I tried compiling a simple Hello World on Windows
with the latest DMD and the latest stable LDC. DMD
compilation results in 239kb executable and LDC
compilation results in 500kb executable (more than
twice bigger). -Os and -flto=full didn't do anything
to the executable size. So I'm wondering why is that?
I thought that LDC compiled binaries should be smaller
due to different kinds of optimizations in LLVM.

On Wednesday, 20 March 2019 at 10:28:24 UTC, Rel wrote:
> I tried compiling a simple Hello World on Windows
> with the latest DMD and the latest stable LDC. DMD
> compilation results in 239kb executable and LDC
> compilation results in 500kb executable (more than
> twice bigger). -Os and -flto=full didn't do anything
> to the executable size. So I'm wondering why is that?
> I thought that LDC compiled binaries should be smaller
> due to different kinds of optimizations in LLVM.

So you're comparing LDC with DMD, not GDC as the title suggests.

DMD on Windows defaults to 32-bit code *and* the DigitalMars C runtime, while a 64-bit LDC to 64-bit code and the MS runtime. Which runtime (static/dynamic) depends on whether you invoke LDC in a VS command prompt or in a naked cmd.exe, and can be tweaked via -mscrtlib. I guess that accounts for most of the size difference; just check your executables' DLL dependencies.

Try coupling `-flto=full` with `-defaultlib=phobos2-ldc-lto,druntime-ldc-lto`, that probably helps in reducing the size further.