From: Steinar H. Gunderson <sesse@debian.org>
Date: Sun, 31 May 2009 23:54:31 +0000 (+0200)
Subject: Microoptimization: Use a form of byteswapping that gcc realizes that is auto-zero... 
X-Git-Url: https://git.sesse.net/?p=fjl;a=commitdiff_plain;h=51c6a589f88aea02b49673ec40745d3a753976cd;hp=0932a33d5fa817a07778f85c05681ed77131bac6

Microoptimization: Use a form of byteswapping that gcc realizes that is auto-zero-extending.
---

diff --git a/bitsource.h b/bitsource.h
index 60744b3..f2f164d 100644
--- a/bitsource.h
+++ b/bitsource.h
@@ -15,9 +15,16 @@
 typedef uint64_t bitreservoir_t;
 typedef uint32_t bitreservoir_fill_t;
 
-static inline bitreservoir_fill_t read_bitreservoir_fill(uint8_t* source)
+// Note: We return bitreservoir_t here, so we can get implicit zero extension on amd64.
+static inline bitreservoir_t read_bitreservoir_fill(uint8_t* source)
 {
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+	bitreservoir_t ret;
+	asm("bswapl %1" : "=r" (ret) : "0" (*(bitreservoir_fill_t*)(source)));
+	return ret;
+#else
 	return ntohl(*(bitreservoir_fill_t*)(source));
+#endif
 }
 
 static const unsigned BITRESERVOIR_SIZE = 8 * sizeof(bitreservoir_t);
@@ -71,7 +78,7 @@ static inline void possibly_refill(struct bit_source* source, unsigned num_bits)
 	// Slower path (~99% of remaining invocations?)
 	assert(source->bits_available + BITRESERVOIR_FILL_SIZE < BITRESERVOIR_SIZE);
 	if (source->bytes_available >= sizeof(bitreservoir_fill_t)) {
-		bitreservoir_fill_t fill = read_bitreservoir_fill(source->byte_read_ptr);
+		bitreservoir_t fill = read_bitreservoir_fill(source->byte_read_ptr);
 		source->byte_read_ptr += sizeof(bitreservoir_fill_t);
 		source->bytes_available -= sizeof(bitreservoir_fill_t);
 		source->bits |= (bitreservoir_t)fill << (BITRESERVOIR_SIZE - BITRESERVOIR_FILL_SIZE - source->bits_available);