wristow added inline comments.
================
Comment at: llvm/lib/Transforms/IPO/PassManagerBuilder.cpp:614
// during ThinLTO and perform the rest of the optimizations afterward.
if (PrepareForThinLTO) {
// Ensure we perform any last passes, but do so before renaming anonymous
----------------
wenlei wrote:
> this also need to be `PrepareForThinLTO || PrepareForLTO` for oldPM?
I agree this is another instance where a balancing act question applies. In
this case, assuming the comment about the concern of code bloat is accurate,
it's not so much about compile-time resources in the full LTO back-end, but
rather about minimizing the ThinLTO bitcode write/read time. So if as this WIP
evolves, it ultimately is a win for SamplePGO to suppress some loop
optimizations (unrolling/vectorization) here, then that will probably also be a
//small // win in full LTO compile time. That said, in addition to these
loop-related optimizations, there are other transformations here that are done
in the full LTO pipeline (but not in the ThinLTO pipeline). So I suspect if
some change to check for `PrepareForThinLTO || PrepareForLTO` (rather than only
`PrepareForThinLTO`) makes sense here from a Sample PGO perspective, then the
change will be more complicated than simply adding the small set of passes here
followed by the early return (that is, I think there are probably things after
the `return` on line 621 that still ought to be enabled for full LTO --
essentially continuing to do them in the pre-link stage for full LTO, to try to
avoid needing to do too much work in the full LTO backend stage, since it's
more of a problem for the full backend to absorb that compile time cost).
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D69732/new/
https://reviews.llvm.org/D69732
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