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digitalmars.D - Any reason why ++1 is not folded to a new constant?

reply Basile B. <b2.temp gmx.com> writes:
I've reached a similar problem in another language and I wanted 
to see what is the D policy. I've been surprised by the result. 
It seems that there's no special case for compile-time-only 
values, eg this case of RValue:
---
void main()
{
     writeln(++1); // NG: cannot modify constant `1`
}
---

is there any reasons why ++1 is not optimized to 2 ?

Thanks for the enlightment.
Oct 20 2020
next sibling parent Basile B. <b2.temp gmx.com> writes:
On Tuesday, 20 October 2020 at 12:41:48 UTC, Basile B. wrote:

 I've reached a similar problem in another language and I wanted 
 to see what is the D policy. I've been surprised by the result. 
 It seems that there's no special case for compile-time-only 
 values, eg this case of RValue:
 ---
 void main()
 {
     writeln(++1); // NG: cannot modify constant `1`
 }
 ---

 is there any reasons why ++1 is not optimized to 2 ?

 Thanks for the enlightment.


well my first guess is that it's because it's lowered to a binass 
operation before optimization.

when you think that the expression optimizer sees

   `writeln(++1)`

actually it sees

   `writeln(1 += 1)`

hence the error...
Oct 20 2020
prev sibling parent reply Walter Bright <newshound2 digitalmars.com> writes:
On 10/20/2020 5:41 AM, Basile B. wrote:

 I've reached a similar problem in another language and I wanted to see what is 
 the D policy. I've been surprised by the result. It seems that there's no 
 special case for compile-time-only values, eg this case of RValue:
 ---
 void main()
 {
      writeln(++1); // NG: cannot modify constant `1`
 }
 ---
 
 is there any reasons why ++1 is not optimized to 2 ?


Because ++ is supposed to operate on an lvalue, and `1` is an rvalue. `++1` is 
nonsense.

I don't see the point to adding a special case for it - special cases are warts 
and need strong justifications to add.
Oct 21 2020
next sibling parent mw <mingwu gmail.com> writes:
On Wednesday, 21 October 2020 at 21:49:11 UTC, Walter Bright 
wrote:

 Because ++ is supposed to operate on an lvalue, and `1` is an 
 rvalue. `++1` is nonsense.

 I don't see the point to adding a special case for it - special 
 cases are warts and need strong justifications to add.


👍, well said.

I hope every D language feature has no special cases.
Oct 21 2020
prev sibling parent reply Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Wednesday, 21 October 2020 at 21:49:11 UTC, Walter Bright 
wrote:

 On 10/20/2020 5:41 AM, Basile B. wrote:

 I've reached a similar problem in another language and I 
 wanted to see what is the D policy. I've been surprised by the 
 result. It seems that there's no special case for 
 compile-time-only values, eg this case of RValue:
 ---
 void main()
 {
      writeln(++1); // NG: cannot modify constant `1`
 }
 ---
 
 is there any reasons why ++1 is not optimized to 2 ?


 Because ++ is supposed to operate on an lvalue, and `1` is an 
 rvalue. `++1` is nonsense.

 I don't see the point to adding a special case for it - special 
 cases are warts and need strong justifications to add.


The question is then: why did it compile?
gcc gives
    error: lvalue required as increment operand

when one tries it in C.
Oct 22 2020
parent Mathias LANG <geod24 gmail.com> writes:
On Thursday, 22 October 2020 at 09:49:32 UTC, Patrick Schluter 
wrote:

 The question is then: why did it compile?
 gcc gives
    error: lvalue required as increment operand

 when one tries it in C.


It doesn't ? https://run.dlang.io/is/sk4kKw
Oct 22 2020