The following is really ugly (until you put it in a macro), but would work:
#define HALF_BITS (sizeof(size_t) * 4U)
#define LO_MASK ((((size_t)1) << HALF_BITS)-1)
#define HI_MASK (LO_MASK<<HALF_BITS)
if (
/* Tim's fast path */
((src1 | src2) & HI_MASK) && (
/* Ugly slower path */
(((src1&LO_MASK) * (src2>>HALF_BITS)) & HI_MASK) |
(((src1>>HALF_BITS) * (src2&LO_MASK)) & HI_MASK) |
(src1>>HALF_BITS) * (src2>>HALF_BITS))
) {
goto overflow;
}
Of course it's hoping for some common subexpression elimination from the
compiler.
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