py/qstr: Add support for MICROPY_QSTR_BYTES_IN_HASH=0.

This disables using qstr hashes altogether, which saves RAM and flash
(two bytes per interned string on a typical build) as well as code size.
On PYBV11 this is worth over 3k flash.

qstr comparison will now be done just by length then data. This affects
qstr_find_strn although this has a negligible performance impact as, for a
given comparison, the length and first character will ~usually be
different anyway.

String hashing (e.g. builtin `hash()` and map.c) now need to compute the
hash dynamically, and for the map case this does come at a performance
cost.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <[email protected]>

py/makeqstrdata.py

@@ -295,7 +295,8 @@ def compute_hash(qstr, bytes_hash):
     for b in qstr:
         hash = (hash * 33) ^ b
     # Make sure that valid hash is never zero, zero means "hash not computed"
-    return (hash & ((1 << (8 * bytes_hash)) - 1)) or 1
+    # if bytes_hash is zero, assume a 16-bit mask (to match qstr.c)
+    return (hash & ((1 << (8 * (bytes_hash or 2))) - 1)) or 1
 
 
 def qstr_escape(qst):

py/qstr.c

@@ -42,7 +42,11 @@
 // A qstr is an index into the qstr pool.
 // The data for a qstr is \0 terminated (so they can be printed using printf)
 
+#if MICROPY_QSTR_BYTES_IN_HASH
 #define Q_HASH_MASK ((1 << (8 * MICROPY_QSTR_BYTES_IN_HASH)) - 1)
+#else
+#define Q_HASH_MASK (0xffff)
+#endif
 
 #if MICROPY_PY_THREAD && !MICROPY_PY_THREAD_GIL
 #define QSTR_ENTER() mp_thread_mutex_lock(&MP_STATE_VM(qstr_mutex), 1)
@@ -77,6 +81,7 @@ size_t qstr_compute_hash(const byte *data, size_t len) {
 // future .mpy version we could re-order them and make it sorted). It also
 // contains additional qstrs that must have IDs <256, see operator_qstr_list
 // in makeqstrdata.py.
+#if MICROPY_QSTR_BYTES_IN_HASH
 const qstr_hash_t mp_qstr_const_hashes_static[] = {
     #ifndef NO_QSTR
 #define QDEF0(id, hash, len, str) hash,
@@ -86,6 +91,7 @@ const qstr_hash_t mp_qstr_const_hashes_static[] = {
 #undef QDEF1
     #endif
 };
+#endif
 
 const qstr_len_t mp_qstr_const_lengths_static[] = {
     #ifndef NO_QSTR
@@ -103,7 +109,9 @@ const qstr_pool_t mp_qstr_const_pool_static = {
     false,              // is_sorted
     MICROPY_ALLOC_QSTR_ENTRIES_INIT,
     MP_QSTRnumber_of_static,   // corresponds to number of strings in array just below
+    #if MICROPY_QSTR_BYTES_IN_HASH
     (qstr_hash_t *)mp_qstr_const_hashes_static,
+    #endif
     (qstr_len_t *)mp_qstr_const_lengths_static,
     {
         #ifndef NO_QSTR
@@ -118,6 +126,7 @@ const qstr_pool_t mp_qstr_const_pool_static = {
 
 // The next pool is the remainder of the qstrs defined in the firmware. This
 // is sorted.
+#if MICROPY_QSTR_BYTES_IN_HASH
 const qstr_hash_t mp_qstr_const_hashes[] = {
     #ifndef NO_QSTR
 #define QDEF0(id, hash, len, str)
@@ -127,6 +136,7 @@ const qstr_hash_t mp_qstr_const_hashes[] = {
 #undef QDEF1
     #endif
 };
+#endif
 
 const qstr_len_t mp_qstr_const_lengths[] = {
     #ifndef NO_QSTR
@@ -144,7 +154,9 @@ const qstr_pool_t mp_qstr_const_pool = {
     true,               // is_sorted
     MICROPY_ALLOC_QSTR_ENTRIES_INIT,
     MP_QSTRnumber_of - MP_QSTRnumber_of_static,   // corresponds to number of strings in array just below
+    #if MICROPY_QSTR_BYTES_IN_HASH
     (qstr_hash_t *)mp_qstr_const_hashes,
+    #endif
     (qstr_len_t *)mp_qstr_const_lengths,
     {
         #ifndef NO_QSTR
@@ -188,8 +200,13 @@ STATIC const qstr_pool_t *find_qstr(qstr *q) {
 }
 
