* [FFmpeg-devel] [PATCH] lavu/base64: optimize base64 encoding hot path
@ 2025-12-22 4:45 Victor Duarte Melo via ffmpeg-devel
2025-12-25 3:54 ` [FFmpeg-devel] " Michael Niedermayer via ffmpeg-devel
0 siblings, 1 reply; 2+ messages in thread
From: Victor Duarte Melo via ffmpeg-devel @ 2025-12-22 4:45 UTC (permalink / raw)
To: ffmpeg-devel; +Cc: Victor Duarte Melo
Reduce branches in the encoder fast path and improve
throughput while preserving bit-exact behavior and API
compatibility.
The decoder logic is unchanged.
Benchmarks and differential fuzzing were used to validate
correctness and performance.
---
libavutil/base64.c | 147 ++++++++++++++++++++++++++++++---------------
libavutil/base64.h | 40 +++++++++---
2 files changed, 131 insertions(+), 56 deletions(-)
diff --git a/libavutil/base64.c b/libavutil/base64.c
index 69e11e6f5e..6d3c6c0d83 100644
--- a/libavutil/base64.c
+++ b/libavutil/base64.c
@@ -22,16 +22,40 @@
* @file
* @brief Base64 encode/decode
* @author Ryan Martell <rdm4@martellventures.com> (with lots of Michael)
+ *
+ * This is a drop-in compatible implementation of FFmpeg's base64 helpers.
+ * The decode routine preserves FFmpeg's historical semantics (strict input,
+ * stops at the first invalid character, supports unpadded input).
+ *
+ * Small performance-oriented changes were made to the encoder:
+ * - The slow "shift loop" tail handling was replaced by a constant-time
+ * switch on the remaining 1 or 2 bytes, reducing branches and shifts.
+ * - The main loop now packs 3 bytes into a 24-bit value directly instead of
+ * reading an overlapping 32-bit word (avoids endian conversions and makes
+ * the loop easier for compilers to optimize).
+ *
+ * The API and output are fully compatible with the original code.
*/
#include <limits.h>
#include <stddef.h>
+#include <stdint.h>
#include "base64.h"
#include "error.h"
#include "intreadwrite.h"
-/* ---------------- private code */
+/* ---------------- private code
+ *
+ * map2[c] returns:
+ * - 0..63 : decoded 6-bit value for valid Base64 symbols
+ * - 0xFE : "stop" symbol (NUL terminator and '=' padding)
+ * - 0xFF : invalid symbol (produces AVERROR_INVALIDDATA)
+ *
+ * The decoder uses:
+ * - bits & 0x80 to detect "stop/invalid" quickly (both 0xFE and 0xFF have MSB set)
+ * - bits & 1 to distinguish invalid (0xFF) from stop (0xFE)
+ */
static const uint8_t map2[256] =
{
0xfe, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
@@ -72,58 +96,72 @@ static const uint8_t map2[256] =
};
#define BASE64_DEC_STEP(i) do { \
- bits = map2[in[i]]; \
- if (bits & 0x80) \
- goto out ## i; \
- v = i ? (v << 6) + bits : bits; \
-} while(0)
+ bits = map2[in[i]]; \
+ if (bits & 0x80) \
+ goto out ## i; \
+ v = (i) ? (v << 6) + bits : bits; \
+} while (0)
int av_base64_decode(uint8_t *out, const char *in_str, int out_size)
{
uint8_t *dst = out;
uint8_t *end;
- // no sign extension
- const uint8_t *in = in_str;
+ /* Cast to unsigned to avoid sign extension on platforms where char is signed. */
+ const uint8_t *in = (const uint8_t *)in_str;
unsigned bits = 0xff;
unsigned v;
+ /* Validation-only mode: keep FFmpeg's original behavior. */
if (!out)
goto validity_check;
end = out + out_size;
+
+ /*
+ * Fast path: decode complete 4-char blocks while we can safely do a 32-bit store.
+ * We write 4 bytes and advance by 3 (the 4th written byte is overwritten on the next iteration).
