tree a4020e8df4137a2de5e338852faaeb95811fce25
parent 3e4aa4e8671523321af51449e0569f455ef3ad43
author Peter Smith <peter.smith@linaro.org> 1545042827 +0000
committer Peter Smith <peter.smith@linaro.org> 1545042827 +0000

[ELF][ARM] Add support for architecture v6m thunks

ARM Architecture v6m is used by the smallest microcontrollers such as the
cortex-m0. It is Thumb only (no Thumb 2) which prevents it from using the
existing Thumb 2 range extension thunks as these use the Thumb 2 movt/movw
instructions. Range extension thunks are not usually needed for
microcontrollers due to the small amount of flash and ram on the device,
however if code is copied from flash into ram then a range extension thunk
is required to call that code.

This change adds support for v6m range extension thunks. The procedure call
standard APCS permits a thunk to corrupt the intra-procedural scratch
register r12 (referred to as ip in the APCS). Most Thumb instructions do
not permit access to high registers (r8 - r15) so the thunks must spill
some low registers (r0 - r7) to perform the control transfer.

Fixes pr39922

Differential Revision: https://reviews.llvm.org/D55555


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@349337 91177308-0d34-0410-b5e6-96231b3b80d8