Re: [RFC] Overflow check in simplifying exit cond comparing two IVs.

[guojiufu <guojiufu@linux.ibm.com>]

I just had a test on ppc64le, this patch pass bootstrap and regtest.
Is this patch OK for trunk?

Thanks for any comments.

BR,
Jiufu

On 2021-10-18 21:37, Jiufu Guo wrote:
> With reference the discussions in:
> https://gcc.gnu.org/pipermail/gcc-patches/2021-July/574334.html
> https://gcc.gnu.org/pipermail/gcc-patches/2021-June/572006.html
> https://gcc.gnu.org/pipermail/gcc-patches/2021-September/578672.html
> 
> Base on the patches in above discussion, we may draft a patch to fix 
> the
> issue.
> 
> In this patch, to make sure it is ok to change '{b0,s0} op {b1,s1}' to
> '{b0,s0-s1} op {b1,0}', we also compute the condition which could 
> assume
> both 2 ivs are not overflow/wrap: the niter "of '{b0,s0-s1} op {b1,0}'"
> < the niter "of untill wrap for iv0 or iv1".
> 
> Does this patch make sense?
> 
> BR,
> Jiufu Guo
> 
> gcc/ChangeLog:
> 
> 	PR tree-optimization/100740
> 	* tree-ssa-loop-niter.c (number_of_iterations_cond): Add
> 	assume condition for combining of two IVs
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.c-torture/execute/pr100740.c: New test.
> ---
>  gcc/tree-ssa-loop-niter.c                     | 103 +++++++++++++++---
>  .../gcc.c-torture/execute/pr100740.c          |  11 ++
>  2 files changed, 99 insertions(+), 15 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.c-torture/execute/pr100740.c
> 
> diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
> index 75109407124..f2987a4448d 100644
> --- a/gcc/tree-ssa-loop-niter.c
> +++ b/gcc/tree-ssa-loop-niter.c
> @@ -1863,29 +1863,102 @@ number_of_iterations_cond (class loop *loop,
> 
>       provided that either below condition is satisfied:
> 
> -       a) the test is NE_EXPR;
> -       b) iv0.step - iv1.step is integer and iv0/iv1 don't overflow.
> +       a) iv0.step - iv1.step is integer and iv0/iv1 don't overflow.
> +       b) assumptions in below table also need to be satisfied.
> +
> +	| iv0     | iv1     | assum (iv0<iv1)     | assum (iv0!=iv1)    |
> +	|---------+---------+---------------------+---------------------|
> +	| (b0,2)  | (b1,1)  | before iv1 overflow | before iv1 overflow |
> +	| (b0,2)  | (b1,-1) | true                | true                |
> +	| (b0,-1) | (b1,-2) | before iv0 overflow | before iv0 overflow |
> +	|         |         |                     |                     |
> +	| (b0,1)  | (b1,2)  | false               | before iv0 overflow |
> +	| (b0,-1) | (b1,2)  | false               | true                |
> +	| (b0,-2) | (b1,-1) | false               | before iv1 overflow |
> +       'true' in above table means no need additional condition.
> +       'false' means this case can not satify the transform.
> +       The first three rows: iv0->step > iv1->step;
> +       The second three rows: iv0->step < iv1->step.
> 
>       This rarely occurs in practice, but it is simple enough to 
> manage.  */
>    if (!integer_zerop (iv0->step) && !integer_zerop (iv1->step))
>      {
> +      if (TREE_CODE (iv0->step) != INTEGER_CST
> +	  || TREE_CODE (iv1->step) != INTEGER_CST)
> +	return false;
> +      if (!iv0->no_overflow || !iv1->no_overflow)
> +	return false;
> +
>        tree step_type = POINTER_TYPE_P (type) ? sizetype : type;
> -      tree step = fold_binary_to_constant (MINUS_EXPR, step_type,
