As pointed out in
https://discourse.llvm.org/t/undeterministic-thin-index-file/69985, the
block count added to distributed ThinLTO index files breaks incremental
builds on ThinLTO - if any linked file has a different number of BBs,
then the accumulated sum placed in the index files will change, causing
all ThinLTO backend compiles to be redone.
The block count is only used for scaling of partial sample profiles, and
was added in D80403 for D79831.
This patch simply removes this field from the index files of non partial
sample profile compiles, which is NFC on the output of the compiler.
We subsequently need to see if this can be removed for partial sample
profiles without signficant performance loss, or redesigned in a way
that does not destroy caching.
Differential Revision: https://reviews.llvm.org/D148746
Allows ThinLTO indices to be written to disk on-the-fly/as-part-of “normal” linker execution. Previously ThinLTO indices could be written via --thinlto-index-only but that would cause the linker to exit early. For MLGO specifically, this enables saving the ThinLTO index files without having to restart the linker to collect data only available at later stages (i.e. output of --save-temps) of the linker's execution.
Note, this option does not currently work with:
--thinlto-object-suffix-replace, as this is intended to be used to consume minimized IR bitcode files while --thinlto-emit-index-files is intended to be run together with InProcessThinLTO (which cannot parse minimized IR).
--thinlto-prefix-replace support is left unimplemented but can be implemented if needed
Differential Revision: https://reviews.llvm.org/D127777
