When this pass was originally implemented, the fix pass was enabled
using a llvm command-line flag. This works fine, except in the case of
LTO, where the flag is not passed into the linker plugin in order to
enable the function pass in the LTO backend.
Now LTO exists, the expectation now is to use target features rather
than command-line arguments to control code generation, as this ensures
that different command-line arguments in different files are correctly
represented, and target-features always get to the LTO plugin as they
are encoded into LLVM IR.
The fall-out of this change is that the fix pass has to always be added
to the backend pass pipeline, so now it makes no changes if the function
does not have the right target feature to enable it. This should make a
minimal difference to compile time.
One advantage is it's now much easier to enable when compiling for a
Cortex-A53, as CPUs imply their own individual sets of target-features,
in a more fine-grained way. I haven't done this yet, but it is an
option, if the fix should be enabled in more places.
Existing tests of the user interface are unaffected, the changes are to
reflect that the argument is now turned into a target feature.
Reviewed By: tmatheson
Differential Revision: https://reviews.llvm.org/D114703
It means the same thing as -mllvm; there isn't any reason to have two
options which do the same thing.
Differential Revision: https://reviews.llvm.org/D45109
llvm-svn: 329965
Some early revisions of the Cortex-A53 have an erratum (835769) whereby it is
possible for a 64-bit multiply-accumulate instruction in AArch64 state to
generate an incorrect result. The details are quite complex and hard to
determine statically, since branches in the code may exist in some
circumstances, but all cases end with a memory (load, store, or prefetch)
instruction followed immediately by the multiply-accumulate operation.
The safest work-around for this issue is to make the compiler avoid emitting
multiply-accumulate instructions immediately after memory instructions and the
simplest way to do this is to insert a NOP.
This patch implements clang options to enable this workaround in the backend.
The work-around code generation is not enabled by default.
llvm-svn: 219604
Archibald Elliott