Re: [PATCH v2] RISC-V: Implement TLS Descriptors.

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/8/23 04:49, Tatsuyuki Ishi via Gcc-patches wrote:
> This implements TLS Descriptors (TLSDESC) as specified in [1].
> 
> In TLSDESC instruction sequence, the first instruction relocates against
> the target TLS variable, while subsequent instructions relocates against
> the address of the first. Such usage of labels are not well-supported
> within GCC. Due to this, the 4-instruction sequence is implemented as a
> single RTX insn.
> 
> The default remains to be the traditional TLS model, but can be configured
> with --with_tls={trad,desc}. The choice can be revisited once toolchain
> and libc support ships.
> 
> [1]: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/373.
> 
> gcc/Changelog:
>      * config/riscv/riscv.opt: Add -mtls-dialect to configure TLS flavor.
>      * config.gcc: Add --with_tls configuration option to change the default
>      TLS flavor.
>      * config/riscv/riscv.h: Add TARGET_TLSDESC determined from
>      -mtls-dialect and with_tls defaults.
>      * config/riscv/riscv-opts.h: Define enum riscv_tls_type for the two TLS
>      flavors.
>      * config/riscv/riscv-protos.h: Define SYMBOL_TLSDESC symbol type.
>      * config/riscv/riscv.md: Add instruction sequence for TLSDESC.
>      * config/riscv/riscv.cc (riscv_symbol_insns): Add instruction sequence
>      length data for TLSDESC.
>      (riscv_legitimize_tls_address): Add lowering of TLSDESC.
> ---

> @@ -4694,6 +4696,17 @@ case "${target}" in
>   				;;
>   			esac
>   		fi
> +		# Handle --with-tls.
> +		case "$with_tls" in
> +        "" \
> +        | trad | desc)
> +            # OK
> +            ;;
> +        *)
> +            echo "Unknown TLS method used in --with-tls=$with_tls" 1>&2
> +            exit 1
> +            ;;
> +        esac
Is there a reason why this isn't formatted like the other cases?




> @@ -1869,6 +1870,24 @@
>     [(set_attr "got" "load")
>      (set_attr "mode" "<MODE>")])
>   
> +(define_insn "@tlsdesc<mode>"
> +  [(set (reg:P A0_REGNUM)
> +	    (unspec:P
> +			[(match_operand:P 0 "symbolic_operand" "")
> +			 (match_operand:P 1 "const_int_operand")]
> +			UNSPEC_TLSDESC))
> +   (clobber (reg:SI T0_REGNUM))]
> +  "TARGET_TLSDESC"
> +  {
> +    return ".LT%1: auipc\ta0, %%tlsdesc_hi(%0)\;"
> +           "<load>\tt0,%%tlsdesc_load_lo(.LT%1)(a0)\;"
> +           "addi\ta0,a0,%%tlsdesc_add_lo(.LT%1)\;"
> +           "jalr\tt0,t0,%%tlsdesc_call(.LT%1)";
> +  }
> +  [(set_attr "type" "multi")
> +   (set_attr "length" "16")
> +   (set_attr "mode" "<MODE>")])
Hmm, I would be a bit worried about explicitly using $a0 here.  That's 
generally frowned upon, but probably unavoidable in this case since this 
is a call under the hood.


This needs changes to invoke.texi since it introduces new options.  I 
don't think it has to be anything terribly verbose.  A one liner is 
probably sufficient and I wouldn't be surprised if other ports have 
suitable text we could copy.

So overall if Kito's OK, then I am with the trivial doc change and 
perhaps the formatting fix in config.guess.

jeff