https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79219
--- Comment #4 from H. Peter Anvin <hpa at zytor dot com> ---
There are a few issues with that:
a) the overflow behavior is inherently different, which is why the inline you
propose doesn't work. Try compiling the attached program, on x86-64 it
produces a call to __udivti3 rather than an inline div instruction (output
shown below). The reason is that the div instruction will trap on overflow.
b) it requires algorithms to be implemented using lengthy (source-code-wise)
multi-precision to be portable between 32- and 64-bit architectures.
c) it seems likely that getting __intN where N > CHAR_BIT*sizeof(uintmax_t)
into a standard would be very hard, and thus would not be possible to
standard-track (although it could be used as an implementation on gcc using a
header inline, of course.)
dbldiv.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <div2to1>:
0: 45 31 c9 xor %r9d,%r9d
3: 49 89 fa mov %rdi,%r10
6: 55 push %rbp
7: 4c 89 cf mov %r9,%rdi
a: 48 89 d1 mov %rdx,%rcx
d: 4c 89 d5 mov %r10,%rbp
10: 31 d2 xor %edx,%edx
12: 48 01 f7 add %rsi,%rdi
15: 48 11 d5 adc %rdx,%rbp
18: 48 89 ca mov %rcx,%rdx
1b: 31 c9 xor %ecx,%ecx
1d: 48 89 ee mov %rbp,%rsi
20: e8 00 00 00 00 callq 25 <div2to1+0x25>
21: R_X86_64_PC32 __udivti3-0x4
25: 5d pop %rbp
26: c3 retq
27: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
2e: 00 00
0000000000000030 <div2to1_expected>:
30: 48 89 d1 mov %rdx,%rcx
33: 48 89 f0 mov %rsi,%rax
36: 48 89 fa mov %rdi,%rdx
39: 48 f7 f2 div %rdx
3c: c3 retq
