rewrite do-while loop as for loop

[WalterBright]

I should have thought of this a long time ago.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#16187"

[WalterBright]

This baffles me:

++ diff -up --strip-trailing-cr runnable/extra-files/runnable-cov2.lst 'D:\a\dmd\dmd\compiler\test\test_results/runnable/runnable-cov2.lst2'

--- runnable/extra-files/runnable-cov2.lst	2024-02-14 23:48:55.360386500 +0000

+++ "D:\\a\\dmd\\dmd\\compiler\\test\\test_results/runnable/runnable-cov2.lst2"	2024-02-14 23:52:09.81466[200](https://github.com/dlang/dmd/actions/runs/7908873893/job/21588901790?pr=16187#step:10:201)0 +0000

@@ -71,4 +71,3 @@

        1|    test24264();

        1|    return 0;

         |}

-cov2.d is 95% covered

The generated .lst file and the expected .lst file are identical.

[ibuclaw]

The generated .lst file and the expected .lst file are identical.

Remove the last line.

-cov2.d is 95% covered

[ibuclaw]

I'm expecting 100 or so lines to be removed then from blockexit, dinterpret, toir, ...

[WalterBright]

@ibuclaw I am, too, I'm just waiting for it to work before taking a meat-axe to it.

[WalterBright]

I'm not getting the fail220.d or the ice17074.d extra error messages on my setup. Neither of those files has a do-while in it, either.

[WalterBright]

Remove the last line.

Thanks, that worked

[WalterBright]

The 40 minute timeouts suggest an infinite loop in the compiler.

[WalterBright]

Which means I need to replace every do-while in the compiler with a while, till I find out which one is broken.

[braddr]

A counter and a begin/end range to control when to do the old vs new behavior would allow skipping the manual code updating and allow for binary searching the codebase.

[WalterBright]

Hi Brad! Nice to see you around.

The real time waster is waiting 40 minutes for the test suite to run for each iteration. There are only a handful of do-while's in the source code, so the manual edits aren't that bad. The culprit may be in druntime, too.

I must have done something stupid with this simple change, but I'm blind to what it is. Grump.

[WalterBright]

I think I'll try your idea...

[WalterBright]

Definitely frustrating. I removed all the compiler uses of 'do' but the problem remains.

[tim-dlang]

The rewrite is not equivalent for do-while loops containing continue. Consider this example:

import std.stdio;
void main()
{
    int i = 10;
    do
    {
        writeln(i);
        continue;
    }
    while(--i > 0);
}

Previously this printed the numbers from 10 to 1, but with this PR it is an endless loop always printing 10.

[WalterBright]

@tim-dlang wow, thanks! How did you figure that out?

[tim-dlang]

@tim-dlang wow, thanks! How did you figure that out?

Sometime ago I also wondered, why the do-while loop is not lowered like other loops.

[WalterBright]

I'll have to think of another way to lower it!

[WalterBright]

The way to make this work is to have the continue point to the if statement. Unfortunately, the continue logic in the compiler is overly complex. Figuring out how to patch it is not a net improvement over just having a do-while construct.

I'm going to reluctantly close this.