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Re: PATCH: simplify ~x + 1 into -x


Geoff Keating <geoffk@geoffk.org> writes:
> Lars Brinkhoff <lars@nocrew.org> writes:
> > This patch simlifies ~x + 1 into -x on two's complement machines
> > (probably all machines supported by GCC).  It also adds a new target
> > macro to indicate the integer format, in case anyone would like to
> > port GCC to a machine without two's complement integers.
> 
> The TARGET_INT_FORMAT part is not OK, because there are lots of other
> places where GCC assumes two's-complement, and so it just adds clutter
> without adding any new features.  (It would be different if you
> actually planned to port GCC to a non-twos-complement machine.)
> 
> The optimisation part might be OK, please submit it separately.

Ok.  Then the patch boils down to just this.

Bootstrapped on powerpc-unknown-linux-gnu and checked without
regressions.

2001-04-24  Lars Brinkhoff  <lars@nocrew.org>

	* simplify-rtx.c (simplify_binary_operation): Simplify ~a + 1
	into -a.

Index: simplify-rtx.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/simplify-rtx.c,v
retrieving revision 1.45
diff -c -3 -p -r1.45 simplify-rtx.c
*** simplify-rtx.c	2001/04/05 02:27:45	1.45
--- simplify-rtx.c	2001/04/24 12:04:48
*************** simplify_binary_operation (code, mode, o
*** 915,920 ****
--- 915,927 ----
  	  else if (GET_CODE (op1) == NEG)
  	    return simplify_gen_binary (MINUS, mode, op0, XEXP (op1, 0));
  
+ 	  /* (~a) + 1 -> -a */
+ 	  if (INTEGRAL_MODE_P (mode)
+ 	      && GET_CODE (op0) == NOT
+ 	      && GET_CODE (op1) == CONST_INT
+ 	      && INTVAL (op1) == 1)
+ 	    return gen_rtx_NEG (mode, XEXP (op0, 0));
+ 
  	  /* Handle both-operands-constant cases.  We can only add
  	     CONST_INTs to constants since the sum of relocatable symbols
  	     can't be handled by most assemblers.  Don't add CONST_INT


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