> -					   iv0->step, iv1->step);
> -
> -      /* No need to check sign of the new step since below code takes 
> care
> -	 of this well.  */
> -      if (code != NE_EXPR
> -	  && (TREE_CODE (step) != INTEGER_CST
> -	      || !iv0->no_overflow || !iv1->no_overflow))
> +      tree step
> +	= fold_binary_to_constant (MINUS_EXPR, step_type, iv0->step, 
> iv1->step);
> +
> +      if (code != NE_EXPR && tree_int_cst_sign_bit (step))
>  	return false;
> 
> -      iv0->step = step;
> -      if (!POINTER_TYPE_P (type))
> -	iv0->no_overflow = false;
> +      bool positive0 = !tree_int_cst_sign_bit (iv0->step);
> +      bool positive1 = !tree_int_cst_sign_bit (iv1->step);
> 
> -      iv1->step = build_int_cst (step_type, 0);
> -      iv1->no_overflow = true;
> +      /* Cases in rows 2 and 4 of above table.  */
> +      if ((positive0 && !positive1) || (!positive0 && positive1))
> +	{
> +	  iv0->step = step;
> +	  iv1->step = build_int_cst (step_type, 0);
> +	  return number_of_iterations_cond (loop, type, iv0, code, iv1,
> +					    niter, only_exit, every_iteration);
> +	}
> +
> +      affine_iv i_0, i_1;
> +      class tree_niter_desc num;
> +      i_0 = *iv0;
> +      i_1 = *iv1;
> +      i_0.step = step;
> +      i_1.step = build_int_cst (step_type, 0);
> +      if (!number_of_iterations_cond (loop, type, &i_0, code, &i_1, 
> &num,
> +				      only_exit, every_iteration))
> +	return false;
> +
> +      affine_iv i0, i1;
> +      class tree_niter_desc num_wrap;
> +      i0 = *iv0;
> +      i1 = *iv1;
> +
> +      /* Reset iv0 and iv1 to calculate the niter which cause 
> overflow.  */
> +      if (tree_int_cst_lt (i1.step, i0.step))
> +	{
> +	  if (positive0 && positive1)
> +	    i0.step = build_int_cst (step_type, 0);
> +	  else if (!positive0 && !positive1)
> +	    i1.step = build_int_cst (step_type, 0);
> +	  if (code == NE_EXPR)
> +	    code = LT_EXPR;
> +	}
> +      else
> +	{
> +	  if (positive0 && positive1)
> +	    i1.step = build_int_cst (step_type, 0);
> +	  else if (!positive0 && !positive1)
> +	    i0.step = build_int_cst (step_type, 0);
> +	  gcc_assert (code == NE_EXPR);
> +	  code = GT_EXPR;
> +	}
> +
> +      /* Calculate the niter which cause overflow.  */
> +      if (!number_of_iterations_cond (loop, type, &i0, code, &i1, 
> &num_wrap,
> +				      only_exit, every_iteration))
> +	return false;
> +
> +      /* Make assumption there is no overflow. */
> +      tree assum
> +	= fold_build2 (LE_EXPR, boolean_type_node, num.niter,
> +		       fold_convert (TREE_TYPE (num.niter), num_wrap.niter));
> +      num.assumptions = fold_build2 (TRUTH_AND_EXPR, 
> boolean_type_node,
> +				      num.assumptions, assum);
> +
> +      *iv0 = i_0;
> +      *iv1 = i_1;
> +      *niter = num;
> +      return true;
>      }
> 
>    /* If the result of the comparison is a constant,  the loop is 
> weird.  More
> diff --git a/gcc/testsuite/gcc.c-torture/execute/pr100740.c
> b/gcc/testsuite/gcc.c-torture/execute/pr100740.c
> new file mode 100644
> index 00000000000..8fcdaffef3b
> --- /dev/null
> +++ b/gcc/testsuite/gcc.c-torture/execute/pr100740.c
> @@ -0,0 +1,11 @@
> +/* PR tree-optimization/100740 */
> +
> +unsigned a, b;
> +int main() {
> +  unsigned c = 0;
> +  for (a = 0; a < 2; a++)
> +    for (b = 0; b < 2; b++)
> +      if (++c < a)
> +        __builtin_abort ();
> +  return 0;
> +}