On 11/15/21 05:06, Florian Weimer wrote:
In the future, we might want to switch GCC not to generate both object code and LTO representation during the build process. For most packages, dual generation is not necessary because no relocatable object files for static linking are included in the RPM (neither as separate ET_REL .o files, nor in static .a archives). Final (non-relocatable) links of any kind will generate object code, so only LTO representation needs to be written by GCC. But in case relocatable object files are produced by the package (e.g., for a -static subpackage for static linking), it is necessary to generate object code for relocatable files as well. The reason is that only object code (not LTO representation) is a stable format, and it's the only way to achieve cross-toolchain linking. The way I envisioned LTO-only building for GCC was to replace the brp-strip-lto script <https://src.fedoraproject.org/rpms/redhat-rpm-config/blob/rawhide/f/brp-strip-lto> with somehting that errors out (fails the build) if any relocatable object files (.o) or static archives (.a) by default, and stop producing object code by default, only LTO representation. If a special redhat-rpm-config flag is set, brp-strip-lto comes back, and GCC is configured to produce both object code and LTO representation (basically what we have today).
If we produced LTO only static archives, does this mean end-users who want to use them would need to build their applications with LTO enabled? -Tom
However, Clang has chosen a different approach: Build object code in the final stages, via the brp-llvm-compile-lto-elf script: <https://src.fedoraproject.org/rpms/redhat-rpm-config/blob/rawhide/f/brp-llvm-compile-lto-elf> This does not really work for GCC in downstream because we have multiple GCCs there, with incompatible LTO representation. We also cannot replicate the exact command line options that have been used during the package-internal build process; we only have the default redhat-rpm-config flags at this point, and whatever has been serialized into the LTO representation. Fedora has multiple LLVMs (e.g., rust in Fedora 35 is at LLVM 12, but /usr/bin/clang is LLVM 13). LLVM bitcode is supposed to be more compatible: <https://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility> But don't know to what extent we test that. I'd prefer to use a single mechanism for both toolchains. But it seems that Clang does not support creating ELF object files that contain LLVM IR (GCC's default mode we use today). Given the problems with post-building object code for GCC, I'm not sure if this is feasible. Thoughts? Thanks, Florian PS: I tried to avoid fat/thin references because the terms are inconsistent across the toolchains.
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