On Tuesday, 5 May 2015 at 17:51:51 UTC, Johannes Pfau wrote:
Am Tue, 05 May 2015 11:37:17 +0000
schrieb "Mike" <n...@none.com>:
So, perhaps in my ignorance, I have to say that I don't need a
-ffreestanding option, but I don't consider myself much of an
expert in this field. If you know of a need for the
-ffreestanding option, please let it be known.
What I really need is more granular control of the language
features, either by adding compiler switches, or delegating
implementation to the runtime. My dream would be to have
runtime .di files inform the compiler what the language
features look like, and have the compiler use that information
to generate optimized code or compiler errors if the runtime
doesn't provide what everything compiler needs.
Isn't this simply two ways of looking at the same thing? You
want to disable language features (I guess mostly those
requiring runtime hooks), -ffreestanding disables all these
features.
I realize you want fine-grained control. But then:
-ffreestanding:
-fno-exceptions
-fno-rtti
-fno-gc
-fno-invariants
-fno-moduleinfo
-fno-string-switch
-fno-utf-support (foreach over utf strings)
-fno-boundscheck
-fno-switch-error
From an implementation point of view these are the same and
fine-grained switches are easy to support.
Access using .di is then simple to add as well. If the compiler
is clever it can look if the required features/hooks are
implemented. This requires some compiler work. Simpler
alternative: Enum values in special d module (e.g. rt.config)
enum RUNTIME_SUPPORTS_GC = false;
At the moment, the most pressing issue for me is the phony
support I have to add for TypeInfo and the removal of dead
code (or lack there of) due to GCC bug 192. Some binaries
I've created are so full of TypeInfo stuff that I can't even
get them to fit in my MCU's flash memory for testing and
debugging. Not to mention the added upload time it takes,
diminishing the efficiency of my development cycle.
I'll implement fno-rtti next weekend. We can think about more
fine-grained solutions in the future but fno-rtti should be a
good start.
I remember from previous discussions that there was work to be
done in binutils to get better LTO and dead-code removal. I'd
be interested in hearing more details about that too.
Thanks for the continued support,
Mike
Current implementation of compilers assumes the existence of some
symbols from libc, which leads to an infinite loop if we want to
implement primitives like "memset" with our own code because the
compiler will optimize consecutive set with "memset". This
suggests that we cannot write a freestanding program without
supports from compiler. With "-betterC" flag, dmd/gdc/ldc also
come into this issue[5], which also applies to C/C++[1] and rust
[2][3][4].
It would be better to provide a standard flag like
"-ffreestanding" (or -fno-builtin?) to disable such optimizations
to facilitate freestanding programming instead of forcing the
developers to hack around different compiler implementations, so
I was wondering is there any progress on this problem?
[1] https://godbolt.org/g/5gVWeN
[2]
https://play.rust-lang.org/?gist=64f2acafa8cec112893633a5f2e12a9a&version=stable&mode=release&edition=2015
[3] https://github.com/rust-lang/rust/issues/10116
[4] https://github.com/thestinger/rust-core#freestanding
[5] https://run.dlang.io/is/nnKWnZ
