On some edge cases such as Chromium compiled with full instrumentation we
have a .text section over twice the size of the maximum branch range and
the instrumented code generation containing many examples of the erratum
sequence. The combination of Thunks and many erratum sequences causes
finalizeAddressDependentContent() to not converge. We end up with:
start
- Thunk Creation (disturbs addresses after thunks, creating more patches)
- Patch Creation (disturbs addresses after patches, creating more thunks)
- goto start
In most images with few thunks and patches the mutual disturbance does not
cause convergence problems. As the .text size and number of patches go up
the risk increases.
A way to prevent the thunk creation from interfering with patch creation is
to round up the size of the thunks to a 4KiB boundary when the
erratum patch is enabled. As the erratum sequence only triggers when an
instruction sequence starts at 0xff8 or 0xffc modulo (4 KiB) by making the
thunks not affect addresses modulo (4 KiB) we prevent thunks from
interfering with the patch.
The patches themselves could be aggregated in the same way that Thunks are
within ThunkSections and we could round up the size in the same way. This
would reduce the number of patches created in a .text section size >
128 MiB but would not likely help convergence problems.
Differential Revision: https://reviews.llvm.org/D71281
fixes (remaining part of) pr44071, other part in D71242
86d24193a9