[ARM] Eliminate redundant "mov rN, sp" instructions in Thumb1.

This takes sequences like "mov r4, sp; str r0, [r4]", and optimizes them
to something like "str r0, [sp]".

For regular stack variables, this optimization was already implemented:
we lower loads and stores using frame indexes, which are expanded later.
However, when constructing a call frame for a call with more than four
arguments, the existing optimization doesn't apply.  We need to use
stores which are actually relative to the current value of sp, and don't
have an associated frame index.

This patch adds a special case to handle that construct.  At the DAG
level, this is an ISD::STORE where the address is a CopyFromReg from SP
(plus a small constant offset).

This applies only to Thumb1: in Thumb2 or ARM mode, a regular store
instruction can access SP directly, so the COPY gets eliminated by
existing code.

The change to ARMDAGToDAGISel::SelectThumbAddrModeSP is a related
cleanup: we shouldn't pretend that it can select anything other than
frame indexes.

Differential Revision: https://reviews.llvm.org/D59568

llvm-svn: 356601

0 files changed, 0 insertions, 0 deletions