[PATCH] bitintlower: Fix .MUL_OVERFLOW overflow checking [PR114038]

[PATCH] bitintlower: Fix .MUL_OVERFLOW overflow checking [PR114038]

[Jakub Jelinek]

Hi!

Currently, bitint_large_huge::lower_mul_overflow uses cnt 1 only if
startlimb == endlimb and in that case doesn't use a loop and handles
everything in a special if:
      unsigned cnt;
      bool use_loop = false;
      if (startlimb == endlimb)
        cnt = 1;
      else if (startlimb + 1 == endlimb)
        cnt = 2;
      else if ((end % limb_prec) == 0)
        {
          cnt = 2;
          use_loop = true;
        }
      else
        {
          cnt = 3;
          use_loop = startlimb + 2 < endlimb;
        }
      if (cnt == 1)
	{
	  ...
	}
      else
The loop handling for the loop exit condition wants to compare if the
incremented index is equal to endlimb, but that is correct only if
end is not divisible by limb_prec and there will be a straight line
check after the loop as well for the most significant limb.  The code
used endlimb + (cnt == 1) for that, but cnt == 1 is never true here,
because cnt is either 2 or 3, so the right check is (cnt == 2).

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-02-22  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/114038
	* gimple-lower-bitint.cc (bitint_large_huge::lower_mul_overflow): Fix
	loop exit condition if end is divisible by limb_prec.

	* gcc.dg/torture/bitint-59.c: New test.

--- gcc/gimple-lower-bitint.cc.jj	2024-02-15 09:52:40.999145971 +0100
+++ gcc/gimple-lower-bitint.cc	2024-02-21 20:04:27.590388930 +0100
@@ -4497,7 +4497,7 @@ bitint_large_huge::lower_mul_overflow (t
 					   size_one_node);
 		  insert_before (g);
 		  g = gimple_build_cond (NE_EXPR, idx_next,
-					 size_int (endlimb + (cnt == 1)),
+					 size_int (endlimb + (cnt == 2)),
 					 NULL_TREE, NULL_TREE);
 		  insert_before (g);
 		  edge true_edge, false_edge;
--- gcc/testsuite/gcc.dg/torture/bitint-59.c.jj	2024-02-21 20:07:11.028142323 +0100
+++ gcc/testsuite/gcc.dg/torture/bitint-59.c	2024-02-21 20:07:57.854498649 +0100
@@ -0,0 +1,22 @@
+/* PR tree-optimization/114038 */
+/* { dg-do run { target bitint } } */
+/* { dg-options "-std=c23 -pedantic-errors" } */
+/* { dg-skip-if "" { ! run_expensive_tests }  { "*" } { "-O0" "-O2" } } */
+/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */
+
+#if __BITINT_MAXWIDTH__ >= 129
+int
+foo (unsigned _BitInt(63) x, unsigned _BitInt(129) y)
+{
+  return __builtin_mul_overflow_p (y, x, 0);
+}
+#endif
+
+int
+main ()
+{
+#if __BITINT_MAXWIDTH__ >= 129
+  if (!foo (90, 0x80000000000000000000000000000000uwb))
+    __builtin_abort ();
+#endif
+}

	Jakub

Re: [PATCH] bitintlower: Fix .MUL_OVERFLOW overflow checking [PR114038]

[Richard Biener]

On Thu, 22 Feb 2024, Jakub Jelinek wrote:

> Hi!
> 
> Currently, bitint_large_huge::lower_mul_overflow uses cnt 1 only if
> startlimb == endlimb and in that case doesn't use a loop and handles
> everything in a special if:
>       unsigned cnt;
>       bool use_loop = false;
>       if (startlimb == endlimb)
>         cnt = 1;
>       else if (startlimb + 1 == endlimb)
>         cnt = 2;
>       else if ((end % limb_prec) == 0)
>         {
>           cnt = 2;
>           use_loop = true;
>         }
>       else
>         {
>           cnt = 3;
>           use_loop = startlimb + 2 < endlimb;
>         }
>       if (cnt == 1)
> 	{
> 	  ...
> 	}
>       else
> The loop handling for the loop exit condition wants to compare if the
> incremented index is equal to endlimb, but that is correct only if
> end is not divisible by limb_prec and there will be a straight line
> check after the loop as well for the most significant limb.  The code
> used endlimb + (cnt == 1) for that, but cnt == 1 is never true here,
> because cnt is either 2 or 3, so the right check is (cnt == 2).
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

Thanks,
Richard.

> 2024-02-22  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR tree-optimization/114038
> 	* gimple-lower-bitint.cc (bitint_large_huge::lower_mul_overflow): Fix
> 	loop exit condition if end is divisible by limb_prec.
> 
> 	* gcc.dg/torture/bitint-59.c: New test.
> 
> --- gcc/gimple-lower-bitint.cc.jj	2024-02-15 09:52:40.999145971 +0100
> +++ gcc/gimple-lower-bitint.cc	2024-02-21 20:04:27.590388930 +0100
> @@ -4497,7 +4497,7 @@ bitint_large_huge::lower_mul_overflow (t
>  					   size_one_node);
>  		  insert_before (g);
>  		  g = gimple_build_cond (NE_EXPR, idx_next,
> -					 size_int (endlimb + (cnt == 1)),
> +					 size_int (endlimb + (cnt == 2)),
>  					 NULL_TREE, NULL_TREE);
>  		  insert_before (g);
>  		  edge true_edge, false_edge;
> --- gcc/testsuite/gcc.dg/torture/bitint-59.c.jj	2024-02-21 20:07:11.028142323 +0100
> +++ gcc/testsuite/gcc.dg/torture/bitint-59.c	2024-02-21 20:07:57.854498649 +0100
> @@ -0,0 +1,22 @@
> +/* PR tree-optimization/114038 */
> +/* { dg-do run { target bitint } } */
> +/* { dg-options "-std=c23 -pedantic-errors" } */
> +/* { dg-skip-if "" { ! run_expensive_tests }  { "*" } { "-O0" "-O2" } } */
> +/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */
> +
> +#if __BITINT_MAXWIDTH__ >= 129
> +int
> +foo (unsigned _BitInt(63) x, unsigned _BitInt(129) y)
> +{
> +  return __builtin_mul_overflow_p (y, x, 0);
> +}
> +#endif
> +
> +int
> +main ()
> +{
> +#if __BITINT_MAXWIDTH__ >= 129
> +  if (!foo (90, 0x80000000000000000000000000000000uwb))
> +    __builtin_abort ();
> +#endif
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)