Widening multiplication, but no narrowing division [i386/AMD64]

[Stefan Kanthak]

"Paul Koning" <paulkoning@comcast.net> wrote:

>> On Jan 9, 2023, at 7:20 AM, Stefan Kanthak <stefan.kanthak@nexgo.de> 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.

regards
Stefan