[RFA:] Simplification transformations in semantics.scm

[Doug Evans <dje@transmeta.com>]

I'd like to have the .cpu file that this is necessary for
so that I can play with it a bit first.  Possible?

IIRC, I decided (either explicitly or otherwise) to not allow
conditionals in the left hand side of an assignment.
It complicates things a lot.
I believe gcc doesn't support them (does it?)
and in general I'm always leary of introducing radical
departures from gcc rtl without a lot of thought and compelling
arguments that it is warranted.

Hans-Peter Nilsson writes:
 > These simplifications make the CGEN semantic compilation see
 > somewhat-trivial sets of registers such that the "profile"
 > support (actually: model unit machinery) marks the right operand
 > for input and output.  In a .cpu file I have, I use (through
 > some levels of pmacros) (set (reg h-hw (index-of (operand op))) src)
 > and the other forms below, which confuse CGEN.
 > 
 > Another way (independently correct AFAICT) to make model support
 > work for me would be to make the model machinery just ignore the
 > semantics of the insn and use the operands I *tell it* to use in
 > the model description, as long as it can find them in the format
 > description.  Right now inputs and outputs of units are filtered
 > through whatever is collected from the semantic compilation, so
 > "op" in (set (reg h-hw (index-of (operand op))) src) is *not*
 > marked as an output operand.  Subsequently, with (op INT -1) in
 > a unit output list, the model function for that unit always get
 > -1 for that operand; op is listed as an input but not as an
 > output.  Gah!  That's a bug, I think; the unit input and output
 > should override what the semantic compiler thinks.

I don't understand.  There's some missing info here.
At face value `op' is not an output operand.
Maybe if I saw the entire define-insn it would be more clear.
Howzaboutit?

