[all-commits] [llvm/llvm-project] 227374: [ARM] generate armv6m eXecute Only (XO) code

[Ties Stuij via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2273741ea2547ecda28cb01d7679d0563b35ac16
      https://github.com/llvm/llvm-project/commit/2273741ea2547ecda28cb01d7679d0563b35ac16
  Author: Ties Stuij <ties.stuij at arm.com>
  Date:   2023-06-23 (Fri, 23 Jun 2023)

  Changed paths:
    M llvm/lib/Target/ARM/ARMAsmPrinter.cpp
    M llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
    M llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
    M llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
    M llvm/lib/Target/ARM/ARMISelLowering.cpp
    M llvm/lib/Target/ARM/ARMInstrThumb.td
    M llvm/lib/Target/ARM/ARMMCInstLower.cpp
    M llvm/lib/Target/ARM/ARMPredicates.td
    M llvm/lib/Target/ARM/ARMSubtarget.cpp
    M llvm/lib/Target/ARM/MCTargetDesc/ARMBaseInfo.h
    M llvm/test/CodeGen/ARM/execute-only-section.ll
    M llvm/test/CodeGen/ARM/execute-only.ll

  Log Message:
  -----------
  [ARM] generate armv6m eXecute Only (XO) code

[ARM] generate armv6m eXecute Only (XO) code for immediates, globals

Previously eXecute Only (XO) support was implemented for targets that support
MOVW/MOVT (~armv7+). See: https://reviews.llvm.org/D27449

XO prevents the compiler from generating data accesses to code sections. This
patch implements XO codegen for armv6-M, which does not support MOVW/MOVT, and
must resort to the following general pattern to avoid loads:

    movs    r3, :upper8_15:foo
    lsls    r3, #8
    adds    r3, :upper0_7:foo
    lsls    r3, #8
    adds    r3, :lower8_15:foo
    lsls    r3, #8
    adds    r3, :lower0_7:foo
    ldr     r3, [r3]

This is equivalent to the code pattern generated by GCC.

The above relocations are new to LLVM and have been implemented in a parent
patch: https://reviews.llvm.org/D149443.

This patch limits itself to implementing codegen for this pattern and enabling
XO for armv6-M in the backend.

Separate patches will follow for:
- switch tables
- replacing specific loads from constant islands which are spread out over the
  ARM backend codebase. Amongst others: FastISel, call lowering, stack frames.

Reviewed By: john.brawn

Differential Revision: https://reviews.llvm.org/D152795