Bad/No Dead Code Elimination about ldc HOT 9 CLOSED

Woops, I didn't mean to close this. Sorry.

I just did a manual compilation and linking of this example with ldc 2.9. The doNothing() code is removed properly so it appears to be a problem with ldc2 only (or at least a newer ldc than the llvm-2.9 based version). Just thought I would let people know that this example used to work, so that the search could be narrowed down to new ldc2/llvm-3.0 changes.

Thanks,
K.Wilson

from ldc.

Ok, then to clarify my configuration, I'm using revision 4e8afa8 of LDC2 (the latest trunk revision at the time of writing) with LLVM3. This issue shows up on both a Debian testing and Ubuntu 10.04 box.

from ldc.

I've reverse-engineered what's going on here, and it's not a bug per se, just different configurations. When DMD is run with the -lib switch, it creates an archive where basically every function gets its own object file. Therefore, object files that contain dead code are never pulled in. By default there is never any dead code elimination within an object file. It only happens at the level of the whole object file.

GDC used to write one object file per source file, which is more coarse-grained but still prevents most manifestations.

LDC puts one gigantic object file in the archive it creates. By default this contains one huge linker section. Therefore, when any of this stuff gets pulled in, all of it does.

To get DMD-like behavior, all you have to do is compile your libraries with -ffunction-sections and link with -L--gc-sections. The former causes every function to be put in its own section in the object file. The latter causes the linker to garbage collect unused sections, and hence not complain when an undefined symbol is referenced from dead code.

from ldc.

Shouldn't this be the default then?

from ldc.

Yes, this probably warrants closer discussion.

from ldc.

...In that case maybe we should build Phobos and Druntime with -ffunction-sections and -fdata-sections (which does the same thing as -ffunction-sections, except for static data instead of functions), too. It might result in smaller binaries.

from ldc.

Yep, with gdc I use phobos compiled with -ffunction-sections -fdata-sections just fine.

from ldc.

Other opinions?

from ldc.

We are building with --gc-sections by default now.

from ldc.