Choosing registers for assembly snippets

[Zixian Cai]

Hi all,

I’m just wondering what the best practices are in terms of choosing registers.

I was looking at the assembly code for tlab allocation (https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L131) and noticed the following.

  1.  tmp1 register is invalid.
  2.  tmp2 register is valid but sometimes clashes with var_size_in_bytes, which requires the workaround here https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L155
  3.  t0 is used as a temporary register despite not passed into the method as a temporary register https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L149

I’d really appreciate if someone could shed insight on how these decisions are made so that I can avoid common pitfalls when writing assembly code. The only resource I found at the moment is the architecture description file https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/riscv.ad#L73 which suggests x5(t0)-x6(t1) can always be used as temporary registers.

Sincerely,
Zixian
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