Encode iseq node_id with LEB128

Node ids are small non-negative integers (-1 for synthetic instructions),
so a LEB128 stores them in 1-2 bytes instead of a 4-byte int.

On Shopify's monolith, this reduces the boot heap memsize by 40MB.

compile.c

@@ -2624,6 +2624,19 @@ fix_sp_depth(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
     return stack_max;
 }
 
+#ifdef USE_ISEQ_NODE_ID
+static void
+add_node_id(uint8_t *node_ids, unsigned int *node_ids_size, int node_id)
+{
+    uint32_t v = (uint32_t)node_id + 1;
+    do {
+        uint8_t byte = v & 0x7f;
+        v >>= 7;
+        node_ids[(*node_ids_size)++] = v ? (byte | 0x80) : byte;
+    } while (v);
+}
+#endif
+
 static int
 add_insn_info(struct iseq_insn_info_entry *insns_info, unsigned int *positions,
               int insns_info_index, int code_index, const INSN *iobj)
@@ -2771,7 +2784,7 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
     const int positions_size = insn_num;
     positions = ALLOC_N(unsigned int, insn_num);
 #ifdef USE_ISEQ_NODE_ID
-    int *node_ids = ALLOC_N(int, insn_num);
+    uint8_t *node_ids = ALLOC_N(uint8_t, (size_t)insn_num * 5);
     unsigned int node_ids_size = 0;
 #endif
     if (ISEQ_IS_SIZE(body)) {
@@ -2952,7 +2965,7 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
                 }
                 if (add_insn_info(insns_info, positions, insns_info_index, code_index, iobj)) insns_info_index++;
 #ifdef USE_ISEQ_NODE_ID
-                node_ids[node_ids_size++] = iobj->insn_info.node_id;
+                add_node_id(node_ids, &node_ids_size, iobj->insn_info.node_id);
 #endif
                 code_index += len;
                 break;
@@ -2990,7 +3003,7 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
                         }
                         if (add_adjust_info(insns_info, positions, insns_info_index, code_index, adjust)) insns_info_index++;
 #ifdef USE_ISEQ_NODE_ID
-                        node_ids[node_ids_size++] = -1;
+                        add_node_id(node_ids, &node_ids_size, -1);
 #endif
                     }
                     if (diff > 1) {
@@ -3006,7 +3019,7 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
                         SIZED_FREE_N(insns_info, insns_info_size);
                         SIZED_FREE_N(positions, positions_size);
 #ifdef USE_ISEQ_NODE_ID
-                        SIZED_FREE_N(node_ids, insns_info_size);
+                        SIZED_FREE_N(node_ids, (size_t)insns_info_size * 5);
 #endif
                         if (ISEQ_MBITS_BUFLEN(code_size) > 1) {
                             SIZED_FREE_N(mark_offset_bits, ISEQ_MBITS_BUFLEN(code_index));
@@ -3058,6 +3071,7 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
     body->insns_info.positions_or_succ_index_table.positions = positions;
     body->insns_info.size = insns_info_index;
 #ifdef USE_ISEQ_NODE_ID
+    SIZED_REALLOC_N(node_ids, uint8_t, node_ids_size, (size_t)insns_info_size * 5);
     body->insns_info.node_ids = node_ids;
     body->insns_info.node_ids_size = node_ids_size;
 #endif
@@ -12800,7 +12814,7 @@ typedef uint32_t ibf_offset_t;
 
 #define IBF_MAJOR_VERSION ISEQ_MAJOR_VERSION
 #ifdef RUBY_DEVEL
-#define IBF_DEVEL_VERSION 10
+#define IBF_DEVEL_VERSION 11
 #define IBF_MINOR_VERSION (ISEQ_MINOR_VERSION * 10000 + IBF_DEVEL_VERSION)
 #else
 #define IBF_MINOR_VERSION ISEQ_MINOR_VERSION
@@ -13528,7 +13542,7 @@ ibf_dump_insns_info_body(struct ibf_dump *dump, const rb_iseq_t *iseq)
     }
 #ifdef USE_ISEQ_NODE_ID
     if (body->insns_info.node_ids_size > 0) {
-        ibf_dump_write(dump, body->insns_info.node_ids, sizeof(int) * body->insns_info.node_ids_size);
+        ibf_dump_write(dump, body->insns_info.node_ids, body->insns_info.node_ids_size);
     }
 #endif
 
