This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [PATCH, VAX] Correct ffs instruction constraint


Ping.

On Tue, Jun 20, 2017 at 08:05:42PM +0000, coypu@sdf.org wrote:
> VAX' FFS as variable-length bit field instruction uses a "base"
> operand of type "vb" meaning "byte address".
> "base" can be 32 bits (SI) and due to the definition of
> ffssi2/__builtin_ffs() with the operand constraint "m", code can be
> emitted which incorrectly implies a mode-dependent (= longword, for
> the 32-bit operand) address.
> File scsipi_base.c compiled with -Os for our VAX install kernel shows:
> 
>     ffs $0x0,$0x20,0x50(r11)[r0],r9
> 
> Apparently, 0x50(r11)[r0] as a longword address is assumed to be
> evaluated in longword context by FFS, but the instruction expects a
> byte address.
> 
> Our fix is to change the operand constraint from "m" to "Q", i. e.
> "operand is a MEM that does not have a mode-dependent address", which
> results in:
> 
>     moval 0x50(r11)[r0],r1
>     ffs $0x0,$0x20,(r1),r9
> 
> MOVAL evaluates the source operand/address in longword context, so
> effectively converts the word address to a byte address for FFS.
> 
> See NetBSD PR port-vax/51761 (http://gnats.netbsd.org/51761) and
> discussion on port-vax mailing list
> (http://mail-index.netbsd.org/port-vax/2017/01/06/msg002954.html).
> 
> Changlog:
> 
> 2017-06-20  Maya Rashish  <coypu@sdf.org>
> 
> 	* gcc/config/vax/builtins.md: Correct ffssi2_internal
> 	instruction constraint.
> 
> 
> ---
>  gcc/config/vax/builtins.md | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/gcc/config/vax/builtins.md b/gcc/config/vax/builtins.md
> index fb0f69acb..b78fb5616 100644
> --- a/gcc/config/vax/builtins.md
> +++ b/gcc/config/vax/builtins.md
> @@ -41,7 +41,7 @@
>  
>  (define_insn "ffssi2_internal"
>    [(set (match_operand:SI 0 "nonimmediate_operand" "=rQ")
> -	(ffs:SI (match_operand:SI 1 "general_operand" "nrmT")))
> +	(ffs:SI (match_operand:SI 1 "general_operand" "nrQT")))
>     (set (cc0) (match_dup 0))]
>    ""
>    "ffs $0,$32,%1,%0")
> -- 
> 2.13.1


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]