lib/aeabi_div: Add compact integer division helpers.

.github/workflows/format.yml

@@ -7,6 +7,8 @@ on:
     paths:
     - ".github/workflows/format.yml"
     - "bricks/**/*.[ch]"
+    - "lib/aeabi_div/**/*.[ch]"
+    - "lib/lego/**/*.[ch]"
     - "lib/pbio/**/*.[ch]"
     - "py/*.[ch]"
     - "pybricks/**/*.[ch]"
@@ -17,6 +19,8 @@ on:
     paths:
     - ".github/workflows/format.yml"
     - "bricks/**/*.[ch]"
+    - "lib/aeabi_div/**/*.[ch]"
+    - "lib/lego/**/*.[ch]"
     - "lib/pbio/**/*.[ch]"
     - "py/*.[ch]"
     - "pybricks/**/*.[ch]"

bricks/_common/common.mk

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: MIT
 # Copyright (c) 2013, 2014 Damien P. George
-# Copyright (c) 2019-2023 The Pybricks Authors
+# Copyright (c) 2019-2026 The Pybricks Authors
 
 # This file is shared by all bare-metal Arm Pybricks ports.
 
@@ -666,6 +666,13 @@ OBJ += $(addprefix $(BUILD)/, $(PBIO_SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(LEGO_SPEC_SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_LIBM:.c=.o))
 
+ifeq ($(PB_LIB_AEABI_DIV),1)
+OBJ += $(addprefix $(BUILD)/, $(AEABI_DIV_SRC_C:.c=.o))
+# Overriding the libgcc division helpers only works when they are not
+# compiled with LTO. See lib/aeabi_div/aeabi_div.c.
+$(BUILD)/lib/aeabi_div/%.o: CFLAGS += -fno-lto
+endif
+
 ifeq ($(PB_LIB_BLUENRG),1)
 OBJ += $(addprefix $(BUILD)/, $(BLUENRG_SRC_C:.c=.o))
 endif

bricks/_common/sources.mk

@@ -1,8 +1,12 @@
 # SPDX-License-Identifier: MIT
-# Copyright (c) 2019-2023 The Pybricks Authors
+# Copyright (c) 2019-2026 The Pybricks Authors
 
 # This file contains the sources common to all Pybricks MicroPython ports.
 
+# Compact AEABI division helpers
+
+AEABI_DIV_SRC_C = lib/aeabi_div/aeabi_div.c
+
 # Ring buffer
 
 LWRB_SRC_C = lib/lwrb/src/lwrb/lwrb.c

bricks/cityhub/Makefile

@@ -6,6 +6,7 @@ PB_MCU_FAMILY = STM32
 PB_MCU_SERIES = F0
 PB_CMSIS_MCU = STM32F030xC
 PB_MCU_EXT_OSC_HZ = 0 # uses internal oscillator
+PB_LIB_AEABI_DIV = 1
 PB_LIB_STM32_HAL = 1
 PB_LIB_BLE5STACK = 1
 PB_FROZEN_MODULES = 1

bricks/movehub/Makefile

@@ -6,6 +6,7 @@ PB_MCU_FAMILY = STM32
 PB_MCU_SERIES = F0
 PB_CMSIS_MCU = STM32F070xB
 PB_MCU_EXT_OSC_HZ = 0 # uses internal oscillator
+PB_LIB_AEABI_DIV = 1
 PB_LIB_STM32_HAL = 1
 PB_LIB_BLUENRG = 1
 PB_FROZEN_MODULES = 0

lib/aeabi_div/aeabi_div.c

@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 The Pybricks Authors
+
+// Compact replacements for the AEABI integer division helpers.
+//
+// Cortex-M0 has no divide instruction, so the compiler turns every 32-bit
+// integer division into a call to one of the helpers below. Since GCC 11, the
+// v6-m libgcc only ships speed-optimized versions: __udivsi3 is fully
+// unrolled and __divsi3 contains its own second copy of the unsigned core
+// instead of sharing it, which together take around 730 bytes. The loop-based
+// versions here produce identical results in around 110 bytes.
+//
+// The cost is speed: roughly 3 to 5 times slower per division. The heaviest
+// user is motor control with around 10 divisions per 5 ms loop, which stays
+// well below 1% of the CPU either way, in line with these hubs already
+// trading speed for size elsewhere (CSUPEROPT = -Os).
+//
+// Defining these symbols in an object file makes the linker prefer them over
+// the libgcc archive members. This file must be compiled without LTO: the
+// compiler generates references to these helpers during LTO code generation,
+// after LTO symbol resolution, so definitions inside an LTO object fail to
+// link.
+//
+// Division by zero returns a quotient of 0 and a remainder equal to the
+// dividend, the same as libgcc's default __aeabi_idiv0 behavior.
+
+#include <stdint.h>
+
+// The *divmod helpers return the quotient in r0 and the remainder in r1,
+// which matches how a uint64_t is returned (low word in r0, high in r1).
+
+uint64_t __aeabi_uidivmod(uint32_t n, uint32_t d) {
+    if (d == 0) {
+        return (uint64_t)n << 32;
+    }
+
+    // Scale the divisor up to the dividend, then do schoolbook long division
+    // in base 2, one quotient bit per iteration.
+    uint32_t q = 0;
+    uint32_t bit = 1;
+    while (d < n && !(d & UINT32_C(0x80000000))) {
+        d <<= 1;
+        bit <<= 1;
+    }
+    while (bit) {
+        if (n >= d) {
+            n -= d;
+            q |= bit;
+        }
+        d >>= 1;
+        bit >>= 1;
+    }
+    return ((uint64_t)n << 32) | q;
+}
+
+uint32_t __aeabi_uidiv(uint32_t n, uint32_t d) {
+    return (uint32_t)__aeabi_uidivmod(n, d);
+}
+uint32_t __udivsi3(uint32_t n, uint32_t d) __attribute__((alias("__aeabi_uidiv")));
+
+uint64_t __aeabi_idivmod(int32_t n, int32_t d) {
+    uint64_t res = __aeabi_uidivmod(n < 0 ? -(uint32_t)n : (uint32_t)n, d < 0 ? -(uint32_t)d : (uint32_t)d);
+    uint32_t q = (uint32_t)res;
+    uint32_t r = (uint32_t)(res >> 32);
+
+    // C99 division truncates toward zero: the quotient is negative when the
+    // signs differ and the remainder takes the sign of the dividend.
+    if ((n ^ d) < 0) {
+        q = -q;
+    }
+    if (n < 0) {
+        r = -r;
+    }
+    return ((uint64_t)r << 32) | q;
+}
+
+int32_t __aeabi_idiv(int32_t n, int32_t d) {
+    return (int32_t)__aeabi_idivmod(n, d);
+}
+int32_t __divsi3(int32_t n, int32_t d) __attribute__((alias("__aeabi_idiv")));

tools/codeformat.py

@@ -18,6 +18,7 @@
 
 codeformat.PATHS = [
     "bricks/**/*.[ch]",
+    "lib/aeabi_div/**/*.[ch]",
     "lib/pbio/**/*.[ch]",
     "lib/lego/**/*.[ch]",
     "py/*.[ch]",