From: Mark Mitchell <mark@codesourcery.com>
To: geoffk@cygnus.com
Cc: gcc@gcc.gnu.org
Subject: Re: ssa bootstrap problem on x86 (cmpstrsi_1 pattern)
Date: Sun, 23 Jul 2000 00:50:00 -0000 [thread overview]
Message-ID: <20000723005001R.mitchell@codesourcery.com> (raw)
In-Reply-To: <200007230424.VAA00564@localhost.cygnus.com>
Hi Mark,
I've looked at the SSA bootstrap problem. It's caused by an insn
in the x86 backend which looks like:
Thanks for tracking this down.
(define_insn "cmpstrsi_1"
[(set (reg:CC 17)
(if_then_else:CC (ne (match_operand:SI 2 "register_operand" "c")
(const_int 0))
(compare:SI (mem:BLK (match_operand:SI 0 "address_operand" "S"))
(mem:BLK (match_operand:SI 1 "address_operand" "D")))
(const_int 0)))
(use (match_operand:SI 3 "immediate_operand" "i"))
(use (reg:CC 17))
(use (reg:SI 19))
(clobber (match_dup 0))
(clobber (match_dup 1))
(clobber (match_dup 2))]
...)
The problem here is the match_dups. IMHO, it would be much better if
these were written as match_scratch with '0', '1', and '2'
constraints.
So we have something like:
[(SET (reg:CC 17)
(if_then_else:CC (ne (reg:SI 35) (const int 0)) ...)
(USE (reg:CC 17))
...
(CLOBBER (reg:SI 35)))
...]
right? This just doesn't have a sensible SSA representation. I don't
see how match_scratch helps. The bottom line is that moving any use
of register 35 past this point is bogus. SSA doesn't really have a
way of representing instructions that kill their input operands.
That's why I think this kind of thing should happen later in the
compiler. Right now, i386.md creates temporaries for the inputs to
cmpstrsi_1, then clobbers the temporaries.
Instead, we should generate some kind of placeholder here. Perhaps
just:
[(set (reg:CC 17)
(if_then_else:CC (ne (match_operand:SI 2 "register_operand" "c")
(const_int 0))
(compare:SI (mem:BLK (match_operand:SI 0 "address_operand" "S"))
(mem:BLK (match_operand:SI 1 "address_operand" "D")))
(const_int 0)))
(use (reg:CC 17))
(use (reg:SI 19))]
Then, at some point after SSA, we should generate instructions to load
the temporaries and add the clobbers.
Does that make sense?
--
Mark Mitchell mark@codesourcery.com
CodeSourcery, LLC http://www.codesourcery.com
next prev parent reply other threads:[~2000-07-23 0:50 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2000-07-22 21:24 Geoff Keating
2000-07-23 0:50 ` Mark Mitchell [this message]
2000-07-24 11:51 ` Geoff Keating
2000-07-25 3:20 ` Richard Earnshaw
2000-07-26 22:55 ` Mark Mitchell
2000-07-27 12:50 ` Geoff Keating
2000-07-27 13:10 ` Mark Mitchell
2000-07-27 14:48 ` Richard Henderson
2000-07-27 15:13 ` Mark Mitchell
2000-07-27 15:14 ` Richard Henderson
2000-07-27 15:28 ` Mark Mitchell
2000-07-27 15:35 ` Richard Henderson
2000-07-27 15:42 ` Mark Mitchell
2000-07-27 15:46 ` Geoff Keating
2000-07-27 15:44 ` Geoff Keating
2000-07-27 16:01 ` Mark Mitchell
2000-07-27 16:59 ` Joern Rennecke
2000-07-27 17:22 ` Mark Mitchell
2000-07-27 17:57 ` Michael Meissner
2000-07-27 18:31 ` Mark Mitchell
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