Re: [PR72835] Incorrect arithmetic optimization involving bitfield arguments

[kugan]

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  <kug...@linaro.org>

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

gcc/ChangeLog:

2016-08-10  Kugan Vivekanandarajah  <kug...@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.
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr72835.c 
b/gcc/testsuite/gcc.dg/tree-ssa/pr72835.c
index e69de29..d7d0a8d 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/pr72835.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr72835.c
@@ -0,0 +1,36 @@
+/* PR tree-optimization/72835 */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+struct struct_1 {
+    unsigned int m1 : 6 ;
+    unsigned int m2 : 24 ;
+    unsigned int m3 : 6 ;
+};
+
+unsigned short var_32 = 0x2d10;
+
+struct struct_1 s1;
+
+void init ()
+{
+  s1.m1 = 4;
+  s1.m2 = 0x7ca4b8;
+  s1.m3 = 24;
+}
+
+void foo ()
+{
+  unsigned int c =
+    ((unsigned int) s1.m2) * (-((unsigned int) s1.m3))
+    + (var_32) * (-((unsigned int) (s1.m1)));
+  if (c != 4098873984)
+    __builtin_abort ();
+}
+
+int main ()
+{
+    init ();
+    foo ();
+    return 0;
+}
diff --git a/gcc/tree-ssa-reassoc.c b/gcc/tree-ssa-reassoc.c
index 7fd7550..b864ed1 100644
--- a/gcc/tree-ssa-reassoc.c
+++ b/gcc/tree-ssa-reassoc.c
@@ -3945,6 +3945,7 @@ rewrite_expr_tree (gimple *stmt, unsigned int opindex,
              gimple_assign_set_rhs1 (stmt, oe1->op);
              gimple_assign_set_rhs2 (stmt, oe2->op);
              update_stmt (stmt);
+             reset_flow_sensitive_info (lhs);
            }
 
          if (rhs1 != oe1->op && rhs1 != oe2->op)
@@ -4193,9 +4194,11 @@ rewrite_expr_tree_parallel (gassign *stmt, int width,
         others are recreated.  */
       if (i == stmt_num - 1)
        {
+         tree lhs = gimple_assign_lhs (stmts[i]);
          gimple_assign_set_rhs1 (stmts[i], op1);
          gimple_assign_set_rhs2 (stmts[i], op2);
          update_stmt (stmts[i]);
+         reset_flow_sensitive_info (lhs);
        }
       else
        {