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Re: [PATCH] Fix bitfield expansion (PR middle-end/52979)


On Wed, 25 Apr 2012, Jakub Jelinek wrote:

> Hi!
> 
> This patch fixes the attached execute/ testcases.  In some cases
> get_best_mode was ignoring bitregion_* restrictions and returned
> mode that overlapped into adjacent non-bitfields, or store_bit_field_1
> for insv could ignore those restrictions completely.
> After fixing that I've run into several issues that the other parts
> of the patch fix, namely that get_best_mode returning VOIDmode more often
> now may lead to store_split_bit_field being called when it wasn't before,
> and when approaching the bitregion_end point we can't just keep using
> large units (modes), but need to decrease their size in order not to
> overwrite adjacent non-bitfields.  The get_best_mode dropping of % align
> change prevents a mutual recursion, e.g. for bitregion_end == 103
> it would prevent using > QImode even far before the bit region boundary,
> even when not touching anything close to that bit region end.
> But the extra
> +             && (bitregion_end == 0
> +                 || bitpos - (bitpos % unit) + unit <= bitregion_end + 1))
> needed to be added after that change, otherwise we'd regress on
> c-c++-common/simulate-thread/bitfields-2.c.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

Ok.

Thanks,
Richard.

> 2012-04-25  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR middle-end/52979
> 	* stor-layout.c (get_best_mode): Don't return mode with bitsize
> 	larger than maxbits.  Don't compute maxbits modulo align.
> 	Also check that unit bytes long store at bitpos / unit * unit
> 	doesn't affect bits beyond bitregion_end.
> 	* expmed.c (store_bit_field_1): Avoid trying insv if OP_MODE MEM
> 	would not fit into bitregion_start ... bitregion_end + 1 bit
> 	region.
> 	(store_split_bit_field): Decrease unit close to end of bitregion_end
> 	if access is restricted in order to avoid mutual recursion.
> 
> 	* gcc.c-torture/compile/pr52979-1.c: New test.
> 	* gcc.c-torture/execute/pr52979-1.c: New test.
> 	* gcc.c-torture/execute/pr52979-2.c: New test.
> 
> --- gcc/stor-layout.c.jj	2012-03-26 11:53:20.000000000 +0200
> +++ gcc/stor-layout.c	2012-04-25 10:26:18.881631697 +0200
> @@ -2624,7 +2624,7 @@ get_best_mode (int bitsize, int bitpos,
>    if (!bitregion_end)
>      maxbits = MAX_FIXED_MODE_SIZE;
>    else
> -    maxbits = (bitregion_end - bitregion_start) % align + 1;
> +    maxbits = bitregion_end - bitregion_start + 1;
>  
>    /* Find the narrowest integer mode that contains the bit field.  */
>    for (mode = GET_CLASS_NARROWEST_MODE (MODE_INT); mode != VOIDmode;
> @@ -2645,7 +2645,10 @@ get_best_mode (int bitsize, int bitpos,
>  	 (Though at least one Unix compiler ignores this problem:
>  	 that on the Sequent 386 machine.  */
>        || MIN (unit, BIGGEST_ALIGNMENT) > align
> -      || (largest_mode != VOIDmode && unit > GET_MODE_BITSIZE (largest_mode)))
> +      || (largest_mode != VOIDmode && unit > GET_MODE_BITSIZE (largest_mode))
> +      || unit > maxbits
> +      || (bitregion_end
> +	  && bitpos - (bitpos % unit) + unit > bitregion_end + 1))
>      return VOIDmode;
>  
>    if ((SLOW_BYTE_ACCESS && ! volatilep)
> @@ -2663,7 +2666,9 @@ get_best_mode (int bitsize, int bitpos,
>  	      && unit <= MIN (align, BIGGEST_ALIGNMENT)
>  	      && unit <= maxbits
>  	      && (largest_mode == VOIDmode
> -		  || unit <= GET_MODE_BITSIZE (largest_mode)))
> +		  || unit <= GET_MODE_BITSIZE (largest_mode))
> +	      && (bitregion_end == 0
> +		  || bitpos - (bitpos % unit) + unit <= bitregion_end + 1))
>  	    wide_mode = tmode;
>  	}
>  
> --- gcc/expmed.c.jj	2012-04-19 11:09:13.000000000 +0200
> +++ gcc/expmed.c	2012-04-24 17:14:34.264897874 +0200