 // qstr_mutex must be taken while in this function
-STATIC qstr qstr_add(mp_uint_t hash, mp_uint_t len, const char *q_ptr) {
+STATIC qstr qstr_add(mp_uint_t len, const char *q_ptr) {
+    #if MICROPY_QSTR_BYTES_IN_HASH
+    mp_uint_t hash = qstr_compute_hash((const byte *)q_ptr, len);
     DEBUG_printf("QSTR: add hash=%d len=%d data=%.*s\n", hash, len, len, q_ptr);
+    #else
+    DEBUG_printf("QSTR: add len=%d data=%.*s\n", len, len, q_ptr);
+    #endif
 
     // make sure we have room in the pool for a new qstr
     if (MP_STATE_VM(last_pool)->len >= MP_STATE_VM(last_pool)->alloc) {
@@ -199,7 +216,11 @@ STATIC qstr qstr_add(mp_uint_t hash, mp_uint_t len, const char *q_ptr) {
         new_alloc = MAX(MICROPY_ALLOC_QSTR_ENTRIES_INIT, new_alloc);
         #endif
         mp_uint_t pool_size = sizeof(qstr_pool_t)
-            + (sizeof(const char *) + sizeof(qstr_hash_t) + sizeof(qstr_len_t)) * new_alloc;
+            + (sizeof(const char *)
+                #if MICROPY_QSTR_BYTES_IN_HASH
+                + sizeof(qstr_hash_t)
+                #endif
+                + sizeof(qstr_len_t)) * new_alloc;
         qstr_pool_t *pool = (qstr_pool_t *)m_malloc_maybe(pool_size);
         if (pool == NULL) {
             // Keep qstr_last_chunk consistent with qstr_pool_t: qstr_last_chunk is not scanned
@@ -211,8 +232,12 @@ STATIC qstr qstr_add(mp_uint_t hash, mp_uint_t len, const char *q_ptr) {
             QSTR_EXIT();
             m_malloc_fail(new_alloc);
         }
+        #if MICROPY_QSTR_BYTES_IN_HASH
         pool->hashes = (qstr_hash_t *)(pool->qstrs + new_alloc);
         pool->lengths = (qstr_len_t *)(pool->hashes + new_alloc);
+        #else
+        pool->lengths = (qstr_len_t *)(pool->qstrs + new_alloc);
+        #endif
         pool->prev = MP_STATE_VM(last_pool);
         pool->total_prev_len = MP_STATE_VM(last_pool)->total_prev_len + MP_STATE_VM(last_pool)->len;
         pool->alloc = new_alloc;
@@ -223,7 +248,9 @@ STATIC qstr qstr_add(mp_uint_t hash, mp_uint_t len, const char *q_ptr) {
 
     // add the new qstr
     mp_uint_t at = MP_STATE_VM(last_pool)->len;
+    #if MICROPY_QSTR_BYTES_IN_HASH
     MP_STATE_VM(last_pool)->hashes[at] = hash;
+    #endif
     MP_STATE_VM(last_pool)->lengths[at] = len;
     MP_STATE_VM(last_pool)->qstrs[at] = q_ptr;
     MP_STATE_VM(last_pool)->len++;
@@ -238,8 +265,10 @@ qstr qstr_find_strn(const char *str, size_t str_len) {
         return MP_QSTR_;
     }
 
+    #if MICROPY_QSTR_BYTES_IN_HASH
     // work out hash of str
     size_t str_hash = qstr_compute_hash((const byte *)str, str_len);
+    #endif
 
     // search pools for the data
     for (const qstr_pool_t *pool = MP_STATE_VM(last_pool); pool != NULL; pool = pool->prev) {
@@ -261,7 +290,11 @@ qstr qstr_find_strn(const char *str, size_t str_len) {
 
         // sequential search for the remaining strings
         for (mp_uint_t at = low; at < high + 1; at++) {
-            if (pool->hashes[at] == str_hash && pool->lengths[at] == str_len
+            if (
+                #if MICROPY_QSTR_BYTES_IN_HASH
+                pool->hashes[at] == str_hash &&
+                #endif
+                pool->lengths[at] == str_len
                 && memcmp(pool->qstrs[at], str, str_len) == 0) {
                 return pool->total_prev_len + at;
             }
@@ -329,18 +362,21 @@ qstr qstr_from_strn(const char *str, size_t len) {
         MP_STATE_VM(qstr_last_used) += n_bytes;
 
