Re: [PATCH] PR middle-end/15069: Partial mode type conversions

[Mark Mitchell <mark at codesourcery dot com>]

Roger Sayle wrote:

> My apologies to Mark.  It turns out that the constant folding
> optimization in PR 15069 triggers latent problems throughout the
> compiler, not only in the C++ front-end but also in the middle-end.
>
> Clearly, the NOP_EXPR vs. CONVERT_EXPR distinction is being blurred,
> even though this may be completely unrelated to the bug in DOM!  There
> may even be problems in RTL expansion and/or the RTL optimizers.
>
>
> Conclusion: Can of worms.
>  
Agreed.

> The following patch has been tested on i686-pc-linux-gnu with a full
> "make bootstrap", all default languages, and regression tested with a
> top-level "make -k check" with no new failures.
>
> Ok for mainline and 3.4.1?
>  
Yes, thanks.

> 2004-05-30  Roger Sayle  <roger@eyesopen.com>
>
> 	PR middle-end/15069
> 	* fold-const.c (fold_single_bit_test): Only perform "(X & C) != 0"
> 	into "X < 0" (where C is the signbit) if X's type is a full mode.
>
> 	* g++.dg/opt/fold3.C: New test case.
>
>
> Index: fold-const.c
> ===================================================================
> RCS file: /cvs/gcc/gcc/gcc/fold-const.c,v
> retrieving revision 1.383
> diff -c -3 -p -r1.383 fold-const.c
> *** fold-const.c	28 May 2004 16:37:00 -0000	1.383
> --- fold-const.c	30 May 2004 03:53:59 -0000
> *************** fold_single_bit_test (enum tree_code cod
> *** 5433,5439 ****
>        /* If we have (A & C) != 0 where C is the sign bit of A, convert
>  	 this into A < 0.  Similarly for (A & C) == 0 into A >= 0.  */
>        arg00 = sign_bit_p (TREE_OPERAND (arg0, 0), TREE_OPERAND (arg0, 1));
> !       if (arg00 != NULL_TREE)
>  	{
>  	  tree stype = lang_hooks.types.signed_type (TREE_TYPE (arg00));
>  	  return fold (build2 (code == EQ_EXPR ? GE_EXPR : LT_EXPR,
> --- 5433,5443 ----
>        /* If we have (A & C) != 0 where C is the sign bit of A, convert
>  	 this into A < 0.  Similarly for (A & C) == 0 into A >= 0.  */
>        arg00 = sign_bit_p (TREE_OPERAND (arg0, 0), TREE_OPERAND (arg0, 1));
> !       if (arg00 != NULL_TREE
> ! 	  /* This is only a win if casting to a signed type is cheap,
> ! 	     i.e. when arg00's type is not a partial mode.  */
> ! 	  && TYPE_PRECISION (TREE_TYPE (arg00))
> ! 	     == GET_MODE_BITSIZE (TYPE_MODE (TREE_TYPE (arg00))))
>  	{
>  	  tree stype = lang_hooks.types.signed_type (TREE_TYPE (arg00));
>  	  return fold (build2 (code == EQ_EXPR ? GE_EXPR : LT_EXPR,
> *************** fold_single_bit_test (enum tree_code cod
> *** 5441,5450 ****
>  			       fold_convert (stype, integer_zero_node)));
>  	}
>
> -       /* At this point, we know that arg0 is not testing the sign bit.  */
> -       if (TYPE_PRECISION (type) - 1 == bitnum)
> - 	abort ();
> -
>        /* Otherwise we have (A & C) != 0 where C is a single bit,
>  	 convert that into ((A >> C2) & 1).  Where C2 = log2(C).
>  	 Similarly for (A & C) == 0.  */
> --- 5445,5450 ----
>
>
> // PR middle-end/15069
> // { dg-do run }
> // { dg-options "-O2" }
>
> extern "C" void abort (void);
>
> typedef enum {
>  FOUR = 4,
>  FIVE = 5
> } direction_t;
>
> int main ()
> {
>  direction_t four = FOUR;
>  int flags = (four & 4L) ? (32L | 128L) : 0;
>  flags &= 32L;
>
>  if (flags == 0)
>    abort ();
> }
>
>
> Roger
> --
> Roger Sayle,                         E-mail: roger@eyesopen.com
> OpenEye Scientific Software,         WWW: http://www.eyesopen.com/
> Suite 1107, 3600 Cerrillos Road,     Tel: (+1) 505-473-7385
> Santa Fe, New Mexico, 87507.         Fax: (+1) 505-473-0833
>
>  


--
Mark Mitchell
CodeSourcery, LLC
(916) 791-8304
mark@codesourcery.com