[PATCH] Use more conservative fences on RISC-V

Palmer Dabbelt palmer@dabbelt.com
Mon Mar 20 16:43:00 GMT 2017


On Fri, 17 Mar 2017 15:52:54 PDT (-0700), Palmer Dabbelt wrote:
> The RISC-V memory model is still in the process of being formally
> specified, so for now we're going to be safe and add the I/O bits to
> userspace fences because there's no way to know if userspace is touching
> memory-mapped I/O regions at compile time.
>
> This will have no impact on existing microarchitecutres because they
> treat all fences conservatively.
>
> gcc/ChangeLog:
>
> 2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com>
>
>         * config/riscv/riscv.c (riscv_print_operand): Use "fence
>         iorw,ow".
>         * config/riscv/sync.mc (mem_thread_fence_1): Use "fence
>         iorw,iorw".
> ---
>  gcc/ChangeLog            | 7 +++++++
>  gcc/config/riscv/riscv.c | 2 +-
>  gcc/config/riscv/sync.md | 2 +-
>  3 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/gcc/ChangeLog b/gcc/ChangeLog
> index 3e108dd..de32689 100644
> --- a/gcc/ChangeLog
> +++ b/gcc/ChangeLog
> @@ -1,3 +1,10 @@
> +2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com>
> +
> +	* config/riscv/riscv.c (riscv_print_operand): Use "fence
> +	iorw,ow".
> +	* config/riscv/sync.mc (mem_thread_fence_1): Use "fence
> +	iorw,iorw".
> +
>  2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com
>
>  	* doc/install.texi (Specific) <riscv32-*-elf>: Add riscv32-*-elf,
> diff --git a/gcc/config/riscv/riscv.c b/gcc/config/riscv/riscv.c
> index 25cc803..fa93c3c 100644
> --- a/gcc/config/riscv/riscv.c
> +++ b/gcc/config/riscv/riscv.c
> @@ -2794,7 +2794,7 @@ riscv_print_operand (FILE *file, rtx op, int letter)
>
>      case 'F':
>        if (riscv_memmodel_needs_release_fence ((enum memmodel) INTVAL (op)))
> -	fputs ("fence rw,w; ", file);
> +	fputs ("fence iorw,ow; ", file);
>        break;
>
>      default:
> diff --git a/gcc/config/riscv/sync.md b/gcc/config/riscv/sync.md
> index 09970b9..cde19e3 100644
> --- a/gcc/config/riscv/sync.md
> +++ b/gcc/config/riscv/sync.md
> @@ -53,7 +53,7 @@
>  	(unspec:BLK [(match_dup 0)] UNSPEC_MEMORY_BARRIER))
>     (match_operand:SI 1 "const_int_operand" "")] ;; model
>    ""
> -  "fence\trw,rw")
> +  "fence\tiorw,iorw")
>
>  ;; Atomic memory operations.

Committed.



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