I’m pretty sure that at the time this code was written, when R was used almost exclusively on 32-bit hardware, this was the option that performed best across a range of platforms. Also this pre-dated having int64_t available, so reliably getting a 64-bit integer variable across all platforms might have been challenging.
You could look at using __builtin_mul_overflow() when available to see if that helps. I suspect it doesn’t by enough to warrant the hassle of configure checks, but might be worth a try. Best, luke > On Jun 26, 2026, at 5:49 PM, Michael Chirico <[email protected]> > wrote: > > Hi all, > > I submit myself humbly before you trying to understand something very > basic in low-level R. > > The routine R_integer_times [1] is used to multiply two integers. It > works by multiplying the two integers _as integers_, then only > afterwards comparing the result of multiplying them _as doubles_, and > giving NA+warning if they don't match. Implicitly, it relies on x*y > wrapping around if x*y doesn't fit in 'int', however, strictly > speaking this is relying on UB. The comment acknowledges this, but > doesn't explain why we rely on it. > > We can trigger this easily: 2L * .Machine$integer.max > > This implementation has been around in R for many years. Why? Isn't it > preferable to use a 64-bit type to compute the multiplication? Even on > 32-bit platforms it should be faster than the conversion to double > that's done now. I feel I must be missing something. > > This patch survives 'make check': > > Index: src/main/arithmetic.c > =================================================================== > --- src/main/arithmetic.c (revision 90193) > +++ src/main/arithmetic.c (working copy) > @@ -348,15 +348,14 @@ > return x - y; > } > > -#define GOODIPROD(x, y, z) ((double) (x) * (double) (y) == (z)) > static R_INLINE int R_integer_times(int x, int y, bool *pnaflag) > { > if (x == NA_INTEGER || y == NA_INTEGER) > return NA_INTEGER; > else { > - int z = x * y; // UBSAN will warn if this overflows (happens in bda) > - if (GOODIPROD(x, y, z) && z != NA_INTEGER) > - return z; > + int64_t z = (int64_t)x * (int64_t)y; > + if (z <= R_INT_MAX && z >= R_INT_MIN) > + return (int)z; > else { > if (pnaflag != NULL) > *pnaflag = true; > > And this script shows the 30% performance improvement: > > x <- y <- rep(10000L, 5e7) > system.time(for(i in 1:100) x * y) > # current trunk > # user system elapsed > # 13.631 1.056 14.691 > # with patch > # user system elapsed > # 9.231 1.207 10.445 > > Mike C > > [1] > https://github.com/r-devel/r-svn/blob/4fa83d820f50676eef08c38fff0cdd6bfc10f5e5/src/main/arithmetic.c#L351-L366 > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
