Hi Gurus,

I am looking for C code for multiplying 32bit by 32bit operands and
getting a 64bit product. i.e.

U32 m1, m2;
U64 p = m1 * m2;

Of course I can use

U64 p = (U64) m1 * m2;

But, disassembly listing shows that generated code calls a 64bit x
64bit multiplication routine.

The target I am working on has a 32bit x 32 bit => 64bit machine
instruction but doesn't have any 64bit x 64 bit instruction. Before I
wet my hand on assembly programming, I love to see any C code solution
to it.

TIA.

Perry Yuan

On May 14, 6:48 pm, "perry.yuan" gmail.com> wrote:

perry.yuan wrote:

"perry.yuan" gmail.com> writes:

On May 14, 10:48 am, "perry.yuan" gmail.com> wrote:

On 5月15日, 上午6时40分, user923005 connx.com> wrote:

On 14 Mai, 19:48, "perry.yuan" gmail.com> wrote:

It might not be exactly what you were looking for, but
years ago I wrote a function to multiply two 64 bit
unsigned numbers based on 32 bit unsigned logic (the
type uinttype in the example below is 32 bit).
This function makes sense when absolutely no 64 bit
arithmetic is available:

#define LOWER_16(A) ((A) & 0177777L)
#define UPPER_16(A) (((A) >> 16) & 0177777L)
#define LOWER_32(A) ((A) & (uinttype) 037777777777L)

static void mult_64 (uinttype a_high, uinttype a_low,
uinttype b_high, uinttype b_low,
uinttype *c_high, uinttype *c_low)

Thanks for all replies and comments. My thinking is: in C language,
operator * for integral types is type-closed (i.e. int32 * int32 =>
int32) but not value-closed (i.e. int32 * int32 => int63). So that
there is no way to do any value-closed integral * operation at C
language level.

In article <9303863a-44db-450c-b2d6-9f09497cd303@l17g2000pri.googlegroups.com>,
perry.yuan gmail.com> wrote:

It is value-closed for the unsigned integral types.

unsigned int R = (unsigned int) A * (unsigned int) B;

You may wish to interpret the result in terms of the original signs
of A and B. R would be interpreted as negative if ((A < 0) ^ (B < 0))
(That's exclusive OR that I used.) If R > INT_MAX and should be
negative, or if R <= INT_MAX and should be positive, you have
quite straight-forward re-interpretations available, but if
R <= INT_MAX and should be negative or R > INT_MAX and should be positive,
it is less clear what the signed equivilent of R should be.