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[ARM] Implement execute-only support in CodeGen

Authored by prakhar on Dec 14 2016, 11:59 PM.

Description

[ARM] Implement execute-only support in CodeGen

Summary:
This implements execute-only support for ARM code generation, which
prevents the compiler from generating data accesses to code sections.
The following changes are involved:

  • Add the CodeGen option "-arm-execute-only" to the ARM code generator.
  • Add the clang flag "-mexecute-only" as well as the GCC-compatible alias "-mpure-code" to enable this option.
  • When enabled, literal pools are replaced with MOVW/MOVT instructions, with VMOV used in addition for floating-point literals. As the MOVT instruction is required, execute-only support is only available in Thumb mode for targets supporting ARMv8-M baseline or Thumb2.
  • Jump tables are placed in data sections when in execute-only mode.
  • The execute-only text section is assigned section ID 0, and is marked as unreadable with the SHF_ARM_PURECODE flag with symbol 'y'. This also overrides selection of ELF sections for globals.

Reviewers: t.p.northover, rengolin

Subscribers: llvm-commits, aemerson

Differential Revision: https://reviews.llvm.org/D27450

llvm-svn: 289786

Details

Committed
prakharDec 14 2016, 11:59 PM
Differential Revision
D27450: [ARM] Implement execute-only support in CodeGen
Parents
rGe640c6f765c8: Allow ELF section flags to be specified numerically
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