Hi,
GCC (and other C compilers too) support the widening multiplication
of i386/AMD64 processors, but DON'T support their narrowing division:
--- demo.c ---
unsigned long long product(unsigned long multiplicand,
unsigned long multiplier)
{
return (unsigned long long) multiplicand * multiplier;
}
unsigned long long quotient(unsigned long long dividend,
unsigned long divisor,
unsigned long *remainder)
{
*remainder = dividend % divisor;
return dividend / divisor;
}
--- EOF ---
GCC 12.2: gcc -m32 -O2 demo.c
# https://godbolt.org/z/1M9dohMcE
product(unsigned long, unsigned long):
mov eax, DWORD PTR [esp+8]
mul DWORD PTR [esp+4]
ret
quotient(unsigned long long, unsigned long, unsigned long*):
push ebx
xor edx, edx
sub esp, 24
mov eax, DWORD PTR [esp+40]
lea ecx, [esp+8]
sub esp, 12
push ecx
push edx
push eax
push DWORD PTR [esp+60]
push DWORD PTR [esp+60]
call __udivmoddi4
mov ebx, DWORD PTR [esp+40]
mov ecx, DWORD PTR [esp+76]
mov DWORD PTR [ecx], ebx
add esp, 56
pop ebx
ret
### Diversion ###
Even worse and completely BRAINDEAD, another compiler calls __udivdi3()
and wastes a multiplication to compute the remainder, ignoring the fact
that __udivdi3() calls __udivmoddi4() which already returns quotient
and remainder:
clang 15.0.0: clang -m32 -O2 demo.c
# https://godbolt.org/z/rv1sTe7xv
product(unsigned long, unsigned long):
mov eax, dword ptr [esp + 8]
mul dword ptr [esp + 4]
ret
quotient(unsigned long long, unsigned long, unsigned long*):
push ebp
push ebx
push edi
push esi
sub esp, 12
call .L1$pb
.L1$pb:
pop ebx
.Ltmp2:
add ebx, offset _GLOBAL_OFFSET_TABLE_+(.Ltmp2-.L1$pb)
mov esi, dword ptr [esp + 44]
mov edi, dword ptr [esp + 32]
mov ebp, dword ptr [esp + 40]
push 0
push ebp
push dword ptr [esp + 44]
push edi
call __udivdi3@PLT
add esp, 16
imul ebp, eax
sub edi, ebp
mov dword ptr [esi], edi
add esp, 12
pop esi
pop edi
pop ebx
pop ebp
ret
### end of diversion ###
Both compilers miss the fact that the i386 processor has a narrowing
integer division and can therefore divide 64-bit / 32-bit numbers,
for example with the well-known "long" alias "schoolbook" division,
returning 64-bit quotient and 32-bit remainder:
.arch generic 32
.code32
.intel_syntax noprefix
.text
quotient:
mov ecx, [esp+12] # ecx = divisor
.if 0
xor edx, edx
mov eax, [esp+8] # edx:eax = high dword of dividend
cmp eax, edx
je 0f # high dword of dividend = 0?
.else
xor eax, eax
mov edx, [esp+8] # eax:edx = high dword of dividend
cmp edx, ecx
jb 0f # high dword of dividend < divisor?
# quotient < 2**32?
xchg eax, edx
.endif
div ecx # eax = high dword of quotient,
0: # edx = high dword of dividend'
push eax
mov eax, [esp+8] # edx:eax = dividend'
div ecx # eax = low dword of quotient,
# edx = remainder
mov ecx, remainder # ecx = address of remainder
mov [ecx], edx
pop edx # edx:eax = quotient
ret
.end
JFTR: dependent on the magnitude of the numbers and the processor
it MIGHT be better to omit comparison and branch: there's a
trade-öff between the latency of the (un-pipelined) division
instruction and the latency of the conditional branch due to
misprediction.
Stefan Kanthak
