"Paul Koning" <[email protected]> wrote:
>> On Jan 9, 2023, at 10:20 AM, Stefan Kanthak <[email protected]> wrote:
>>
>> "Paul Koning" <[email protected]> wrote:
>>
>>>> On Jan 9, 2023, at 7:20 AM, Stefan Kanthak <[email protected]> wrote:
>>>>
>>>> Hi,
>>>>
>>>> GCC (and other C compilers too) support the widening multiplication
>>>> of i386/AMD64 processors, but DON'T support their narrowing division:
>>>
>>> I wonder if this changed in the recent past.
>>> I have a pattern for this type of thing in pdp11.md:
>> [...]
>>> and I'm pretty sure this worked at some point in the past.
>>
>> Unfortunately the C standard defines that the smaller operand (of lesser
>> conversion rank), here divisor, has to undergo a conversion to the "real
>> common type", i.e. the broader operand (of higher conversion rank), here
>> dividend. Unless the information about promotion/conversion is handed over
>> to the code generator it can't apply such patterns -- as demonstrated by
>> the demo code.
> Yes, I was thinking the same. But I spent a while on that pattern -- I
> wanted to support div/mod as a single operation because the machine has
> that primitive. And I'm pretty sure I saw it work before I committed
> that change. That's why I'm wondering if something changed.
I can't tell from the past how GCC once worked, but today it can't
(or doesn't) use such patterns, at least not on i386/AMD64 processors.
To give another example where the necessary information is most
obviously NOT propagated from front end to back end:
--- clmul.c ---
// widening carry-less multiplication
unsigned long long clmul(unsigned long p, unsigned long q)
{
unsigned long long r = 0;
unsigned long s = 1UL << 31;
do {
r <<= 1;
if (q & s)
#ifdef _MSC_VER
(unsigned long) r ^= p;
#else
r ^= p; // no need to promote/convert p here!
#endif
} while (s >>= 1);
return r;
}
--- EOF ---
# https://gcc.godbolt.org/z/E99v7fEP3
clmul(unsigned long, unsigned long):
push ebp
mov ecx, -2147483648
xor eax, eax
xor edx, edx
push edi # OOPS: superfluous
xor edi, edi # OOPS: superfluous
push esi
push ebx # OUCH: WTF?
mov ebp, DWORD PTR [esp+24]
mov ebx, 32 # OUCH: WTF?
mov esi, DWORD PTR [esp+20]
.L3:
shld edx, eax, 1
add eax, eax
test ebp, ecx
je .L2
xor eax, esi
xor edx, edi # OOPS: superfluous
.L2:
shr ecx, 1
sub ebx, 1 # OUCH: WTF?
jne .L3
pop ebx # OUCH: WTF?
pop esi
pop edi # OOPS: superfluous
pop ebp
ret
8 superfluous instructions out of the total 25 instructions!
NOT AMUSED
Stefan