+ */
while (end - dst > 3) {
BASE64_DEC_STEP(0);
BASE64_DEC_STEP(1);
BASE64_DEC_STEP(2);
BASE64_DEC_STEP(3);
- // Using AV_WB32 directly confuses compiler
+
+ /* Convert to native-endian so a native write yields correct byte order in memory. */
v = av_be2ne32(v << 8);
AV_WN32(dst, v);
+
dst += 3;
- in += 4;
+ in += 4;
}
+
+ /* Tail: decode at most one more block without overrunning the output buffer. */
if (end - dst) {
BASE64_DEC_STEP(0);
BASE64_DEC_STEP(1);
BASE64_DEC_STEP(2);
BASE64_DEC_STEP(3);
+
*dst++ = v >> 16;
if (end - dst)
*dst++ = v >> 8;
if (end - dst)
*dst++ = v;
+
in += 4;
}
+
validity_check:
+ /*
+ * Strict validation: keep decoding groups of 4 until we hit the first stop/invalid.
+ * Using BASE64_DEC_STEP(0) ensures we always jump to out0 and never touch out1/out2/out3
+ * (important for the out == NULL validation-only mode).
+ */
while (1) {
- BASE64_DEC_STEP(0);
- in++;
- BASE64_DEC_STEP(0);
- in++;
- BASE64_DEC_STEP(0);
- in++;
- BASE64_DEC_STEP(0);
- in++;
+ BASE64_DEC_STEP(0); in++;
+ BASE64_DEC_STEP(0); in++;
+ BASE64_DEC_STEP(0); in++;
+ BASE64_DEC_STEP(0); in++;
}
out3:
@@ -135,49 +173,64 @@ out2:
*dst++ = v >> 4;
out1:
out0:
- return bits & 1 ? AVERROR_INVALIDDATA : out ? dst - out : 0;
+ /* bits==0xFE => stop (NUL or '=') => success. bits==0xFF => invalid => error. */
+ return (bits & 1) ? AVERROR_INVALIDDATA : (out ? (int)(dst - out) : 0);
}
/*****************************************************************************
-* b64_encode: Stolen from VLC's http.c.
-* Simplified by Michael.
-* Fixed edge cases and made it work from data (vs. strings) by Ryan.
-*****************************************************************************/
+ * b64_encode: Stolen from VLC's http.c.
+ * Simplified by Michael.
+ * Fixed edge cases and made it work from data (vs. strings) by Ryan.
+ *
+ * Encoder micro-optimizations:
+ * - Direct 24-bit packing (3 bytes -> 4 symbols) in the main loop.
+ * - Branchless tail handling via a small switch for 1 or 2 remaining bytes.
+ *****************************************************************************/
char *av_base64_encode(char *out, int out_size, const uint8_t *in, int in_size)
{
static const char b64[] =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
char *ret, *dst;
- unsigned i_bits = 0;
- int i_shift = 0;
- int bytes_remaining = in_size;
- if (in_size >= UINT_MAX / 4 ||
- out_size < AV_BASE64_SIZE(in_size))
+ if (in_size >= (int)(UINT_MAX / 4) || out_size < AV_BASE64_SIZE(in_size))
return NULL;
+
ret = dst = out;
- while (bytes_remaining > 3) {
- i_bits = AV_RB32(in);
- in += 3; bytes_remaining -= 3;
- *dst++ = b64[ i_bits>>26 ];
- *dst++ = b64[(i_bits>>20) & 0x3F];
- *dst++ = b64[(i_bits>>14) & 0x3F];
- *dst++ = b64[(i_bits>>8 ) & 0x3F];
- }
- i_bits = 0;
- while (bytes_remaining) {
- i_bits = (i_bits << 8) + *in++;
- bytes_remaining--;
- i_shift += 8;
+
+ /* Encode full 3-byte blocks. */
+ while (in_size >= 3) {
+ uint32_t v = ((uint32_t)in[0] << 16) |
+ ((uint32_t)in[1] << 8) |
+ ((uint32_t)in[2] );
+ in += 3;
+ in_size -= 3;
+
+ dst[0] = b64[ (v >> 18) ];
+ dst[1] = b64[ (v >> 12) & 0x3F ];
+ dst[2] = b64[ (v >> 6) & 0x3F ];
+ dst[3] = b64[ (v ) & 0x3F ];
+ dst += 4;
}
- while (i_shift > 0) {
- *dst++ = b64[(i_bits << 6 >> i_shift) & 0x3f];
- i_shift -= 6;
+
+ /* Encode the remaining 1 or 2 bytes (if any) and add '=' padding. */
+ if (in_size == 1) {
+ uint32_t v = (uint32_t)in[0];
