PDP-11: unsigned long multiplication is incorrect

[kwhr0]

When you compile and run the source below, 0xc0000000 is displayed, but the correct value is 0x40000000.

#include <stdio.h>
int main(void) {
	unsigned long a = 0x8000L, b = 0x8000L;
	printf("0x%lx\n", a * b);
	return 0;
}

Since PDP-11 only has signed multiplication instruction, replacing mach/pdp/libem/mlu4.s with something equivalent to the code below will work correctly, but it is large and slow.

uint32_t mlu4(uint32_t x, uint32_t y) {
	int16_t a0 = x, a1 = x >> 16, b0 = y, b1 = y >> 16; 
	int16_t a0l = a0 & 0x7fff, a0m = a0 < 0;
	int16_t a1l = a1 & 0x7fff, a1m = a1 < 0;
	int16_t b0l = b0 & 0x7fff, b0m = b0 < 0;
	int16_t b1l = b1 & 0x7fff, b1m = b1 < 0;
	uint32_t r = (int32_t)a0l * b0l;
	if (a0m) r += (int32_t)b0l << 15;
	if (b0m) r += (int32_t)a0l << 15;
	if (a0m && b0m) r += 0x40000000U;
	r += (int32_t)a1l * b0l << 16;
	if (a1m && b0l & 1) r += 0x80000000U;
	if (b0m && a1l & 1) r += 0x80000000U;
	r += (int32_t)a0l * b1l << 16;
	if (a0m && b1l & 1) r += 0x80000000U;
	if (b1m && a0l & 1) r += 0x80000000U;
	return r;
}

Is there a better way?