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digitalmars.D.learn - Doubt about compiler: Front-End vs Back-End

reply Curious <just curious.com> writes:
Hi, could anyone shed a light about the Frond-End and Back-End?

I understood the Back-End part, where "roughly" each compiler 
will generate a machine code based on a IR (Intermediate 
Representation) generated by the Front-End.

But I'm having trouble with the Front-End part, and for what I 
gather here: https://wiki.dlang.org/DMD_Source_Guide


 The front-end (DMD-FE) implements all things D-specific: lexing 
 and parsing D syntax, instantiating templates, producing error 
 messages, etc. The same front-end code is used by DMD, GDC and 
 LDC.


Note the ending: "The same front-end code is used by DMD, GDC and 
LDC."

But what the meaning of this sentence?

These are 3 different projects so how they share the SAME 
front-end or how they are connected? Is it a source written in 
*.CPP for example and the 3 compilers uses the same source? Or 
the "SAME" means each compiler needs to implement the same code?

Looking over Github: https://github.com/dlang/ I see DMD, Phobos 
and so on, but no separated project called Front-End.

Thanks in advance.
Feb 16 2020
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Monday, 17 February 2020 at 02:55:56 UTC, Curious wrote:

 Note the ending: "The same front-end code is used by DMD, GDC 
 and LDC."

 But what the meaning of this sentence?


They literally share like 97% of the code.


 Looking over Github: https://github.com/dlang/ I see DMD, 
 Phobos and so on, but no separated project called Front-End.


Compare dmd:
https://github.com/dlang/dmd/

to ldc:
https://github.com/ldc-developers/ldc/

dmd has a `src` folder. ldc has a `dmd` folder. inside those 
you'll find most the content is identical - written in D btw.

gdc is a little different because it is based on an older version 
of dmd when it was written in C++ before it was ported to D. it 
will be moving to the D version probably later this year too, but 
it had to be C++ at first for gcc inclusion. But even so, it is 
still almost all the same code and even has some patches from new 
D versions ported back to c++ for it.


But basically all three compilers are branches of the same 
codebase with most the same patches applied going forward so they 
share the vast, vast majority of frontend code.
Feb 16 2020
parent Curious <just curious.com> writes:
On Monday, 17 February 2020 at 03:07:33 UTC, Adam D. Ruppe wrote:

 On Monday, 17 February 2020 at 02:55:56 UTC, Curious wrote:

 Note the ending: "The same front-end code is used by DMD, GDC 
 and LDC."

 But what the meaning of this sentence?


 They literally share like 97% of the code.


 Looking over Github: https://github.com/dlang/ I see DMD, 
 Phobos and so on, but no separated project called Front-End.


 Compare dmd:
 https://github.com/dlang/dmd/

 to ldc:
 https://github.com/ldc-developers/ldc/

 dmd has a `src` folder. ldc has a `dmd` folder. inside those 
 you'll find most the content is identical - written in D btw.

 gdc is a little different because it is based on an older 
 version of dmd when it was written in C++ before it was ported 
 to D. it will be moving to the D version probably later this 
 year too, but it had to be C++ at first for gcc inclusion. But 
 even so, it is still almost all the same code and even has some 
 patches from new D versions ported back to c++ for it.


 But basically all three compilers are branches of the same 
 codebase with most the same patches applied going forward so 
 they share the vast, vast majority of frontend code.


Hmm I see now, so DMD compiler has both Front-End and Back-End, 
the LDC in this case is getting only the Front-End part from DMD 
project.

So since the Front-End written in D and the GDC is still the 
older version (C++), then they need to replicate the new features 
from D to C++. Well this seems a pain.

By the way since the Front-End is in D, so it's occurring 
bootstrapping, interesting.

Thanks.
Feb 16 2020