         // store the interned strings' data
-        size_t hash = qstr_compute_hash((const byte *)str, len);
         memcpy(q_ptr, str, len);
         q_ptr[len] = '\0';
-        q = qstr_add(hash, len, q_ptr);
+        q = qstr_add(len, q_ptr);
     }
     QSTR_EXIT();
     return q;
 }
 
 mp_uint_t qstr_hash(qstr q) {
     const qstr_pool_t *pool = find_qstr(&q);
+    #if MICROPY_QSTR_BYTES_IN_HASH
     return pool->hashes[q];
+    #else
+    return qstr_compute_hash((byte *)pool->qstrs[q], pool->lengths[q]);
+    #endif
 }
 
 size_t qstr_len(qstr q) {
@@ -375,7 +411,11 @@ void qstr_pool_info(size_t *n_pool, size_t *n_qstr, size_t *n_str_data_bytes, si
         *n_total_bytes += gc_nbytes(pool); // this counts actual bytes used in heap
         #else
         *n_total_bytes += sizeof(qstr_pool_t)
-            + (sizeof(const char *) + sizeof(qstr_hash_t) + sizeof(qstr_len_t)) * pool->alloc;
+            + (sizeof(const char *)
+                #if MICROPY_QSTR_BYTES_IN_HASH
+                + sizeof(qstr_hash_t)
+                #endif
+                + sizeof(qstr_len_t)) * pool->alloc;
         #endif
     }
     *n_total_bytes += *n_str_data_bytes;

py/qstr.h

@@ -60,7 +60,9 @@ enum {
 typedef size_t qstr;
 typedef uint16_t qstr_short_t;
 
-#if MICROPY_QSTR_BYTES_IN_HASH == 1
+#if MICROPY_QSTR_BYTES_IN_HASH == 0
+// No qstr_hash_t type needed.
+#elif MICROPY_QSTR_BYTES_IN_HASH == 1
 typedef uint8_t qstr_hash_t;
 #elif MICROPY_QSTR_BYTES_IN_HASH == 2
 typedef uint16_t qstr_hash_t;
@@ -82,7 +84,9 @@ typedef struct _qstr_pool_t {
     size_t is_sorted : 1;
     size_t alloc;
     size_t len;
+    #if MICROPY_QSTR_BYTES_IN_HASH
     qstr_hash_t *hashes;
+    #endif
     qstr_len_t *lengths;
     const char *qstrs[];
 } qstr_pool_t;
@@ -92,6 +96,7 @@ typedef struct _qstr_pool_t {
 void qstr_init(void);
 
 size_t qstr_compute_hash(const byte *data, size_t len);
+
 qstr qstr_find_strn(const char *str, size_t str_len); // returns MP_QSTRnull if not found
 
 qstr qstr_from_str(const char *str);

tools/mpy-tool.py

@@ -1473,21 +1473,20 @@ def freeze_mpy(firmware_qstr_idents, compiled_modules):
     raw_code_count = 0
     raw_code_content = 0
 
-    print()
-    print("const qstr_hash_t mp_qstr_frozen_const_hashes[] = {")
-    qstr_size = {"metadata": 0, "data": 0}
-    for _, _, _, qbytes in new:
-        qhash = qstrutil.compute_hash(qbytes, config.MICROPY_QSTR_BYTES_IN_HASH)
-        print("    %d," % qhash)
-    print("};")
+    if config.MICROPY_QSTR_BYTES_IN_HASH:
+        print()
+        print("const qstr_hash_t mp_qstr_frozen_const_hashes[] = {")
+        for _, _, _, qbytes in new:
+            qhash = qstrutil.compute_hash(qbytes, config.MICROPY_QSTR_BYTES_IN_HASH)
+            print("    %d," % qhash)
+            qstr_content += config.MICROPY_QSTR_BYTES_IN_HASH
+        print("};")
     print()
     print("const qstr_len_t mp_qstr_frozen_const_lengths[] = {")
     for _, _, _, qbytes in new:
         print("    %d," % len(qbytes))
-        qstr_size["metadata"] += (
-            config.MICROPY_QSTR_BYTES_IN_LEN + config.MICROPY_QSTR_BYTES_IN_HASH
-        )
-        qstr_size["data"] += len(qbytes)
+        qstr_content += config.MICROPY_QSTR_BYTES_IN_LEN
+        qstr_content += len(qbytes) + 1  # include NUL
     print("};")
     print()
     print("extern const qstr_pool_t mp_qstr_const_pool;")
@@ -1497,14 +1496,12 @@ def freeze_mpy(firmware_qstr_idents, compiled_modules):
     print("    true, // is_sorted")
     print("    %u, // allocated entries" % qstr_pool_alloc)
     print("    %u, // used entries" % len(new))
-    print("    (qstr_hash_t *)mp_qstr_frozen_const_hashes,")
+    if config.MICROPY_QSTR_BYTES_IN_HASH:
+        print("    (qstr_hash_t *)mp_qstr_frozen_const_hashes,")
     print("    (qstr_len_t *)mp_qstr_frozen_const_lengths,")
     print("    {")
     for _, _, qstr, qbytes in new:
         print('        "%s",' % qstrutil.escape_bytes(qstr, qbytes))
-        qstr_content += (
-            config.MICROPY_QSTR_BYTES_IN_LEN + config.MICROPY_QSTR_BYTES_IN_HASH + len(qbytes) + 1
-        )
     print("    },")
     print("};")