Tested with 7.2.0 and 8.1.0. The following example unnecessarily calls ldivmod twice: struct foo { long a, b; }; struct foo test(long long x) { return (struct foo){x / 77, x % 77}; } armv7m-linux-musleabi-gcc -c -O2 test.c armv7m-linux-musleabi-objdump -d test.o 00000000 <test>: 0: b5d0 push {r4, r6, r7, lr} 2: 4616 mov r6, r2 4: 461f mov r7, r3 6: 4604 mov r4, r0 8: 224d movs r2, #77 ; 0x4d a: 2300 movs r3, #0 c: 4630 mov r0, r6 e: 4639 mov r1, r7 10: f7ff fffe bl 0 <__aeabi_ldivmod> 14: 4639 mov r1, r7 16: 6020 str r0, [r4, #0] 18: 224d movs r2, #77 ; 0x4d 1a: 4630 mov r0, r6 1c: 2300 movs r3, #0 1e: f7ff fffe bl 0 <__aeabi_ldivmod> 22: 4620 mov r0, r4 24: 6062 str r2, [r4, #4] 26: bdd0 pop {r4, r6, r7, pc} If the test is rearranged so that the denominator is a function argument the generated code is as expected: struct foo { long a, b; }; struct foo test(long long x, long den) { return (struct foo){x / den, x % den}; } armv7m-linux-musleabi-gcc -c -O2 test.c armv7m-linux-musleabi-objdump -d test.o 00000000 <test>: 0: b5d0 push {r4, r6, r7, lr} 2: 4616 mov r6, r2 4: 461f mov r7, r3 6: 9a04 ldr r2, [sp, #16] 8: 4604 mov r4, r0 a: 4639 mov r1, r7 c: 4630 mov r0, r6 e: 17d3 asrs r3, r2, #31 10: f7ff fffe bl 0 <__aeabi_ldivmod> 14: e9c4 0200 strd r0, r2, [r4] 18: 4620 mov r0, r4 1a: bdd0 pop {r4, r6, r7, pc}
I think this was fixed with GCC 8. Can you check?
Sure, # cat test.c struct foo { long a, b; }; struct foo test(long long x) { return (struct foo){x / 77, x % 77}; } # gcc --version gcc (GCC) 8.1.0 Copyright (C) 2018 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. # gcc -c -O2 test.c # objdump -d test.o test.o: file format elf32-littlearm Disassembly of section .text: 00000000 <test>: 0: e92d40d0 push {r4, r6, r7, lr} 4: e1a06002 mov r6, r2 8: e1a07003 mov r7, r3 c: e1a04000 mov r4, r0 10: e3a0204d mov r2, #77 ; 0x4d 14: e3a03000 mov r3, #0 18: e1a00006 mov r0, r6 1c: e1a01007 mov r1, r7 20: ebfffffe bl 0 <__aeabi_ldivmod> 24: e1a01007 mov r1, r7 28: e3a0204d mov r2, #77 ; 0x4d 2c: e3a03000 mov r3, #0 30: e5840000 str r0, [r4] 34: e1a00006 mov r0, r6 38: ebfffffe bl 0 <__aeabi_ldivmod> 3c: e1a00004 mov r0, r4 40: e5842004 str r2, [r4, #4] 44: e8bd80d0 pop {r4, r6, r7, pc} Looks like the same problem is still there.
Still present in 9.2.0.
Fixed in GCC 11: lsrs r1, r2, #30 push {r4, lr} orr r1, r1, r3, lsl #2 bic lr, r1, #-1073741824 bic r1, r2, #-1073741824 add r1, r1, lr asrs r4, r3, #31 add r3, r1, r3, lsr #28 and r1, r4, #137 add r3, r3, r1 movw r1, #57025 movt r1, 13617 bic r4, r4, #75 umull lr, r1, r1, r3 mov lr, #77 lsrs r1, r1, #4 mls r3, lr, r1, r3 movw r1, #14469 movt r1, 16171 add r3, r3, r4 str r3, [r0, #4] subs r2, r2, r3 umull r2, r1, r2, r1 str r2, [r0] pop {r4, pc} For: #define N 7723 struct foo { long a, b; }; struct foo test(long long x) { return (struct foo){x / N, x % N}; } GCC 11+ Produces: push {r4, lr} mov r4, r0 mov r1, r3 mov r0, r2 movs r3, #0 movw r2, #7723 bl __aeabi_ldivmod strd r0, r2, [r4] mov r0, r4 pop {r4, pc} Which is exactly what you want too.