Sketch of a few ways to store 8 or 16 elements indexed by a packed byte array.

resource_table.cc

#include <xmmintrin.h>
#include <stdint.h>

struct ResourceTable8
{
	uint64_t type_bits_;
	uintptr_t elements_[8];

	inline uint64_t mask_for_type(uint8_t type)
	{
		const uint64_t type_bits = type_bits_;
		const uint64_t splatted_type = 0x0101010101010101ull * type;
		const uint64_t zeroes = type_bits ^ splatted_type;

		// http://www.azillionmonkeys.com/qed/asmexample.html

		const uint64_t mask_lo = 0x7F7F7F7F7F7F7F7Full;
		const uint64_t mask_hi = 0x8080808080808080ull;
		const uint64_t inv_matches = (((zeroes & mask_lo) + mask_lo) | zeroes) & mask_hi;
		const uint64_t matches = inv_matches ^ mask_hi;
		return matches;
	}

	inline uint32_t mask_for_type_mm(uint8_t type)
	{
		const __m64 type_bits = _mm_cvtsi64_m64(type_bits_);
		const __m64 splatted_type = _mm_set1_pi8(type);
		const __m64 mask = _mm_cmpeq_pi8(type_bits, splatted_type);
		return _mm_movemask_pi8(mask);
	}

	void forall(uint8_t type);
	void forall_mm(uint8_t type);
};

struct ResourceTable16
{
	__m128i type_bits_;
	uintptr_t elements_[16];

	inline uint32_t mask_for_type_xmm(uint8_t type)
	{
		const __m128i type_bits = _mm_loadu_si128(&type_bits_);
		const __m128i splatted_type = _mm_set1_epi8(type);
		const __m128i mask = _mm_cmpeq_epi8(type_bits, splatted_type);
		return _mm_movemask_epi8(mask);
	}

	void forall_xmm(uint8_t type);
};

#include <stdio.h>

void ResourceTable8::forall(uint8_t type)
{
	uint64_t mask = mask_for_type(type);
	while (mask)
	{
		const int bit_index = __builtin_ctzll(mask);
		const int index = (bit_index - 7) >> 3;

		printf("ResourceTable8::forall %d\n", index);

		mask &= mask - 1;
	}
}

void ResourceTable8::forall_mm(uint8_t type)
{
	uint32_t mask = mask_for_type_mm(type);
	while (mask)
	{
		const int index = __builtin_ctz(mask);

		printf("ResourceTable8::forall_mm %d\n", index);

		mask &= mask - 1;
	}
}

void ResourceTable16::forall_xmm(uint8_t type)
{
	uint32_t mask = mask_for_type_xmm(type);
	while (mask)
	{
		const int index = __builtin_ctz(mask);

		printf("ResourceTable16::forall_xmm %d\n", index);

		mask &= mask - 1;
	}
}