> @@ -640,7 +640,13 @@ store_bit_field_1 (rtx str_rtx, unsigned
>        && !(MEM_P (op0) && MEM_VOLATILE_P (op0)
>  	   && flag_strict_volatile_bitfields > 0)
>        && ! ((REG_P (op0) || GET_CODE (op0) == SUBREG)
> -	    && (bitsize + bitpos > GET_MODE_BITSIZE (op_mode))))
> +	    && (bitsize + bitpos > GET_MODE_BITSIZE (op_mode)))
> +      /* Do not use insv if the bit region is restricted and
> +	 op_mode integer at offset doesn't fit into the
> +	 restricted region.  */
> +      && !(MEM_P (op0) && bitregion_end
> +	   && bitnum - bitpos + GET_MODE_BITSIZE (op_mode)
> +	      > bitregion_end + 1))
>      {
>        struct expand_operand ops[4];
>        int xbitpos = bitpos;
> @@ -760,7 +766,7 @@ store_bit_field_1 (rtx str_rtx, unsigned
>  	  || GET_MODE_BITSIZE (GET_MODE (op0)) > maxbits
>  	  || (op_mode != MAX_MACHINE_MODE
>  	      && GET_MODE_SIZE (GET_MODE (op0)) > GET_MODE_SIZE (op_mode)))
> -	bestmode = get_best_mode  (bitsize, bitnum,
> +	bestmode = get_best_mode (bitsize, bitnum,
>  				  bitregion_start, bitregion_end,
>  				  MEM_ALIGN (op0),
>  				  (op_mode == MAX_MACHINE_MODE
> @@ -1096,6 +1102,16 @@ store_split_bit_field (rtx op0, unsigned
>        offset = (bitpos + bitsdone) / unit;
>        thispos = (bitpos + bitsdone) % unit;
>  
> +      /* When region of bytes we can touch is restricted, decrease
> +	 UNIT close to the end of the region as needed.  */
> +      if (bitregion_end
> +	  && unit > BITS_PER_UNIT
> +	  && bitpos + bitsdone - thispos + unit > bitregion_end + 1)
> +	{
> +	  unit = unit / 2;
> +	  continue;
> +	}
> +
>        /* THISSIZE must not overrun a word boundary.  Otherwise,
>  	 store_fixed_bit_field will call us again, and we will mutually
>  	 recurse forever.  */
> --- gcc/testsuite/gcc.c-torture/compile/pr52979-1.c.jj	2012-04-25 08:27:48.188902802 +0200
> +++ gcc/testsuite/gcc.c-torture/compile/pr52979-1.c	2012-04-25 08:27:29.000000000 +0200
> @@ -0,0 +1,15 @@
> +/* PR middle-end/52979 */
> +
> +struct S
> +{
> +  unsigned int a : 16, b : 16, c : 16, d : 16, e : 14;
> +  unsigned int f : 4, g : 14, h : 8;
> +  char i;
> +  int j;
> +};
> +
> +void
> +foo (struct S *s)
> +{
> +  s->f = 1;
> +}
> --- gcc/testsuite/gcc.c-torture/execute/pr52979-1.c.jj	2012-04-24 17:20:51.246662745 +0200
> +++ gcc/testsuite/gcc.c-torture/execute/pr52979-1.c	2012-04-24 17:21:03.526586921 +0200
> @@ -0,0 +1,40 @@
> +/* PR middle-end/52979 */
> +
> +extern void abort (void);
> +int c, d, e;
> +
> +void
> +foo (void)
> +{
> +}
> +
> +struct __attribute__((packed)) S { int g : 31; int h : 6; };
> +struct S a = { 1 };
> +static struct S b = { 1 };
> +
> +void
> +bar (void)
> +{
> +  a.h = 1;
> +  struct S f = { };
> +  b = f;
> +  e = 0;
> +  if (d)
> +    c = a.g;
> +}
> +
> +void
> +baz (void)
> +{
> +  bar ();
> +  a = b;
> +}
> +
> +int
> +main ()
> +{
> +  baz ();
> +  if (a.g)
> +    abort ();
> +  return 0;
> +}
> --- gcc/testsuite/gcc.c-torture/execute/pr52979-2.c.jj	2012-04-24 17:20:54.603642467 +0200
> +++ gcc/testsuite/gcc.c-torture/execute/pr52979-2.c	2012-04-24 17:19:15.000000000 +0200
> @@ -0,0 +1,40 @@
> +/* PR middle-end/52979 */
> +
> +extern void abort (void);
> +int c, d, e;
> +
> +void
> +foo (void)
> +{
> +}
> +
> +struct __attribute__((packed)) S { int g : 31; int h : 6; };
> +static struct S b = { 1 };
> +struct S a = { 1 };
> +
> +void
> +bar (void)
> +{
> +  a.h = 1;
> +  struct S f = { };
> +  b = f;
> +  e = 0;
> +  if (d)
> +    c = a.g;
> +}
> +
> +void
> +baz (void)
> +{
> +  bar ();
> +  a = b;
> +}
> +
> +int
> +main ()
> +{
> +  baz ();
> +  if (a.g)
> +    abort ();
> +  return 0;
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Guenther <rguenther@suse.de>
SUSE / SUSE Labs
SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer

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