perf: lower the default allocation for bignums

rts/Makefile

@@ -45,6 +45,7 @@ TOMMATH_FLAGS = \
   -DMP_FREE=mp_free \
   -DMP_MEMSET=0 \
   -DMP_FIXED_CUTOFFS \
+  -DMP_PREC=4 \
   -DMP_NO_FILE \
   -D__STDC_IEC_559__ \
 

rts/motoko-rts/src/bigint.rs

@@ -51,6 +51,7 @@ unsafe fn mp_alloc<M: Memory>(mem: &mut M, size: Bytes<u32>) -> *mut u8 {
     let size = size.as_usize();
     debug_assert_eq!((size % core::mem::size_of::<mp_digit>()), 0);
     (*blob).mp_int.alloc = (size / core::mem::size_of::<mp_digit>()) as i32;
+    (*blob).mp_int.used = (*blob).mp_int.alloc; // Otherwise, `block_size() in `allocation_barrier()` may already shorten the object.
     allocation_barrier(ptr);
     blob.payload_addr() as *mut u8
 }
@@ -169,7 +170,11 @@ unsafe fn persist_bigint(i: mp_int) -> Value {
     }
     let r = BigInt::from_payload(i.dp);
     if (*r).mp_int.alloc != i.alloc {
-        panic!("persist_bigint: alloc changed?");
+        panic!(
+            "persist_bigint: alloc changed? heap {} stack {}",
+            (*r).mp_int.alloc,
+            i.alloc
+        );
     }
     (*r).mp_int = i;
     Value::from_ptr(r as usize)

rts/motoko-rts/src/types.rs

@@ -921,6 +921,7 @@ pub(crate) unsafe fn block_size(address: usize) -> Words<u32> {
 
         TAG_BIGINT => {
             let bigint = address as *mut BigInt;
+            (*bigint).mp_int.alloc = (*bigint).mp_int.used;
             size_of::<BigInt>() + bigint.len().to_words()
         }
 

test/bench/bignum.mo

@@ -0,0 +1,37 @@
+//MOC-FLAG --force-gc
+import { Array_tabulate; performanceCounter; rts_heap_size; debugPrint } = "mo:⛔";
+
+actor Bignum {
+    var steps = 0;
+    var nat = 3;
+
+    func counters() : (Int, Nat64) = (rts_heap_size(), performanceCounter(0));
+
+    let arr : [var Nat] = [var 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
+    var arr1 : ?[Nat] = null;
+
+    public func go() : async () {
+        let (size0, perf0) = counters();
+
+        while (steps < 12) {
+            nat := nat * 5 * nat;
+            arr[steps] := nat;
+            steps := steps + 1
+        };
+
+        let (size1, perf1) = counters();
+
+        debugPrint(debug_show {size = size1 - size0; cycles = perf1 - perf0 });
+    };
+
+    public func candid() : async () {
+        let (size0, perf0) = counters();
+        let ser = to_candid(Array_tabulate<Nat>(arr.size(), func i = arr[i]));
+        arr1 := from_candid(ser);
+        let (size1, perf1) = counters();
+        debugPrint(debug_show {size = size1 - size0; cycles = perf1 - perf0 });
+    }
+}
+
+//CALL ingress go 0x4449444C0000
+//CALL ingress candid 0x4449444C0000

test/bench/ok/bignum.drun-run.ok

@@ -0,0 +1,6 @@
+ingress Completed: Reply: 0x4449444c016c01b3c4b1f204680100010a00000000000000000101
+ingress Completed: Reply: 0x4449444c0000
+debug.print: {cycles = 2_494_559; size = +59_652}
+ingress Completed: Reply: 0x4449444c0000
+debug.print: {cycles = 103_059_688; size = +1_817_872}
+ingress Completed: Reply: 0x4449444c0000

test/bench/ok/heap-32.drun-run.ok

@@ -1,5 +1,5 @@
 ingress Completed: Reply: 0x4449444c016c01b3c4b1f204680100010a00000000000000000101
 ingress Completed: Reply: 0x4449444c0000
-debug.print: (50_227, +91_377_860, 1_434_593_325)
-debug.print: (50_070, +102_586_000, 1_506_674_763)
+debug.print: (50_227, +30_261_252, 676_332_164)
+debug.print: (50_070, +32_992_212, 730_267_012)
 ingress Completed: Reply: 0x4449444c0000