 > In summary, this patch simplifies (as the comments say):
 > 
 > (reg XMODE h-hw (index-of YMODE (operand XMODE op)))
 >  into
 > (operand XMODE op)))
 > 
 > (SET (if IFMODE COND ltrue lfalse) src)
 >  into (for SET = {set, set-quiet}):
 > (if COND (SET ltrue dst) (SET lfalse dst))
 > 
 > (reg h-hw RMODE (if COND ntrue nfalse))
 >  into
 > (if RMODE COND (reg h-hw RMODE ntrue)
 > 	       (reg h-hw RMODE nfalse))
 >  but only if simplification makes that latter expression
 >  not take that form, i.e. (if COND (reg...) (reg...))
 > 
 > If you accept this change but consider it so large that
 > copyright is an issue, we have to work something out, since
 > there's no copyright agreement between Axis Communications and
 > Red Hat.  Suggestions welcome.  FWIW, I can only think of this
 > way to accomplish what I describe above, so I hope to get away
 > with it as obvious enough.  (I wish, I wish, that Red Hat would
 > find a way to assign copyright for CGEN to the FSF.  Oh yes, and
 > peace of earth.)
 > 
 > 	* semantics.scm (-simplify-expr-fn) <set, set-quiet, reg>: New
 > 	simplifications.
 > 
 > Index: semantics.scm
 > ===================================================================
 > RCS file: /cvs/src/src/cgen/semantics.scm,v
 > retrieving revision 1.1.1.1
 > diff -c -p -r1.1.1.1 semantics.scm
 > *** semantics.scm	28 Jul 2000 04:11:52 -0000	1.1.1.1
 > --- semantics.scm	7 Dec 2002 02:48:45 -0000
 > ***************
 > *** 256,261 ****
 > --- 256,392 ----
 >   	   known-val ; (rtx-make 'const 'INT known-val)
 >   	   #f)))
 >   
 > +     ((set set-quiet)
 > +      ; Transform: (set (if IFMODE COND ltrue lfalse) src)
 > +      ; into: (if COND (set ltrue dst) (set lfalse dst))
 > +      ; hoping for further simplification through the
 > +      ; regno-operand-to-operand simplification below.
 > +      ; This conditional-lvalue destination is expected from the
 > +      ; if-transformations below rather than from user input. 
 > +      ; FIXME: Other conditionals than "if" may be useful
 > +      ; too.  Add them as the need arises.  Maybe check src for 'if too.
 > +      (let ((ifexpr (-rtx-traverse (rtx-set-dest expr)
 > + 				    'RTX 'DFLT expr 2 tstate appstuff))
 > + 	     (src (-rtx-traverse (rtx-set-src expr)
 > + 					'RTX 'DFLT expr 3 tstate appstuff)))
 > + 	 (if (rtx-kind? 'if ifexpr)
 > + 	     (let* ((name (rtx-name expr))
 > + 		    (simplified-result
 > + 		     (rtx-make
 > + 		      'if 'DFLT (rtx-if-test ifexpr)
 > + 		      (rtx-make name
 > + 				(rtx-if-then ifexpr) src)
 > + 		      (rtx-make name
 > + 				(rtx-if-else ifexpr) src))))
 > + 	       (logit 4
 > + 		      "SET -: " (rtx-dump expr) "\n"
 > + 		      "SET +: " (rtx-dump simplified-result) "\n")
 > + 	       simplified-result)
 > + 	     #f)))
 > + 
 > +     ((reg)
 > +      (if (rtx-reg-number expr)
 > + 	 (let* ((regname (rtx-reg-name expr))
 > + 		(regmode (rtx-mode expr))
 > + 		(regno (-rtx-traverse (rtx-reg-number expr)
 > + 				      'RTX 'DFLT expr 3 tstate appstuff)))
 > + 	   (case (rtx-name regno)
 > + 	     ((index-of)
 > + 
 > + 	      ; When op is a h-hw, simplify
 > + 	      ; (reg XMODE h-hw (index-of YMODE (operand XMODE op)))
 > + 	      ; into (operand XMODE op).  For example, (reg h-gr
 > + 	      ; (regno Rd)) into Rd, when Rd is a h-gr and likewise
 > + 	      ; (reg SI h-gr (regno Rd)) provided that h-gr holds
 > + 	      ; registers in SImode.
 > + 	      (if (and
 > + 		   (rtx-operand? (rtx-index-of-value regno))
 > + 		   (equal?
 > + 		    (op:hw-name (rtx-operand-obj (rtx-index-of-value regno)))
 > + 		    regname)
 > + 
 > + 		   ; For the modes specified on reg and the operand,
 > + 		   ; only do the simplification when both have the
 > + 		   ; same mode, or if either is DFLT, the other one
 > + 		   ; having the "natural" mode of the register.
 > + 		   ;  We can only check modes of the hardware,
 > + 		   ; i.e. (op:mode ), when keeping just one mach, so
 > + 		   ; we'll assume we can't make the simplification at
 > + 		   ; other times.  One occasion we will be called with
 > + 		   ; multiple machs active is when generating
 > + 		   ; ARCH-desc.h for the simulator.
 > + 		   (not (keep-multiple?))
 > + 		   (let ((xmode (rtx-mode (rtx-index-of-value regno)))
 > + 			 (natmode (op:mode (rtx-operand-obj
 > + 					    (rtx-index-of-value regno)))))
 > + 		     (or (mode:eq? regmode xmode)
 > + 			 (and (mode:eq? 'DFLT regmode)
 > + 			      (mode:eq? natmode xmode))
 > + 			 (and (mode:eq? 'DFLT xmode)
 > + 			      (mode:eq? natmode regmode)))))
 > + 		  (begin
 > + 		    (logit 4
 > + 			   "REG INDEX-OF: -" (rtx-dump expr) "\n"
 > + 			   "REG INDEX-OF: +" (rtx-dump
 > + 					      (rtx-index-of-value regno))
 > + 			   "\n")
 > + 
 > + 		    (-rtx-traverse
 > + 		     (rtx-index-of-value regno)
 > + 		     'RTX 'DFLT expr op-pos tstate appstuff))
 > + 		    #f))
 > + 
 > + 	       ((if)
 > + 
 > + 		; Transform (reg h-hw RMODE (if COND ntrue nfalse)) into
 > + 		; (if RMODE COND (reg h-hw RMODE ntrue)
 > + 		;		 (reg h-hw RMODE nfalse))
 > + 		; if further simplifications eliminate terms in ntrue
 > + 		; and nfalse, as with the index-of case above.
 > + 		;  RMODE has to be collected from the natural mode of
 > + 		; the register, if it's DFLT in the original
 > + 		; expression.  Again, we can only check modes of the
 > + 		; hardware, i.e. (op:mode ), when keeping just one
 > + 		; mach, so we'll assume we can't make the
 > + 		; simplification at other times.
 > + 		(if (keep-multiple?)
 > + 		    #f
 > + 		    (let*
 > + 			((regmode (rtx-mode expr))
 > + 			 (rmode
 > + 			  (if (mode:eq? regmode 'DFLT)
 > + 			      (obj:name (hw-mode (current-hw-sem-lookup-1
 > + 						  regname)))
 > + 			      regmode))
 > + 			 (simplified-result
 > + 			  (-rtx-traverse
 > + 			   (rtx-make
 > + 			    'if
 > + 			    rmode
 > + 			    (rtx-if-test regno)
 > + 			    (rtx-make 'reg regmode
 > + 				      regname (rtx-if-then regno))
 > + 			    (rtx-make 'reg regmode
 > + 				      regname (rtx-if-else regno)))
 > + 			   'RTX rmode expr op-pos tstate appstuff)))
 > + 
 > + 		      ; Only pass on the simplification if (at least) the
 > + 		      ; reg parts were simplified away.
 > + 		      (if (or (not (rtx-kind? 'if simplified-result))
 > + 			      (and (not (rtx-kind? 'reg (rtx-if-then
 > + 							 simplified-result)))
 > + 				   (not (rtx-kind? 'reg (rtx-if-then
 > + 							 simplified-result)))))
 > + 			  (begin
 > + 			    (logit 4
 > + 				   "REG-IF -: " (rtx-dump expr) "\n"
 > + 				   "REG-IF +: " (rtx-dump simplified-result)
 > + 				   "\n")
 > + 			    simplified-result)
 > + 			  #f))))
 > + 	       (else #f)))
 > + 	 #f))
 > + 
 >       ; Leave EXPR unchanged and continue.
 >       (else #f))
 >   )
 > 
 > brgds, H-P
 >