From:	"A. Sinan Unur" <asu1 AT c-o-r-n-e-l-l DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Problem with multiplications
Date:	21 Sep 2002 14:33:28 GMT
Organization:	Cornell University
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"Carlo Bramini" <cbramix AT libero DOT it> wrote in
news:M2%i9.25731$Av4 DOT 488132 AT twister2 DOT libero DOT it: 

> Hi to eveyone,
> I have a problem with DJGPP, but I'm going to think it's common with
> all i386 target versions of this compiler.
> I have two 32 bit numbers and I want to do:
> 
> c = (a*b) >> 32;

<snip>

> It's quite strange, because on other platforms I got correct results.
> I tried MIPS, ARM, H8 and SH4, and it works as expected.

"as expected" is the key phrase here. i would not expect a 32 bit number 
to hold 64 bits.

that said:

int main(void)
{
	unsigned a;
	unsigned b = 0xf0f0f0f0, c=01010101;
	a = ((unsigned long long) b*c) >> 32;
	return 0;
}

gcc -S i.c results in:

        .file   "i.c"
        .section .text
        .p2align 1
.globl _main
_main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        andl    $-16, %esp
        movl    $0, %eax
        subl    %eax, %esp
        movl    $-252645136, -8(%ebp)
        movl    $266305, -12(%ebp)
        movl    -12(%ebp), %eax
        mull    -8(%ebp)
        movl    %edx, %eax
        movl    $0, %edx
        movl    %eax, -4(%ebp)
        movl    $0, %eax
        leave
        ret
        .ident  "GCC: (GNU) 3.1"

adding optimization, and doing some tricks to avoid gcc computing the 
result at compile time lead to a longer listing but still one mull.

-- 
A. Sinan Unur
asu1 AT c-o-r-n-e-l-l DOT edu
Remove dashes for address
Spam bait: mailto:uce AT ftc DOT gov

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