nqdasm: reduce size of line structure

This structure represents essentially all the run-time memory use of the
disassembler, with 32k instances of it allocated.

On 64-bit systems, this cuts the size of a single instance from 16 bytes to
8 bytes.  On a 32-bit system, it goes from 12 bytes to 8 bytes.

1 files changed, 13 insertions, 11 deletions

diff --git a/nqdasm.c b/nqdasm.c
index 5449c06..f918136 100644
--- a/nqdasm.c
+++ b/nqdasm.c
@@ -25,27 +25,28 @@ const char *progname;
 	} while (0)
 
 static struct line {
-	const struct mnemonic *mnem;
-	uint16_t value;
 	int      label;
+	uint16_t value;
+	uint8_t  mnem;
 } lines[0x7fff];
 
 static size_t maxpc;
 
 #define LINE(addr)  &lines[(uint16_t)(addr) >> 1]
+#define MNEM(line)  ((line)->mnem < MNEM_LAST ? &mnemonics[(line)->mnem] : NULL)
 
-static const struct mnemonic *match_instruction(unsigned bits)
+static const uint8_t match_instruction(unsigned bits)
 {
 	for (int i = 0; i < MNEM_LAST; i++) {
 		const struct mnemonic *m = mnemonics + i;
-		if ((bits & m->mask) == m->bits) return m;
+		if ((bits & m->mask) == m->bits) return i;
 	}
-	return NULL;
+	return MNEM_LAST;
 }
 
 static inline int get_argvalue(const struct line *line, int argidx)
 {
-	const struct operand *op = line->mnem->operands + argidx;
+	const struct operand *op = MNEM(line)->operands + argidx;
 	switch (op->type) {
 	case REG:
 	case REGPTR: return       7 & (line->value >> op->shift);
@@ -65,10 +66,11 @@ static void assign_labels(void)
 {
 	/* mark those addresses that need a label */
 	for (size_t pc = 0; pc < maxpc; pc += 2) {
-		const struct line *l = LINE(pc);
-		if (!l->mnem) continue;
+		const struct line     *l = LINE(pc);
+		const struct mnemonic *m = MNEM(l);
+		if (!m) continue;
 
-		const struct operand *ops = l->mnem->operands;
+		const struct operand *ops = m->operands;
 		for (int j = 0; j < 3 && ops[j].type; j++) {
 			if (ops[j].type == PCOFF) {
 				struct line *target = get_pcoff_target(pc, j);
@@ -88,7 +90,7 @@ static void assign_labels(void)
 static void print_operand(FILE *out, size_t pc, size_t argidx)
 {
 	const struct line  *inst = LINE(pc);
-	const struct operand *op = inst->mnem->operands + argidx;
+	const struct operand *op = MNEM(inst)->operands + argidx;
 
 	switch (op->type) {
 	case REG:    fprintf(out,  "r%d", get_argvalue(inst, argidx)); break;
@@ -132,7 +134,7 @@ static void disassemble(FILE *in, FILE *out)
 	for (size_t pc = 0; pc < maxpc; pc += 2) {
 		const struct line *line  = LINE(pc);
 		unsigned bits            = line->value;
-		const struct mnemonic *m = line->mnem;
+		const struct mnemonic *m = MNEM(line);
 
 		if (line->label >= 0) fprintf(out, "L%d:", line->label);