[PATCH] RISC-V: Document some -march special cases

[PATCH] RISC-V: Document some -march special cases

[Palmer Dabbelt]

Vineet just got bit by the autovec params, and a bunch of us forgot
about the second one.

gcc/ChangeLog:

	* doc/invoke.texi (RISC-V Options): Add some -march special
	cases.
---
 gcc/doc/invoke.texi | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index a32dabf0405..4a6cad21dcb 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -29242,10 +29242,22 @@ The default is @option{-misa-spec=20191213} unless GCC has been configured
 with @option{--with-isa-spec=} specifying a different default version.
 
 @opindex march
-@item -march=@var{ISA-string}
-Generate code for given RISC-V ISA (e.g.@: @samp{rv64im}).  ISA strings must be
-lower-case.  Examples include @samp{rv64i}, @samp{rv32g}, @samp{rv32e}, and
-@samp{rv32imaf}.
+@item -march=@var{subtarget}
+Generate code for given RISC-V ISA (e.g.@: @samp{rv64im}).  The @var{subtarget}
+string is similar to an ISA string, but has various subtle differences.
+@var{subtarget} strings must be lower case.  Extension ordering and
+dependencies may differ from the specification.  Additionally, the following
+extensions are special cases:
+@table @code
+@item V
+Enabling the V extension does not enable autovecorization, users must select
+either @samp{--param=riscv-autovec-preference=scalable} or
+@samp{--param=riscv-autovec-preference=fixed-vlmax} to decide between VLA and
+VLS autovectorization.
+@item Zihintpause
+This extension is always enabled.
+@end table
+Examples include @samp{rv64i}, @samp{rv32g}, @samp{rv32e}, and @samp{rv32imaf}.
 
 When @option{-march=} is not specified, use the setting from @option{-mcpu}.
 
-- 
2.41.0

Re: [PATCH] RISC-V: Document some -march special cases

[Kito Cheng]

I would prefer NOT to expose those --param on user manual since
generally those options are used for internal only, we should add -m
option and enable `--param=riscv-autovec-preference=scalable` by
default once we think it's stable enough.

On Wed, Aug 30, 2023 at 1:04 PM Palmer Dabbelt <palmer@rivosinc.com> wrote:
>
> Vineet just got bit by the autovec params, and a bunch of us forgot
> about the second one.
>
> gcc/ChangeLog:
>
>         * doc/invoke.texi (RISC-V Options): Add some -march special
>         cases.
> ---
>  gcc/doc/invoke.texi | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index a32dabf0405..4a6cad21dcb 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -29242,10 +29242,22 @@ The default is @option{-misa-spec=20191213} unless GCC has been configured
>  with @option{--with-isa-spec=} specifying a different default version.
>
>  @opindex march
> -@item -march=@var{ISA-string}
> -Generate code for given RISC-V ISA (e.g.@: @samp{rv64im}).  ISA strings must be
> -lower-case.  Examples include @samp{rv64i}, @samp{rv32g}, @samp{rv32e}, and
> -@samp{rv32imaf}.
> +@item -march=@var{subtarget}
> +Generate code for given RISC-V ISA (e.g.@: @samp{rv64im}).  The @var{subtarget}
> +string is similar to an ISA string, but has various subtle differences.
> +@var{subtarget} strings must be lower case.  Extension ordering and
> +dependencies may differ from the specification.  Additionally, the following
> +extensions are special cases:
> +@table @code
> +@item V
> +Enabling the V extension does not enable autovecorization, users must select
> +either @samp{--param=riscv-autovec-preference=scalable} or
> +@samp{--param=riscv-autovec-preference=fixed-vlmax} to decide between VLA and
> +VLS autovectorization.
> +@item Zihintpause
> +This extension is always enabled.
> +@end table
> +Examples include @samp{rv64i}, @samp{rv32g}, @samp{rv32e}, and @samp{rv32imaf}.
>
>  When @option{-march=} is not specified, use the setting from @option{-mcpu}.
>
> --
> 2.41.0
>

Re: [PATCH] RISC-V: Document some -march special cases

[Jeff Law]

On 8/29/23 23:52, Kito Cheng wrote:
> I would prefer NOT to expose those --param on user manual since
> generally those options are used for internal only, we should add -m
> option and enable `--param=riscv-autovec-preference=scalable` by
> default once we think it's stable enough.
I tend to agree.

The params controlling this stuff are in my mind a stopgap as we work 
through those issues.  Ideally we get to a point where they're no longer 
needed and we just remove them.  THat gets harder if they get exposed in 
a release, even more so if they get documented in the release.

Jeff