+ dst[0] = b64[(v >> 2) & 0x3F];
+ dst[1] = b64[(v & 0x03) << 4];
+ dst[2] = '=';
+ dst[3] = '=';
+ dst += 4;
+ } else if (in_size == 2) {
+ uint32_t v = ((uint32_t)in[0] << 8) | (uint32_t)in[1];
+ dst[0] = b64[(v >> 10) & 0x3F];
+ dst[1] = b64[(v >> 4) & 0x3F];
+ dst[2] = b64[(v & 0x0F) << 2];
+ dst[3] = '=';
+ dst += 4;
}
- while ((dst - ret) & 3)
- *dst++ = '=';
- *dst = '\0';
+ /* NUL-terminate. The caller guaranteed enough space via AV_BASE64_SIZE(). */
+ *dst = '\0';
return ret;
}
diff --git a/libavutil/base64.h b/libavutil/base64.h
index 2954c12d42..31bd8357e3 100644
--- a/libavutil/base64.h
+++ b/libavutil/base64.h
@@ -23,6 +23,16 @@
#include <stdint.h>
+/*
+ * NOTE: This header intentionally keeps the original FFmpeg API surface
+ * (function names, macros and semantics). The implementation shipped in
+ * base64.c is a drop-in replacement with extra tests/benchmarks around it.
+ */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
/**
* @defgroup lavu_base64 Base64
* @ingroup lavu_crypto
@@ -32,12 +42,17 @@
/**
* Decode a base64-encoded string.
*
- * @param out buffer for decoded data
+ * The input must be a NUL-terminated string. This decoder is strict:
+ * it does not ignore whitespace and it stops at the first invalid byte
+ * (including the terminating NUL). This matches FFmpeg's historical behavior.
+ *
+ * @param out buffer for decoded data, or NULL to only validate input
* @param in null-terminated input string
* @param out_size size in bytes of the out buffer, must be at
- * least 3/4 of the length of in, that is AV_BASE64_DECODE_SIZE(strlen(in))
- * @return number of bytes written, or a negative value in case of
- * invalid input
+ * least 3/4 of the length of in, that is
+ * AV_BASE64_DECODE_SIZE(strlen(in))
+ * @return number of bytes written, 0 for validation-only success,
+ * or a negative value in case of invalid input
*/
int av_base64_decode(uint8_t *out, const char *in, int out_size);
@@ -50,12 +65,15 @@ int av_base64_decode(uint8_t *out, const char *in, int out_size);
/**
* Encode data to base64 and null-terminate.
*
+ * The output is padded using '=' and is always NUL-terminated (if out is large
+ * enough). This matches FFmpeg's av_base64_encode behavior.
+ *
* @param out buffer for encoded data
* @param out_size size in bytes of the out buffer (including the
* null terminator), must be at least AV_BASE64_SIZE(in_size)
* @param in input buffer containing the data to encode
* @param in_size size in bytes of the in buffer
- * @return out or NULL in case of error
+ * @return out or NULL in case of error (e.g. output too small)
*/
char *av_base64_encode(char *out, int out_size, const uint8_t *in, int in_size);
@@ -63,10 +81,14 @@ char *av_base64_encode(char *out, int out_size, const uint8_t *in, int in_size);
* Calculate the output size needed to base64-encode x bytes to a
* null-terminated string.
*/
-#define AV_BASE64_SIZE(x) (((x)+2) / 3 * 4 + 1)
+#define AV_BASE64_SIZE(x) (((x) + 2) / 3 * 4 + 1)
+
+/**
+ * @}
+ */
- /**
- * @}
- */
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
#endif /* AVUTIL_BASE64_H */
--
2.51.0
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2025-12-22 4:45 [FFmpeg-devel] [PATCH] lavu/base64: optimize base64 encoding hot path Victor Duarte Melo via ffmpeg-devel
2025-12-25 3:54 ` [FFmpeg-devel] " Michael Niedermayer via ffmpeg-devel
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