Bad codegen for comparison with postfix increment

[Iain Buclaw]

On Thursday, 9 May 2024 at 18:24:12 UTC, Iain Buclaw wrote:
> On Wednesday, 8 May 2024 at 14:26:25 UTC, Paul Backus wrote:
>> When the following program is compiled with GDC 14.1, the 
>> assertion fails:
>>
>>     void main() {
>>         int d = 42;
>>         bool o = d > d++;
>>         assert(o == false);
>>     }
>>
>> (Godbolt: https://d.godbolt.org/z/e63xbb9Td)
>>
>> The assertion passes when the program is compiled with DMD 
>> 2.108.0.
>>
>> The same bug was recently reported for LDC: 
>> https://github.com/ldc-developers/ldc/issues/4651
>
> I want to say that rewriting `d > d++` to `d++ < d` is a valid 
> optimization.
>
> See also `opCmp` rewriting `a < b` into `b.opCmp(a)`
>
> https://dlang.org/spec/operatoroverloading.html#compare

Even when it doesn't swap order, DMD doesn't store the left hand 
of the expression to a target/temporary when operator overloading 
is involved.

https://d.godbolt.org/z/jYYr1WMhn


```
o = d.opCmp(d.opUnary!"++"())
```

Effectively
```
arg = d.opUnary!"++"(); // d mutates
o = d.opCmp(arg) > 0;   // true
```