HomePhabricator

Revert "[CodeGen] [WinException] Only produce handler data at the end of the…

Authored by hans on Nov 3 2020, 4:01 AM.

Description

Revert "[CodeGen] [WinException] Only produce handler data at the end of the function if needed"

This caused an explosion in ICF times during linking on Windows when libfuzzer
instrumentation is enabled. For a small binary we see ICF time go from ~0 to
~10 s. For a large binary it goes from ~1 s to forevert (I gave up after 30
minutes).

See comment on the code review.

If we are going to write handler data (that is written as variable
length data following after the unwind info in .xdata), we need to
emit the handler data immediately, but for cases where no such
info is going to be written, skip emitting it right away. (Unwind
info for all remaining functions that hasn't gotten it emitted
directly is emitted at the end.)

This does slightly change the ordering of sections (triggering a
bunch of updates to DebugInfo/COFF tests), but the change should be
benign.

This also matches GCC's assembly output, which doesn't output
.seh_handlerdata unless it actually is needed.

For ARM64, the unwind info can be packed into the runtime function
entry itself (leaving no data in the .xdata section at all), but
that can only be done if there's no follow-on data in the .xdata
section. If emission of the unwind info is triggered via
EmitWinEHHandlerData (or the .seh_handlerdata directive), which
implicitly switches to the .xdata section, there's a chance of the
caller wanting to pass further data there, so the packed format
can't be used in that case.

Differential Revision: https://reviews.llvm.org/D87448

This reverts commit 36c64af9d7f97414d48681b74352c9684077259b.