72301a9863
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> |
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