This is the mail archive of the
mailing list for the GCC project.
Re: [PATCH, VAX] Correct ffs instruction constraint
- From: coypu at sdf dot org
- To: gcc-patches at gcc dot gnu dot org
- Cc: matt at 3am-software dot com
- Date: Thu, 29 Jun 2017 15:47:05 +0000
- Subject: Re: [PATCH, VAX] Correct ffs instruction constraint
- Authentication-results: sourceware.org; auth=none
- References: <20170620200542.GA17979@SDF.ORG>
On Tue, Jun 20, 2017 at 08:05:42PM +0000, email@example.com 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
> 2017-06-20 Maya Rashish <firstname.lastname@example.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")