[base] Add lz4 decompression

Add the lz4 decompression algorithm. This algorithm costs around 400
bytes.

Signed-off-by: Siemen Dhooghe <sdhooghe@google.com>

Author: siemen11

sw/device/lib/base/BUILD

@@ -266,6 +266,15 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "lz4",
+    srcs = ["lz4.c"],
+    hdrs = ["lz4.h"],
+    deps = [
+        "//sw/device/lib/base:macros",
+    ],
+)
+
 cc_test(
     name = "random_order_unittest",
     srcs = ["random_order_unittest.cc"],

sw/device/lib/base/lz4.c

@@ -0,0 +1,87 @@
+// Copyright lowRISC contributors (OpenTitan project).
+// Licensed under the Apache License, Version 2.0, see LICENSE for details.
+// SPDX-License-Identifier: Apache-2.0
+
+#include "sw/device/lib/base/lz4.h"
+
+#include <stdint.h>
+
+int LZ4_decompress(const char *src_in, char *dst_in, int compressed_size,
+                   int dst_capacity) {
+  const uint8_t *src = (const uint8_t *)src_in;
+  const uint8_t *src_end = src + compressed_size;
+  uint8_t *dst = (uint8_t *)dst_in;
+  uint8_t *dst_end = dst + dst_capacity;
+  uint8_t *dst_ptr = dst;
+
+  while (src < src_end) {
+    uint8_t token = *src++;
+
+    // Process literals
+    unsigned int lit_len = token >> 4;
+    if (lit_len == 15) {
+      uint8_t s;
+      do {
+        if (src >= src_end)
+          return -1;  // Unexpected end of src
+        s = *src++;
+        lit_len += s;
+      } while (s == 255);
+    }
+
+    // Check bounds for literals
+    if ((size_t)(src_end - src) < lit_len ||
+        (size_t)(dst_end - dst_ptr) < lit_len) {
+      return -1;  // Literal length exceeds remaining space
+    }
+
+    // Copy literals
+    for (unsigned int i = 0; i < lit_len; i++) {
+      *dst_ptr++ = *src++;
+    }
+
+    // The last sequence stops immediately after the literals.
+    if (src >= src_end) {
+      break;
+    }
+
+    // Process match offset
+    if (src + 2 > src_end)
+      return -1;  // Offset missing
+    // LZ4 offset is strictly little-endian
+    uint16_t offset = (uint16_t)(src[0] | (src[1] << 8));
+    src += 2;
+
+    if (offset == 0 || offset > (size_t)(dst_ptr - dst)) {
+      return -1;  // Offset out of bounds
+    }
+
+    // Process match length
+    unsigned int match_len = token & 0x0F;
+    if (match_len == 15) {
+      uint8_t s;
+      do {
+        if (src >= src_end)
+          return -1;  // Unexpected end of src
+        s = *src++;
+        match_len += s;
+      } while (s == 255);
+    }
+    match_len += 4;  // LZ4 min-match length is 4
+
+    // Check bounds for match destination
+    if ((size_t)(dst_end - dst_ptr) < match_len) {
+      return -1;  // Match exceeds destination capacity
+    }
+
+    // This must be a byte-by-byte copy, not memcpy,
+    // because LZ4 heavily relies on overlapping copies where the match length
+    // is longer than the match offset.
+    const uint8_t *match_ptr = dst_ptr - offset;
+    for (unsigned int i = 0; i < match_len; i++) {
+      *dst_ptr++ = *match_ptr++;
+    }
+  }
+
+  return (int)(dst_ptr - dst);
+}

sw/device/lib/base/lz4.h

@@ -0,0 +1,34 @@
+// Copyright lowRISC contributors (OpenTitan project).
+// Licensed under the Apache License, Version 2.0, see LICENSE for details.
+// SPDX-License-Identifier: Apache-2.0
+
+#ifndef OPENTITAN_SW_DEVICE_LIB_BASE_LZ4_H_
+#define OPENTITAN_SW_DEVICE_LIB_BASE_LZ4_H_
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Decompresses a raw LZ4 block into a destination buffer.
+ *
+ * This implementation is designed specifically for constrained embedded
+ * environments. It performs no dynamic allocation and requires no
+ * framing metadata.
+ *
+ * @param src Pointer to the compressed data.
+ * @param dst Pointer to the destination buffer.
+ * @param compressed_size The exact size of the compressed data.
+ * @param dst_capacity The maximum capacity of the destination buffer.
+ * @return The number of bytes decompressed, or a negative value on error.
+ */
+int LZ4_decompress(const char *src, char *dst, int compressed_size,
+                   int dst_capacity);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // OPENTITAN_SW_DEVICE_LIB_BASE_LZ4_H_