foundation, Inc.

$ gcc -v
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-cygwin/4.8.3/lto-wrapper.exe
dest arch: i686-pc-cygwin

$ file a.exe
a.exe: PE32 executable (console) Intel 80386, for MS Windows
+3


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1 answer


You currently have:

uint64_t res = (uint32_t) a * (uint32_t) b;


You will need to promote the arguments to 64 bit numbers before multiplying. Therefore:

uint64_t res = (uint64_t) a * b;
+9


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