[Issue 20983] New: Insufficient garbage collection in std.format context

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20983

          Issue ID: 20983
           Summary: Insufficient garbage collection in std.format context
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: kinke at gmx.net

Testcase:
-----
import core.memory;
import core.stdc.stdio;
import std.range;
import std.format;

int toMB(ulong size) { return cast(int) (size / 1048576.0 + 0.5); }

void printGCStats()
{
    const stats = GC.stats;
    const used = toMB(stats.usedSize);
    const free = toMB(stats.freeSize);
    const total = toMB(stats.usedSize + stats.freeSize);
    printf("  GC stats: %dM used, %dM free, %dM total\n", used, free, total);
}

void formatRange()
{
    string str = format("%s", iota(100_000_000));
    printf("  string size: %dM\n", toMB(str.length));
}

void main()
{
    printGCStats();

    while (true)
    {
        puts("Starting");
        formatRange();
        GC.collect();
        printGCStats();
    }
}
-----

On Windows, with DMD 2.092:

> dmd -m64 test.d
> test.exe
  GC stats: 0M used, 1M free, 1M total
Starting
  string size: 943M
  GC stats: 1168M used, 1139M free, 2306M total
Starting
  string size: 943M
  GC stats: 1168M used, 2456M free, 3623M total
Starting
  string size: 943M
  GC stats: 1168M used, 2456M free, 3623M total
Starting
  string size: 943M
  GC stats: 1168M used, 2456M free, 3623M total
Starting
  string size: 943M
  GC stats: 1168M used, 2456M free, 3623M total
...

The expected behavior is that there should be no used GC memory after an
iteration - the huge string is built in another function and doesn't escape,
and the manual collection should mark all memory as being unused.

Using the precise GC via `test.exe --DRT-gcopt=gc:precise` makes no difference.

No idea whether that's a GC issue (druntime) or std.format doing weird things.

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