@@ -13552,11 +13566,11 @@ ibf_load_insns_info_body(const struct ibf_load *load, ibf_offset_t body_offset,
     }
 #ifdef USE_ISEQ_NODE_ID
     if (node_ids_size > 0) {
-        if ((size_t)node_ids_size > (load->current_buffer->size - reading_pos) / sizeof(int)) {
+        if (node_ids_size > load->current_buffer->size - reading_pos) {
             rb_raise(rb_eRuntimeError, "invalid byte sequence");
         }
-        int *node_ids = ALLOC_N(int, node_ids_size);
-        memcpy(node_ids, load->current_buffer->buff + reading_pos, sizeof(int) * node_ids_size);
+        uint8_t *node_ids = ALLOC_N(uint8_t, node_ids_size);
+        memcpy(node_ids, load->current_buffer->buff + reading_pos, node_ids_size);
         load_body->insns_info.node_ids = node_ids;
         load_body->insns_info.node_ids_size = node_ids_size;
     }

iseq.c

@@ -597,7 +597,7 @@ rb_iseq_memsize(const rb_iseq_t *iseq)
         size += body->iseq_size * sizeof(VALUE);
         size += body->insns_info.size * (sizeof(struct iseq_insn_info_entry) + sizeof(unsigned int));
 #ifdef USE_ISEQ_NODE_ID
-        size += body->insns_info.node_ids_size * sizeof(int);
+        size += body->insns_info.node_ids_size;
 #endif
         size += body->local_table_size * sizeof(ID); // body->local_table
         if (!ISEQ_LVAR_STATES_EMBED_P(body->local_table_size) && body->lvar_states.list) {
@@ -2578,6 +2578,21 @@ rb_iseq_line_no(const rb_iseq_t *iseq, size_t pos)
 }
 
 #ifdef USE_ISEQ_NODE_ID
+static int
+node_id_stream_read(const uint8_t **ptr, const uint8_t *end)
+{
+    uint32_t v = 0;
+    unsigned int shift = 0;
+    while (*ptr < end) {
+        uint8_t byte = **ptr;
+        (*ptr)++;
+        v |= (uint32_t)(byte & 0x7f) << shift;
+        if (!(byte & 0x80)) break;
+        shift += 7;
+    }
+    return (int)v - 1;
+}
+
 int
 rb_iseq_node_id(const rb_iseq_t *iseq, size_t pos)
 {
@@ -2587,17 +2602,17 @@ rb_iseq_node_id(const rb_iseq_t *iseq, size_t pos)
     }
 
     const VALUE *code = rb_iseq_original_iseq(iseq);
-    const int *node_ids = body->insns_info.node_ids;
-    unsigned int i = 0;
+    const uint8_t *ptr = body->insns_info.node_ids;
+    const uint8_t *end = ptr + body->insns_info.node_ids_size;
     size_t code_pos = 0;
 
-    while (i < body->insns_info.node_ids_size && code_pos < body->iseq_size) {
+    while (ptr < end && code_pos < body->iseq_size) {
         size_t len = (size_t)insn_len(code[code_pos]);
+        int node_id = node_id_stream_read(&ptr, end);
         if (pos < code_pos + len) {
-            return node_ids[i];
+            return node_id;
         }
         code_pos += len;
-        i++;
     }
 
     VM_ASSERT(0); /* node_ids out of sync with the instruction sequence */
@@ -3783,6 +3798,10 @@ iseq_data_to_ary(const rb_iseq_t *iseq)
     prev_insn_info = NULL;
 #ifdef USE_ISEQ_NODE_ID
     VALUE node_ids = rb_ary_new();
+    /* The node id stream holds one varint per instruction, in the same
+     * order as the instructions in nbody. */
+    const uint8_t *nid_ptr = iseq_body->insns_info.node_ids;
+    const uint8_t *nid_end = nid_ptr ? nid_ptr + iseq_body->insns_info.node_ids_size : NULL;
 #endif
 
     for (l=0, pos=0; l<RARRAY_LEN(nbody); l++) {
@@ -3796,9 +3815,9 @@ iseq_data_to_ary(const rb_iseq_t *iseq)
 
         info = get_insn_info(iseq, pos);
 #ifdef USE_ISEQ_NODE_ID
-        VM_ASSERT(l < (long)iseq_body->insns_info.node_ids_size);
-        rb_ary_push(node_ids, INT2FIX(l < (long)iseq_body->insns_info.node_ids_size ?
-                                      iseq_body->insns_info.node_ids[l] : -1));
+        VM_ASSERT(nid_ptr && nid_ptr < nid_end);
+        rb_ary_push(node_ids, INT2FIX(nid_ptr && nid_ptr < nid_end ?
+                                      node_id_stream_read(&nid_ptr, nid_end) : -1));
 #endif
 
         if (prev_insn_info != info) {

vm_core.h

@@ -533,7 +533,7 @@ struct rb_iseq_constant_body {
         } positions_or_succ_index_table;
         unsigned int size;
 #ifdef USE_ISEQ_NODE_ID
-        const int *node_ids;
+        const uint8_t *node_ids;
         unsigned int node_ids_size;
 #endif
     } insns_info;

zjit/src/cruby_bindings.inc.rs

@@ -1276,7 +1276,7 @@ pub struct rb_iseq_constant_body_iseq_insn_info {
     pub body: *const iseq_insn_info_entry,
     pub positions_or_succ_index_table: rb_iseq_constant_body_iseq_insn_info__bindgen_ty_1,
     pub size: ::std::os::raw::c_uint,
-    pub node_ids: *const ::std::os::raw::c_int,
+    pub node_ids: *const u8,
     pub node_ids_size: ::std::os::raw::c_uint,
 }
 #[repr(C)]