liblzma: LZMA decoder: Optimize loop comparison.

But now it needs one more local variable.

src/liblzma/lzma/lzma_decoder.c

@@ -261,7 +261,7 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
 	const size_t dict_start = dict.pos;
 
 	// Range decoder
-	rc_to_local(coder->rc, *in_pos);
+	rc_to_local(coder->rc, *in_pos, LZMA_IN_REQUIRED);
 
 	// State
 	uint32_t state = coder->state;
@@ -340,8 +340,7 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
 
 		// If there is not enough room for another LZMA symbol
 		// go to Resumable mode.
-		if (unlikely(rc_in_end - rc_in_ptr < LZMA_IN_REQUIRED
-				|| dict.pos == dict.limit))
+		if (unlikely(!rc_is_fast_allowed() || dict.pos == dict.limit))
 			goto slow;
 
 		// Decode the first bit from the next LZMA symbol.

src/liblzma/rangecoder/range_decoder.h

@@ -61,13 +61,21 @@ rc_read_init(lzma_range_decoder *rc, const uint8_t *restrict in,
 /// Makes local copies of range decoder and *in_pos variables. Doing this
 /// improves speed significantly. The range decoder macros expect also
 /// variables 'in' and 'in_size' to be defined.
-#define rc_to_local(range_decoder, in_pos) \
+#define rc_to_local(range_decoder, in_pos, fast_mode_in_required) \
 	lzma_range_decoder rc = range_decoder; \
 	const uint8_t *rc_in_ptr = in + (in_pos); \
 	const uint8_t *rc_in_end = in + in_size; \
+	const uint8_t *rc_in_fast_end \
+			= (rc_in_end - rc_in_ptr) <= (fast_mode_in_required) \
+			? rc_in_ptr \
+			: rc_in_end - (fast_mode_in_required); \
 	uint32_t rc_bound
 
 
+/// Evaluates to true if there is enough input remaining to use fast mode.
+#define rc_is_fast_allowed() (rc_in_ptr < rc_in_fast_end)
+
+
 /// Stores the local copes back to the range decoder structure.
 #define rc_from_local(range_decoder, in_pos) \
 do { \