Optimize signed division in assembly — divide the code size too

App/CMakeLists.txt

@@ -5,6 +5,8 @@ target_link_libraries(App INTERFACE PY32F071_Driver)
 target_compile_definitions(App INTERFACE PRINTF_INCLUDE_CONFIG_H)
 
 target_sources(App INTERFACE
+    # libgcc override (signed division)
+    compact_div.S
     # Drivers
     driver/backlight.c
     driver/bk4829.c

App/compact_div.S

@@ -0,0 +1,68 @@
+/* Copyright 2024 Armel F4HWN
+ * https://github.com/armel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *     Unless required by applicable law or agreed to in writing, software
+ *     distributed under the License is distributed on an "AS IS" BASIS,
+ *     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *     See the License for the specific language governing permissions and
+ *     limitations under the License.
+ */
+
+/*
+ * Signed 32-bit division for the Cortex-M0+ (no hardware divider).
+ *
+ * libgcc's _divsi3.o (468 B) carries its own copy of the unrolled division
+ * loop that _udivsi3.o (276 B) already brings for unsigned divisions. These
+ * entry points only fold the operand signs around __aeabi_uidivmod, which
+ * returns the quotient in r0 and the remainder in r1 (~30 extra cycles per
+ * signed division).
+ *
+ * Signs are kept as masks m (0 or -1): (x ^ m) - m negates x when m is -1.
+ * Results match libgcc, including the undefined cases:
+ *   - quotient truncated toward zero, remainder has the sign of the dividend;
+ *   - INT_MIN / -1 = INT_MIN, remainder 0 (|INT_MIN| is 0x80000000 unsigned);
+ *   - x / 0 = 0, remainder x (libgcc's __aeabi_uidivmod returns 0 and |x|).
+ */
+
+.syntax unified
+.thumb
+
+.section .text.__aeabi_idiv, "ax", %progbits
+.align 2
+
+.global __aeabi_idiv
+.global __aeabi_idivmod
+.global __divsi3
+.type __aeabi_idiv, %function
+.type __aeabi_idivmod, %function
+.type __divsi3, %function
+.thumb_func
+__aeabi_idiv:
+.thumb_func
+__aeabi_idivmod:
+.thumb_func
+__divsi3:
+    push    {r3, r4, r5, lr}    /* r3 only keeps the stack 8-byte aligned */
+    asrs    r4, r0, #31         /* r4 = dividend sign mask */
+    eors    r0, r4
+    subs    r0, r0, r4          /* r0 = |dividend| */
+    asrs    r5, r1, #31         /* r5 = divisor sign mask */
+    eors    r1, r5
+    subs    r1, r1, r5          /* r1 = |divisor| */
+    eors    r5, r4              /* r5 = quotient sign mask */
+    bl      __aeabi_uidivmod    /* r0 = quotient, r1 = remainder */
+    eors    r0, r5
+    subs    r0, r0, r5          /* quotient is negative when the signs differ */
+    eors    r1, r4
+    subs    r1, r1, r4          /* remainder takes the dividend sign */
+    pop     {r3, r4, r5, pc}
+
+.size __aeabi_idiv, . - __aeabi_idiv
+.size __aeabi_idivmod, . - __aeabi_idivmod
+.size __divsi3, . - __divsi3