On 09/08/16 23:43, kugan wrote:
> Hi,
>
> The test-case in PR72835 is failing with -O2 and passing with
> -fno-tree-reassoc. It also passes with -O2 -fno-tree-vrp.
>
> diff of .115t.dse2 and .116t.reassoc1 for the c++ testcase is as
> follows, which looks OK.
>
> +  unsigned int _16;
> +  unsigned int _17;
> +  unsigned int _18;
>
>     <bb 2>:
>     _1 = s1.m2;
>     _2 = (unsigned int) _1;
>     _3 = s1.m3;
>     _4 = (unsigned int) _3;
> -  _5 = -_4;
> -  _6 = _2 * _5;
> +  _5 = _4;
> +  _6 = _5 * _2;
>     var_32.0_7 = var_32;
>     _8 = (unsigned int) var_32.0_7;
>     _9 = s1.m1;
>     _10 = (unsigned int) _9;
> -  _11 = -_10;
> -  _12 = _8 * _11;
> -  c_14 = _6 + _12;
> +  _11 = _10;
> +  _12 = _11 * _8;
> +  _16 = _12 + _6;
> +  _18 = _16;
> +  _17 = -_18;
> +  c_14 = _17;
>     if (c_14 != 4098873984)
>
>
> However, I noticed that when we re-associate and assign different
> operands to the stmts, we are not resetting the flow information to the
> LHS. This looks wrong. Attached patch resets it. With this, the
> testcases in TH PR is now working.
>
>
> Bootstrap and regression testing for x86_64-linux-gnu is in progress. Is
> this OK for trunk if there is no regression.

There was no new regression while testing. I also moved the testcase 
from gcc.dg/torture/pr72835.c to gcc.dg/tree-ssa/pr72835.c. Is this OK 
for trunk?

Thanks,
Kugan

gcc/testsuite/ChangeLog:

2016-08-10  Kugan Vivekanandarajah  <kuganv@linaro.org>

	PR tree-optimization/72835
	* gcc.dg/tree-ssa/pr72835.c: New test.

gcc/ChangeLog:

2016-08-10  Kugan Vivekanandarajah  <kuganv@linaro.org>

	PR tree-optimization/72835
	* tree-ssa-reassoc.c (rewrite_expr_tree): Reset value_range of LHS when
	operands are changed.
	(rewrite_expr_tree_parallel): Likewise.