Quote from: jgarzik on November 28, 2012, 03:11:49 AM

It continues to be true for all known compilers I've seen or heard of, when compared with a simple, native machine integer as used by the current implementation.

OK, show of hands. You must have heard of the existience of Microsoft Visual Studio C++/C compiler. I wrote a short test program that tests strong-typed little-endian integer on the Intel architecture, which will be the most important case.

Code:

#include

union le_int {
unsigned char b[4];
int i;
void operator =(int a) { i = a; }
operator int() { return i; }
};

int lehmer1(int n)
{
int r = 1;
while (n--)
r = (16807 * r) % 2147483647;
return r;
}

int lehmer2(int n)
{
union le_int r;

r = 1;
while (n--)
r = (16807 * r) % 2147483647;
return r;
}

void main()
{
printf("%d\n",lehmer1(2112));
printf("%d\n",lehmer2(2112));
}

and the generated "Release Win32" code, with everything at default:

Code:

00311000 push edi
printf("%d\n",lehmer1(2112));
00311001 mov ecx,840h
00311006 mov edx,1
0031100B mov edi,7FFFFFFFh
00311010 imul edx,edx,41A7h
00311016 mov eax,edx
00311018 cdq
00311019 idiv eax,edi
0031101B dec ecx
0031101C jne main+10h (0311010h)
0031101E push edx
0031101F push 312100h
00311024 call dword ptr ds:[312090h]
0031102A add esp,8
printf("%d\n",lehmer2(2112));
0031102D mov ecx,840h
00311032 mov edx,1
00311037 imul edx,edx,41A7h
0031103D mov eax,edx
0031103F cdq
00311040 idiv eax,edi
00311042 dec ecx
00311043 jne main+37h (0311037h)
00311045 push edx
00311046 push 312100h
0031104B call dword ptr ds:[312090h]
00311051 add esp,8

Could anyone here can give an example of a popular compiler that produces different code for those two little functions? Maybe the test program needs to be somehow changed to trip up the optimizer? I'm seriously asking. I have code like this (and pure C equivalent with #define macros instead of operators()) in wide deployment for many years now on a variety of platforms.

I really only have my Windows laptop with me. I would have to ask people to unpack and boot my usual development environment for remote access.

Meanwhile I'm going to see if I can trip-up the Visual Studio somehow by rewriting this code in various ways.