Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!bu-cs!ncsuvx!mcnc!rti!xyzzy!wood
From: wood@dg-rtp.dg.com (Tom Wood)
Newsgroups: gnu.gcc.bug
Subject: Re: Is (subreg:SI (mem:QI ...)) valid or what
Message-ID: <2025@xyzzy.UUCP>
Date: 20 Oct 89 21:49:13 GMT
References: <2012@xyzzy.UUCP>
Sender: usenet@xyzzy.UUCP
Reply-To: wood@bert.dg.com (Tom Wood)
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Organization: Data General Corporation, Research Triangle Park, NC
Lines: 101

I found more about what is causing the widening subreg problem.  A
change to expand_binop\optabs.c attempts to use widening subreg around
narrow operands to logical operators instead of only using convert_move
as was the case in 1.35.  This works fine, and in fact saves
instructions in the non-optimized case.  The trouble is that combine is
unable to deal with this construct in the same way it deals with
sign_extend & zero_extend.  Here again is the test case:

	typedef struct { unsigned int a:5; double b; } T;
	
	void set_T(p)
	     T *p; 
	{
	  p->a = 5;
	  p->b = 2.0;
	}

Here's the RTL after flow analysis (just before combine):
		
	(insn 8 6 9 (set (reg:QI 33)
	       (mem:QI (reg/v:SI 32))) -1 (insn_list 4 (nil))
	   (nil))
	
	(insn 9 8 10 (set (reg:SI 34)
	       (and:SI (subreg:SI (reg:QI 33) 0)
		   (const_int 7))) -1 (insn_list 8 (nil))
	   (expr_list:REG_DEAD (reg:QI 33)
	       (nil)))
	
	(insn 10 9 11 (set (reg:QI 35)
	       (subreg:QI (reg:SI 34) 0)) -1 (insn_list 9 (nil))
	   (expr_list:REG_DEAD (reg:SI 34)
	       (nil)))
	
Combine produces this:	
	
	(insn 8 6 9 (set (reg:QI 33)
	       (mem:QI (reg/v:SI 32))) -1 (insn_list 4 (nil))
	   (nil))
	
	(insn 9 8 10 (set (reg:SI 34)
	       (and:SI (subreg:SI (reg:QI 33) 0)
		   (const_int 7))) -1 (insn_list 8 (nil))
	   (expr_list:REG_DEAD (reg:QI 33)
	       (nil)))
	
The trouble is that unless these are combined further, later passes
go wrong.  The RTL wants to look like this.

	(insn 12 11 13 (set (reg:SI 38)
	       (and:SI (subreg:SI (mem:QI (reg/v:SI 32)) 0)
		   (const_int 7))) 236 (insn_list 11 (insn_list 10
					(insn_list 9 (insn_list 8
					 (insn_list 4 (nil))))))
	   (nil))

My fix is truly a kludge.  If you're not optimizing, go ahead with
the widening subreg form.  Otherwise, explictitly use zero_extend or
sign_extend which combine is able to deal with.  Here's the patch:

*** optabs.c.orig	Fri Oct 20 17:37:58 1989
--- optabs.c	Fri Oct 20 17:41:17 1989
***************
*** 308,314 ****
  		  if (no_extend)
  		    {
  		      temp = force_reg (GET_MODE (xop0), xop0);
! 		      xop0 = gen_rtx (optimize ? unsignedp ? ZERO_EXTEND : SIGN_EXTEND : SUBREG, wider_mode, temp, 0);
  		    }
  		  else
  		    {
--- 308,314 ----
  		  if (no_extend)
  		    {
  		      temp = force_reg (GET_MODE (xop0), xop0);
! 		      xop0 = gen_rtx (SUBREG, wider_mode, temp, 0);
  		    }
  		  else
  		    {
***************
*** 322,328 ****
  		  if (no_extend)
  		    {
  		      temp = force_reg (GET_MODE (xop1), xop1);
! 		      xop1 = gen_rtx (optimize ? unsignedp ? ZERO_EXTEND : SIGN_EXTEND : SUBREG, wider_mode, temp, 0);
  		    }
  		  else
  		    {
--- 322,328 ----
  		  if (no_extend)
  		    {
  		      temp = force_reg (GET_MODE (xop1), xop1);
! 		      xop1 = gen_rtx (SUBREG, wider_mode, temp, 0);
  		    }
  		  else
  		    {

Comments?
---
			Tom Wood	(919) 248-6067
			Data General, Research Triangle Park, NC
			{the known world}!rti!xyzzy!wood