cmd/internal/obj: use prefix insn in MOV* opcodes for GOPPC64=power10
As background, Power10 adds prefixed load, store, and add immediate
instructions which encode 34b signed displacements. Likewise, they
also give the option to compute addresses against the PC. This enables
using simpler PC relative (PC-rel) relocations instead of maintaining a
dedicated pointer (the TOC) to the code/data blob on PPC64/linux.
Similary, there are several Go opcodes where it can be advantageous to
use prefixed instructions instead of composite sequences like oris/ori/add
to implement "MOVD <big const>, Rx" or "ADD <big const>, Rx, Ry", or
large offset load/stores like "MOVD <big constant>(Rx), Ry" using the same
framework which dynamically configures optab.
When selecting prefixed instruction forms, the assembler must also use
new relocations. These new relocations are always PC-rel by design, thus
code assembled as such has no implicit requirement to maintain a TOC
pointer when assembling shared objects. Thus, we can safely avoid
situations where some Go objects use a TOC pointer, and some do not. This
greatly simplifies linking Go objects. For more details about the
challenges of linking TOC and PC-rel compiled code, see the PPC64 ELFv2
ABI.
The TOC pointer in R2 is still maintained in those build configurations
which previously required it (e.x buildmode=pie). However, Go code built
with PC-rel relocations does not require the TOC pointer. A future
change could remove the overhead of maintaining a TOC pointer in those
build configurations.
This is enabled only for power10/ppc64le/linux.
A final noteworthy difference between the prefixed and regular load/store
instruction forms is the removal of the DS/DQ form restrictions. That
is, the immediate operand does not need to be aligned.
Updates #44549
Change-Id: If59c216d203c3eed963bfa08855e21771e6ed669
Reviewed-on: https://go-review.googlesource.com/c/go/+/355150 Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>