Merge pull request #1056 from bettio/support-extended-x-regs

This PR is focused into removing 2 main limitations:
1. Functions can have any number of parameters
2. Up to 1023 live x registers are supported

This restriction has been easily workarounded so far, but removing this
limitation closes a gap that makes development smoother.

Since so far everything worked nicely with just 16 registers, this
implementation threats "extended x registers" as an uncommon case, so using
them is way slower, but still possible, while reducing memory footprint to bare
minimum.

There is still a limitation: NIFs with more than 16 parameters are still not
allowed, but this limitation will be likely stay unnoticed.

This PR also fixes an unlikely corruption when having exactly 16 live registers,
and enables support of more than > 16 typed y registers (I never observed any
crash due to this limitation).

See also for #943 for an alternative approach, decoding is based on Paul's work
from that PR.

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added Pico cmake option `AVM_WAIT_BOOTSEL_ON_EXIT` (default `ON`) to allow tools to use automated `BOOTSEL` mode after main application exits
 - Use UTF-8 encoding for atoms when using `erlang:term_to_binary/1`, in conformance with OTP-26
 - Pico: Wait for USB serial connection `cmake` configuration option `AVM_USB_WAIT_SECONDS` added with 20 second default.
+- Support for code that makes use of more than 16 live registers, such as functions with > 16
+parameters and complex pattern matchings.
 
 ### Fixed
 
@@ -25,6 +27,8 @@ used)
 - ESP32: GPIO driver fix bug that would accept invalid `pull` direction, and silently set `pull` direction to `floating` without issuing an error.
 - ESP32: fixed bug in gpio driver that would accept invalid pin numbers (either negative, or too large)
 - RP2040: fixed bug in `gpio:set_pin_pull/2` that would accept any parameter as a valid `pull` mode.
+- Support to function with 10 or more parameters
+- Very unlikely but possible corruption caused by generated code that uses 16 live registers
 
 ### Changed
 

src/libAtomVM/atom.c

@@ -54,6 +54,39 @@ int atom_are_equals(AtomString a, AtomString b)
     }
 }
 
+static void utoa10(unsigned int int_value, char *integer_string)
+{
+    /*
+        Generic version:
+
+        size_t integer_string_len = 0;
+        unsigned int v = int_value;
+        do {
+            v = v / 10;
+            integer_string_len++;
+        } while (v != 0);
+    */
+
+    // let's avoid divisions
+    size_t integer_string_len = 1;
+    if (UNLIKELY(integer_string_len >= 1000)) {
+        integer_string_len = 4;
+    } else if (UNLIKELY(integer_string_len >= 100)) {
+        integer_string_len = 3;
+    } else if (integer_string_len >= 10) {
+        integer_string_len = 2;
+    }
+
+    int ix = 1;
+    do {
+        unsigned int digit = int_value % 10;
+        integer_string[integer_string_len - ix] = '0' + digit;
+        int_value = int_value / 10;
+        ix++;
+    } while (int_value != 0);
+    integer_string[integer_string_len] = '\0';
+}
+
 void atom_write_mfa(char *buf, size_t buf_size, AtomString module, AtomString function, unsigned int arity)
 {
     size_t module_name_len = atom_string_len(module);
@@ -62,14 +95,12 @@ void atom_write_mfa(char *buf, size_t buf_size, AtomString module, AtomString fu
     buf[module_name_len] = ':';
 
     size_t function_name_len = atom_string_len(function);
-    if (UNLIKELY((arity > 9) || (module_name_len + function_name_len + 4 > buf_size))) {
+    if (UNLIKELY((module_name_len + 1 + function_name_len + 1 + 4 + 1 > buf_size))) {
         fprintf(stderr, "Insufficient room to write mfa.\n");
         AVM_ABORT();
     }
     memcpy(buf + module_name_len + 1, atom_string_data(function), function_name_len);
 
-    // TODO: handle functions with more than 9 parameters
     buf[module_name_len + function_name_len + 1] = '/';
-    buf[module_name_len + function_name_len + 2] = (char) ('0' + arity);
-    buf[module_name_len + function_name_len + 3] = 0;
+    utoa10(arity, buf + module_name_len + 1 + function_name_len + 1);
 }

src/libAtomVM/context.c

@@ -43,6 +43,7 @@
 #define BYTES_PER_TERM (TERM_BITS / 8)
 
 static struct ResourceMonitor *context_monitors_handle_terminate(Context *ctx);
+static void destroy_extended_registers(Context *ctx, unsigned int live);
 
 Context *context_new(GlobalContext *glb)
 {
@@ -61,6 +62,7 @@ Context *context_new(GlobalContext *glb)
     ctx->e = ctx->heap.heap_end;
 
     context_clean_registers(ctx, 0);
+    list_init(&ctx->extended_x_regs);
 
     ctx->fr = NULL;
 
@@ -151,6 +153,7 @@ void context_destroy(Context *ctx)
     // Any other process released our mailbox, so we can clear it.
     mailbox_destroy(&ctx->mailbox, &ctx->heap);
 
+    destroy_extended_registers(ctx, 0);
     free(ctx->fr);
 
     memory_destroy_heap(&ctx->heap, ctx->global);

src/libAtomVM/context.h

@@ -76,6 +76,8 @@ enum HeapGrowthStrategy
 
 // Max number of x(N) & fr(N) registers
 // BEAM sets this to 1024.
+// x regs have an additional register that is used for storing an additional working element
+// so the number of registers is MAX_REG + 1, but only MAX_REG are addressable as x registers
 #define MAX_REG 16
 
 struct Context
@@ -84,7 +86,8 @@ struct Context
     GlobalContext *global;
     Heap heap;
     term *e;
-    term x[MAX_REG];
+    term x[MAX_REG + 1];
+    struct ListHead extended_x_regs;
 
     struct ListHead processes_list_head;
 
@@ -167,6 +170,13 @@ struct ResourceMonitor
     struct ListHead resource_list_head;
 };
 
+struct ExtendedRegister
+{
+    struct ListHead head;
+    unsigned int index;
+    term value;
+};
+
 /**
  * @brief Creates a new context
  *

src/libAtomVM/memory.c

@@ -264,6 +264,13 @@ static enum MemoryGCResult memory_gc(Context *ctx, size_t new_size, size_t num_r
         entry->value = memory_shallow_copy_term(old_root_fragment, entry->value, &ctx->heap.heap_ptr, true);
     }
 
+    TRACE("- Running copy GC on process extended registers\n");
+    LIST_FOR_EACH (item, &ctx->extended_x_regs) {
+        struct ExtendedRegister *ext_reg = GET_LIST_ENTRY(item, struct ExtendedRegister, head);
+        ext_reg->value = memory_shallow_copy_term(
+            old_root_fragment, ext_reg->value, &ctx->heap.heap_ptr, true);
+    }
+
     TRACE("- Running copy GC on exit reason\n");
     ctx->exit_reason = memory_shallow_copy_term(old_root_fragment, ctx->exit_reason, &ctx->heap.heap_ptr, true);
 

src/libAtomVM/nifs.c

@@ -791,7 +791,8 @@ const struct Nif *nifs_get(AtomString module, AtomString function, int arity)
 
     int function_name_len = atom_string_len(function);
     if (UNLIKELY((arity > 9) || (module_name_len + function_name_len + 4 > MAX_NIF_NAME_LEN))) {
-        AVM_ABORT();
+        // In AtomVM NIFs are limited to 9 parameters
+        return NULL;
     }
     memcpy(nifname + module_name_len + 1, atom_string_data(function), function_name_len);