From e446ab7a18abfde18f8d1cf02a914df72b1370e3 Mon Sep 17 00:00:00 2001
From: Jia Tan <jiat0218@gmail.com>
Date: Mon, 12 Feb 2024 17:09:10 +0200
Subject: liblzma: Creates separate "safe" range decoder mode.

The new "safe" range decoder mode is the same as old range decoder, but
now the default behavior of the range decoder will not check if there is
enough input or output to complete the operation. When the buffers are
close to fully consumed, the "safe" operations must be used instead. This
will improve speed because it will reduce the number of branches needed
for most of the range decoder operations.
---
 src/liblzma/rangecoder/range_decoder.h | 77 +++++++++++++++++++++++++---------
 1 file changed, 57 insertions(+), 20 deletions(-)

(limited to 'src/liblzma/rangecoder')

diff --git a/src/liblzma/rangecoder/range_decoder.h b/src/liblzma/rangecoder/range_decoder.h
index a38e6aa3..5e813f56 100644
--- a/src/liblzma/rangecoder/range_decoder.h
+++ b/src/liblzma/rangecoder/range_decoder.h
@@ -80,10 +80,22 @@ do { \
 	((range_decoder).code == 0)
 
 
-/// Read the next input byte if needed. If more input is needed but there is
+// Read the next input byte if needed.
+#define rc_normalize() \
+do { \
+	if (rc.range < RC_TOP_VALUE) { \
+		rc.range <<= RC_SHIFT_BITS; \
+		rc.code = (rc.code << RC_SHIFT_BITS) | in[rc_in_pos++]; \
+	} \
+} while (0)
+
+
+/// If more input is needed but there is
 /// no more input available, "goto out" is used to jump out of the main
-/// decoder loop.
-#define rc_normalize(seq) \
+/// decoder loop. The "_safe" macros are used in the Resumable decoder
+/// mode in order to save the sequence to continue decoding from that
+/// point later.
+#define rc_normalize_safe(seq) \
 do { \
 	if (rc.range < RC_TOP_VALUE) { \
 		if (unlikely(rc_in_pos == in_size)) { \
@@ -99,7 +111,7 @@ do { \
 /// Start decoding a bit. This must be used together with rc_update_0()
 /// and rc_update_1():
 ///
-///     rc_if_0(prob, seq) {
+///     rc_if_0(prob) {
 ///         rc_update_0(prob);
 ///         // Do something
 ///     } else {
@@ -107,8 +119,14 @@ do { \
 ///         // Do something else
 ///     }
 ///
-#define rc_if_0(prob, seq) \
-	rc_normalize(seq); \
+#define rc_if_0(prob) \
+	rc_normalize(); \
+	rc_bound = (rc.range >> RC_BIT_MODEL_TOTAL_BITS) * (prob); \
+	if (rc.code < rc_bound)
+
+
+#define rc_if_0_safe(prob, seq) \
+	rc_normalize_safe(seq); \
 	rc_bound = (rc.range >> RC_BIT_MODEL_TOTAL_BITS) * (prob); \
 	if (rc.code < rc_bound)
 
@@ -136,9 +154,21 @@ do { \
 /// This macro is used as the last step in bittree reverse decoders since
 /// those don't use "symbol" for anything else than indexing the probability
 /// arrays.
-#define rc_bit_last(prob, action0, action1, seq) \
+#define rc_bit_last(prob, action0, action1) \
 do { \
-	rc_if_0(prob, seq) { \
+	rc_if_0(prob) { \
+		rc_update_0(prob); \
+		action0; \
+	} else { \
+		rc_update_1(prob); \
+		action1; \
+	} \
+} while (0)
+
+
+#define rc_bit_last_safe(prob, action0, action1, seq) \
+do { \
+	rc_if_0_safe(prob, seq) { \
 		rc_update_0(prob); \
 		action0; \
 	} else { \
@@ -150,26 +180,33 @@ do { \
 
 /// Decodes one bit, updates "symbol", and runs action0 or action1 depending
 /// on the decoded bit.
-#define rc_bit(prob, action0, action1, seq) \
+#define rc_bit(prob, action0, action1) \
 	rc_bit_last(prob, \
+		symbol <<= 1; action0, \
+		symbol = (symbol << 1) + 1; action1);
+
+
+#define rc_bit_safe(prob, action0, action1, seq) \
+	rc_bit_last_safe(prob, \
 		symbol <<= 1; action0, \
 		symbol = (symbol << 1) + 1; action1, \
 		seq);
 
-
-/// Like rc_bit() but add "case seq:" as a prefix. This makes the unrolled
-/// loops more readable because the code isn't littered with "case"
-/// statements. On the other hand this also makes it less readable, since
-/// spotting the places where the decoder loop may be restarted is less
-/// obvious.
-#define rc_bit_case(prob, action0, action1, seq) \
-	case seq: rc_bit(prob, action0, action1, seq)
+/// Decode a bit without using a probability.
+#define rc_direct(dest) \
+do { \
+	rc_normalize(); \
+	rc.range >>= 1; \
+	rc.code -= rc.range; \
+	rc_bound = UINT32_C(0) - (rc.code >> 31); \
+	rc.code += rc.range & rc_bound; \
+	dest = (dest << 1) + (rc_bound + 1); \
+} while (0)
 
 
-/// Decode a bit without using a probability.
-#define rc_direct(dest, seq) \
+#define rc_direct_safe(dest, seq) \
 do { \
-	rc_normalize(seq); \
+	rc_normalize_safe(seq); \
 	rc.range >>= 1; \
 	rc.code -= rc.range; \
 	rc_bound = UINT32_C(0) - (rc.code >> 31); \
-- 
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