Place intrinsics in individual object files

[tmiasko]

No description provided.

[bjorn3]

I think you will need to add #[inline] to all the functions wrapped by the intrinsics to ensure that they are compiled in the same codegen unit as the intrinsic.

[tmiasko]

The interaction with other attributes is not ideal. I had to disable attribute expansions in win64_128bit_abi_hack, this seems fine now, but could be a footgun in a future.

@bjorn3 So far I didn't found it necessary for release builds (based on nm and objdump -r --disassemble). Is it different for you?

[alexcrichton]

Should we perhaps go ahead and just place all intrinsics in their own codegen unit?

And yes it is not necessary to use #[inline] due to ThinLTO.

[tmiasko]

I added #[inline] on undefined symbols from nm to get similar results regardless of ThinLTO.

I cross-validated partitioning with libgcc.a installed on my system. They match with default features, and there is a small mismatch when c feature is enabled. I don't feel much need to change things further, unless there is some specific target that would like me to test against.

[alexcrichton]

I don't think there's any realistic need for more than one symbol per CGU, even if libgcc does something similar. I also think it would be better to automatically place everything with one-symbol-per-object rather than having to manually annotate each function with a new attribute.

[alexcrichton]

Is the #[inline] necessary here for all these functions? I would say that we should probably avoid adding that if we can, only adding it if we profile at some point and see that LLVM isn't doing great-enough inlining.

[tmiasko]

The #[inline] is necessary to get code that doesn't have large amount of trivial indirections for non-ThinLTO builds. I can drop those changes if you want.

[alexcrichton]

I agree yeah that if ThinLTO is disabled it's a problem, but I think ThinLTO is enabled everywhere, so I'm not sure it's a case we need to worry about?

[tmiasko]

In context of official builds, I would expect this to be the case. Hard to say how it used more generally. In any case, we can revisit this aspect later if necessary. I removed inlining changes for now.

[alexcrichton]

Ok I've published 0.1.27 with this now too