LTO (was: Compiling multiple source files --- each file separately, or all together?)

[Iain Buclaw]

On Friday, 26 December 2025 at 17:38:55 UTC, Adam Wilson wrote:
> On Friday, 26 December 2025 at 10:44:21 UTC, David Given wrote:
>> When compiling C or C++, and you have multiple source files, 
>> you typically compile each source file independently into a .o 
>> file and then link them together. This is either done manually 
>> in the build script or automatically if you pass multiple 
>> source files into the compiler program.
>
> I was speaking with Walter a couple weeks ago and he thinks 
> that future of DMD is unified compilation. There are a number 
> of things the compiler can do when compiling all the source 
> files at once that you just cannot do with independent 
> compilation and Walter would like to be able to do those things.
>
> So the general recommendation from Walter is unified 
> compilation.

This is the one of the key points of LTO (-flto) though - 
something that I'm sure of having mentioned at most DConf's since 
2013. :-)

Serialise the post-semantic (lightweight?) backend representation 
into the individual `.o` files, then read it all in at the end 
for unified compilation (with `-O` turned on) before linking.