In addition to being rather hard to follow, there isn't a good reason
why FatLTO shouldn't just share the same code for setting module flags
for (Thin)LTO. This patch simplifies the logic and makes sure we use set
these flags in a consistent way, independent of FatLTO.
Additionally, we now test that output in the .llvm.lto section actually
matches the output from Full and Thin LTO compilation.
Currently, the UnifiedLTO pipeline seems to have trouble with several
LTO features, like SplitLTO units, which means we cannot use important
optimizations like Whole Program Devirtualization or security hardening
instrumentation like CFI.
This patch reverts FatLTO to using distinct pipelines for Full LTO and
ThinLTO. It still avoids module cloning, since that was error prone.
Since FatLTO now uses the UnifiedLTO pipeline, we should not set the
ThinLTO module flag to true, since it may cause an assertion failure.
See https://github.com/llvm/llvm-project/issues/70703 for context.
https://github.com/llvm/llvm-project/issues/70703 pointed out that
cloning LLVM modules could lead to miscompiles when using FatLTO.
This is due to an existing issue when cloning modules with labels (see
#55991 and #47769). Since this can lead to miscompilation, we can avoid
cloning the LLVM modules, which was desirable anyway.
This patch modifies the EmbedBitcodePass to no longer clone the module
or run an input pipeline over it. Further, it make FatLTO always perform
UnifiedLTO, so we can still defer the Thin/Full LTO decision to
link-time. Lastly, it removes dead/obsolete code related to now defunct
options that do not work with the EmbedBitcodePass implementation any
longer.
Fat LTO objects contain both LTO compatible IR, as well as generated
object code. This allows users to defer the choice of whether to use LTO
or not to link-time. This is a feature available in GCC for some time,
and makes the existing -ffat-lto-objects flag functional in the same
way as GCC's.
This patch adds support for that flag in the driver, as well as setting the
necessary codegen options for the backend. Largely, this means we select
the newly added pass pipeline for generating fat objects.
Users are expected to pass -ffat-lto-objects to clang in addition to one
of the -flto variants. Without the -flto flag, -ffat-lto-objects has no
effect.
// Compile and link. Use the object code from the fat object w/o LTO.
clang -fno-lto -ffat-lto-objects -fuse-ld=lld foo.c
// Compile and link. Select full LTO at link time.
clang -flto -ffat-lto-objects -fuse-ld=lld foo.c
// Compile and link. Select ThinLTO at link time.
clang -flto=thin -ffat-lto-objects -fuse-ld=lld foo.c
// Compile and link. Use ThinLTO with the UnifiedLTO pipeline.
clang -flto=thin -ffat-lto-objects -funified-lto -fuse-ld=lld foo.c
// Compile and link. Use full LTO with the UnifiedLTO pipeline.
clang -flto -ffat-lto-objects -funified-lto -fuse-ld=lld foo.c
// Link separately, using ThinLTO.
clang -c -flto=thin -ffat-lto-objects foo.c
clang -flto=thin -fuse-ld=lld foo.o -ffat-lto-objects # pass --lto=thin --fat-lto-objects to ld.lld
// Link separately, using full LTO.
clang -c -flto -ffat-lto-objects foo.c
clang -flto -fuse-ld=lld foo.o # pass --lto=full --fat-lto-objects to ld.lld
Original RFC: https://discourse.llvm.org/t/rfc-ffat-lto-objects-support/63977
Depends on D146776
Reviewed By: tejohnson, MaskRay
Differential Revision: https://reviews.llvm.org/D146777
