An identity COPY like this:
%AL = COPY %AL, %EAX<imp-def>
has no semantic effect, but encodes liveness information: Further users
of %EAX only depend on this instruction even though it does not define
the full register.
Replace the COPY with a KILL instruction in those cases to maintain this
liveness information. (This reverts a small part of r238588 but this
time adds a comment explaining why a KILL instruction is useful).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274952 91177308-0d34-0410-b5e6-96231b3b80d8
The red zone consists of 128 bytes beyond the stack pointer so that the
allocation of objects in leaf functions doesn't require decrementing
rsp. In r255656, we introduced an optimization that would cheaply
materialize certain constants via push/pop. Push decrements the stack
pointer and stores it's result at what is now the top of the stack.
However, this means that using push/pop would encroach on the red zone.
PR26023 gives an example where this corrupts an object in the red zone.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256808 91177308-0d34-0410-b5e6-96231b3b80d8
Use the 3-byte (4 with REX prefix) push-pop sequence for materializing
small constants. This is smaller than using a mov (5, 6 or 7 bytes
depending on size and REX prefix), but it's likely to be slower, so
only used for 'minsize'.
This is a follow-up to r255656.
Differential Revision: http://reviews.llvm.org/D15549
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255936 91177308-0d34-0410-b5e6-96231b3b80d8
Matthias Braun