ARM64 atomics assembly survey

This survey only concerns header files that are involved in atomic operations.

commit ae8b3a83fb9de394f609035041cd7a668fda2ab3

Headers of interest

• arch/arm64/include/asm/atomic.h which includes
  • arch/arm64/include/asm/cmpxchg.h
  • arch/arm64/include/asm/lse.h
  • arch/arm64/include/asm/atomic-ll-sc.h
  • arch/arm64/include/asm/barrier.h

Syntactical variants in use

Key operand modifiers

The following translation is derived from Rust reference on inline assembly.

.. on Output

• =r
  • corresponds to Rust lateout(reg)
• =&r
  • corresponds to Rust out(reg)
• +&r
  • corresponds to Rust inout(reg)
• =Q
  • specific to AArch64
  • discard existing value in memory reference
  • corresponds to Rust latein(reg) with rewrite into a memref
    • Translation needs to know assembly dialect, which is Intel syntax mostly
    • Let's say it is translated into [{..}]
• +Q
  • specific to AArch64
  • may use existing value in memory reference
  • corresponds to Rust latein(reg) with rewrite into a memref
    • Translation needs to know assembly dialect

.. on Input

• rZ
  • This is specific to some machine like AArch64
    • … which is just xzr/wzr
  • Requires rewrite into Rust dialect, in this case just replacing it with xzr/wzr
• r
  • well… register
• one of {I,J,K,L} + r: See atomic_ll_sc.h
  • This is specific to AArch64
  • Modifies the size of immediate value
    • There is no clear Rust equivalent
    • In fact, when Ir or rI is specified, LLVM prefers to consider inserting an immediate of width I/J/K/L first, then a register as per r second
      • I has higher precedence over r
      • See llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp …
        • TargetLowering::getConstraintPreferences called by …
        • TargetLowering::ComputeConstraintToUse called by …
        • llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp: SelectionDAGBuilder::visitInlineAsm whose purpose is quite clear
    • It is safe to translate it into r-equivalent constraint, unless benchmarks show deficiency.

Clobbers

• einfach nix
  • Translates to Rust options(nomem, pure)
• memory
  • Rust has this by default, opt out with options(nomem)
• cc + memory
  • atomic_ll_sc, e.g. __ll_sc_atomic64_dec_if_positive
  • cc maps to options(preserve_flags)
• we probably should add options(nostack)

Categories of C operands

Input

• uninitized …
  • unsigned long temporaries with =&r
• counter like v->counter where v is atomic_t*
  • +Q

Output

Simple place expression really, either being pointer to a place or a variable used for return value.

Observation

asm blocks appears in various context

• it can be fully expanded into static inline functions;
  • we probably already link into today with helpers
• it can be embedded in do-expression blocks, in a macro
  • we can similarly materialise them, too