From: "Krzysztof Matula" <K.Matula@adb.pl>
To: <Richard.Earnshaw@arm.com>
Cc: <gcc@gcc.gnu.org>
Subject: Re: porting GCC
Date: Fri, 02 Feb 2001 06:07:00 -0000 [thread overview]
Message-ID: <00b001c08d21$48f99fd0$3704a4c3@adb.pl> (raw)
In-Reply-To: <200102021200.MAA10437@cam-mail2.cambridge.arm.com>
----- Original Message -----
From: "Richard Earnshaw" <rearnsha@arm.com>
Sent: Friday, February 02, 2001 13:00
> > I am generating several insn-s inside define_expand. The meaning of the
code
> > is not exactly the same, as the rtl matched. The problem is that GCC
always
> > notes the last insn I generate with REG_EQUAL expression, corresponding
to
> > the ORIGINAL code... I don't know how to avoid this...
>
> Why is this a problem? Gcc annotates the end of the sequence with a note
> of the equivalent it collapses to. If you think of (symbol_ref
> ("__clz_tab")) as the final address that will be dereferenced then the
> expression
>
> (unspec [(symbol_ref ("__clz_tab"))] 3)
> should be thought of as being equivalent to
>
> (minus (symbol_ref ("__clz_tab")) (reg:SI base_reg))
I'm not sure if I understand this... Maybe there's something wrong that GCC
thinks of (symbol_ref ("__clz_tab")) as absolute address and the assembly
instructions not:
(expr_list:REG_EQUAL (plus:SI (mem:SI (reg:SI 9))
(symbol_ref/v:SI ("__clz_tab")))
(reg:SI 9) here represents the index to the __clz_tab, not the segment
base...
Referencing an element requires addition of segment base ((reg:SI 14) in the
example below), the index (reg:SI 9) and the relative starting offset of the
array: (symbol_ref/v:SI ("__clz_tab")). When I write the __clz_tab reference
into otput file I get the offset only...
The segment base is not a single register. It is always accessible as first
hidden argument to every function. It always needs to execute code to get it
into register... I don't think I can do this inside define_insn...
When I compile my example with -O2 GCC makes use of the REG_EQUAL and
produces rlt that doesn't match any insn (and in fact is incorrect):
(insn 10 8 11 (set (reg:SI 15)
(plus:SI (reg/v:SI 13)
(symbol_ref/v:SI ("__clz_tab")))) -1 (nil)
(nil))
(reg/v:SI 13) stands here for the index to the array. No segment base
addition takes place.
> [we don't express it this way since then the compiler might try to
> simplify the expression in a way that would be incorrect, but
> mathematically when the base_reg value is added back on we end up with the
> address we really want.]
If only the value is added...
> >
> > (insn 9 8 11 (set (reg:SI 15) ;;real array address
> > (plus:SI (reg:SI 14) ;;segment base
> >
pec:SI[
> > (symbol_ref/v:SI ("__clz_tab"))
> > ] 3))) -1 (nil)
> > (nil))
> >
> > (insn 11 9 12 (set (reg:SI 16) ;;index to the array
> > (mem:SI (reg:SI 9))) -1 (nil)
> > (nil))
> >
> > (insn 12 11 14 (set (reg:SI 13)
> > (plus:SI (reg:SI 16)
> > (reg:SI 15))) -1 (nil)
> > (nil))
> >
> > (insn 14 12 16 (set (reg:SI 13)
> > (reg:SI 13)) -1 (nil)
> > (expr_list:REG_EQUAL (plus:SI (mem:SI (reg:SI 9))
> > (symbol_ref/v:SI ("__clz_tab")))
> > (nil)))
> >
KM
next prev parent reply other threads:[~2001-02-02 6:07 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-02-01 2:37 Krzysztof Matula
2001-02-01 3:10 ` Richard Earnshaw
2001-02-02 3:43 ` Krzysztof Matula
2001-02-02 4:00 ` Richard Earnshaw
2001-02-02 6:07 ` Krzysztof Matula [this message]
2001-02-02 7:08 ` Richard Earnshaw
2001-02-15 8:41 porting gcc Filippo Basso
2001-02-15 9:50 ` Fergus Henderson
2002-10-22 9:53 Grégoire Dooms
2002-10-22 9:56 ` Grégoire Dooms
2002-10-22 14:40 ` Mike Stump
2002-10-22 14:43 ` Andrew Haley
2002-10-22 15:16 ` Michael Matz
2002-10-22 15:27 ` Svein E. Seldal
2002-10-22 18:02 ` Mike Stump
2002-10-23 10:05 ` Andrew Haley
2002-10-22 15:50 ` Denis Chertykov
2003-08-13 11:15 Porting gcc Petar Penchev
2003-08-13 16:07 ` Ben Elliston
2004-05-21 13:09 Mile Davidovic
2004-05-21 13:43 ` Dave Korn
2004-06-02 16:49 Porting GCC Gyle Yearsley
2004-06-03 15:38 ` Richard Sandiford
2004-06-04 15:24 Gyle Yearsley
2008-03-12 6:11 Porting gcc Schmave
2008-03-12 6:15 ` Ben Elliston
2008-10-20 10:20 porting gcc Aurélien Buhrig
2008-10-20 19:34 ` Ian Lance Taylor
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