When we emit a tail call for Armv8-M, but then discover that the caller needs to
save/restore `LR`, we convert the tail call to an ordinary one, since restoring
`LR` takes extra instructions, which may negate the benefits of the tail
call. If the callee, however, takes stack arguments, this conversion is
incorrect, since nothing has been done to pass the stack arguments.
Thus the patch reverts https://reviews.llvm.org/rL294000
Also, we improve the instruction sequence for popping `LR` in the case when we
couldn't immediately find a scratch low register, but we can use as a temporary
one of the callee-saved low registers and restore `LR` before popping other
callee-saves.
Differential Revision: https://reviews.llvm.org/D39599
llvm-svn: 318143
Summary:
The tail call optimisation is performed before register allocation, so
at that point we don't know if LR is being spilt or not. If LR was spilt
to the stack, then we cannot do a tail call optimisation. That would
involve popping back into LR which is not possible in Thumb1 code.
Reviewers: rengolin, jmolloy, rovka, olista01
Reviewed By: olista01
Subscribers: llvm-commits, aemerson
Differential Revision: https://reviews.llvm.org/D29020
llvm-svn: 294000
Momchil Velikov