[RFC 0/7] RISCV: Implement ISA Manual Table A.6 Mappings

Patrick O'Neill patrick@rivosinc.com
Thu Apr 7 18:33:44 GMT 2022


This series should not be applied as it causes an ABI break.

This RFC aims to bring the RISCV atomics implementation in line with
the recommended mapping present in table A.6 of the ISA manual.

https://github.com/riscv/riscv-isa-manual/blob/c7cf84547b3aefacab5463add1734c1602b67a49/src/memory.tex#L1083-L1157

This mapping being implemented would result in an ABI break due to
libatomic's LR.aq/SC.rl mapping and the A.6's SEQ_CST store mapping
not enforcing SEQ_CST when placed next to eachother.

This can be seen using the following Herd7 litmus test:

RISCV W-RMW

(*
Seq_cst store along with LR.aq/SC.rl is insufficient for a
seq_cst store, seq_cst RMW mapping.
*)

{
0:x7=A; 0:x8=B;
1:x7=A; 1:x8=B;
}

   P0                  | P1          ;
   ori x1,x0,1         | ori x1,x0,1 ;
   fence rw,w          | fence rw,rw ;
   sw x1,0(x8)         | sw x1,0(x7) ;
   lr.w.aq x3,0(x7)    | fence rw,rw ;
   sc.w.rl x1,x1,0(x7) | lw x2,0(x8) ;

exists (0:x3=0 /\ 1:x2=0)

In GCC for SEQ_CST store, we currently emit fence iorw,ow + amoswap.aq,
which successfully enforces ordering for the given litmus test. This
will only be a problem in GCC if we move the SEQ_CST store to the A.6
mapping.

Note: LLVM implements fence rw,w + sw
https://godbolt.org/z/n68P7ne1W

That means that LLVM isn't compatible with libatomic's LR.aq/SC.rl.

* PR target/89835: The RISC-V target uses amoswap.w for relaxed stores

Patrick O'Neill (7):
  RISCV: Enforce Libatomic LR/SC SEQ_CST
  RISCV: Enforce Atomic Compare Exchange SEQ_CST
  RISCV: Add AMO release bits
  RISCV: Optimize AMO Ops
  RISCV: Optimize LR/SC Pairs
  RISCV: Optimize Atomic Stores
  RISCV: Relax mem_thread_fence

 gcc/config/riscv/riscv-protos.h               |  6 ++
 gcc/config/riscv/riscv.cc                     | 93 +++++++++++++++++--
 gcc/config/riscv/sync.md                      | 46 ++++++---
 .../gcc.target/riscv/amo-thread-fence-1.c     |  6 ++
 .../gcc.target/riscv/amo-thread-fence-2.c     |  6 ++
 .../gcc.target/riscv/amo-thread-fence-3.c     |  6 ++
 .../gcc.target/riscv/amo-thread-fence-4.c     |  6 ++
 .../gcc.target/riscv/amo-thread-fence-5.c     |  6 ++
 .../gcc.target/riscv/inline-atomics-model-1.c | 12 +++
 .../gcc.target/riscv/inline-atomics-model-2.c | 12 +++
 .../gcc.target/riscv/inline-atomics-model-3.c | 12 +++
 .../gcc.target/riscv/inline-atomics-model-4.c | 12 +++
 .../gcc.target/riscv/inline-atomics-model-5.c | 12 +++
 gcc/testsuite/gcc.target/riscv/pr89835.c      |  9 ++
 libgcc/config/riscv/atomic.c                  |  4 +-
 15 files changed, 223 insertions(+), 25 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/riscv/amo-thread-fence-1.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/amo-thread-fence-2.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/amo-thread-fence-3.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/amo-thread-fence-4.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/amo-thread-fence-5.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/inline-atomics-model-1.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/inline-atomics-model-2.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/inline-atomics-model-3.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/inline-atomics-model-4.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/inline-atomics-model-5.c
 create mode 100644 gcc/testsuite/gcc.target/riscv/pr89835.c

-- 
2.25.1



More information about the Gcc-patches mailing list