how to generate x86 "narrowing" divide instruction

[Jay Foad]

On 7 March 2011 18:32, Ian Lance Taylor <iant@google.com> wrote:
>> $ cat rand.c
>> #include <stdint.h>
>> uint32_t rand(uint32_t x) { return (uint64_t)x * 16807 % 0x7FFFFFFF; }

> The div instruction is difficult for gcc to use for 64->32 division
> because it generates an exception if the quotient is too large.  For
> code like the above gcc would have to insert runtime checks to make sure
> that the division did not overflow, even though the code only cares
> about the remainder.

I should have said explicitly: the division above can't overflow,
because the dividend can't be bigger than about 2^31 * 16807 (in
absolute value), so the quotient won't be bigger than about 16807. I
was hoping that value range propagation could work that out, so that
the compiler would know it's safe to use div.

Thanks,
Jay.