[Bug tree-optimization/47707] New: Loss of step type precision leads to bad scalar evolution

[xinliangli at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47707

           Summary: Loss of step type precision leads to bad scalar
                    evolution
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: xinlian...@gmail.com


Compiling the following program with O2 and -fno-tree-vrp, the program aborts
at runtime

#include <assert.h>

struct CH
{
  unsigned char ch : 3;
} ch;

__attribute__((noinline)) void MakeCheckOp (unsigned int *v1, unsigned int *v2)
{
 assert (*v1 == *v2);

}

int main (void)
{

  int len;

  for (len = 4; len >= 1; len--)
  {
     unsigned v1, v2;
     ch.ch = len;
     v1 = ch.ch;
     v2 = len;
     MakeCheckOp (&v1, &v2);
  }
}

The following is the ir dump before ivopt:

int main() ()
{
  unsigned int ivtmp.12;
  int pretmp.11;
  unsigned int pretmp.10;
  <unnamed-unsigned:3> pretmp.9;
  unsigned char pretmp.8;
  <unnamed-unsigned:3> pretmp.7;
  unsigned char pretmp.6;
  unsigned int v2;
  unsigned int v1;
  int len;
  unsigned int len.1;
  unsigned int v1.0;
  <unnamed-unsigned:3> D.2142;
  unsigned char D.2141;

<bb 2>:

<bb 3>:
Invalid sum of incoming frequencies 8400, should be 8000
  # len_17 = PHI <len_9(4), 4(2)>
  # ivtmp.12_21 = PHI <ivtmp.12_20(4), 4(2)>
  D.2141_4 = (unsigned char) len_17;
  D.2142_5 = (<unnamed-unsigned:3>) D.2141_4;
  ch.ch = D.2142_5;
  v1.0_7 = (unsigned int) D.2142_5;
  v1 = v1.0_7;
  len.1_8 = (unsigned int) len_17;
  v2 = len.1_8;
  MakeCheckOp (&v1, &v2);
  len_9 = len_17 - 1;
  ivtmp.12_20 = ivtmp.12_21 - 1;
  if (ivtmp.12_20 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

<bb 4>:
  goto <bb 3>;

<bb 5>:
Invalid sum of incoming frequencies 1600, should be 2000
  return 0;

}


The scalar evolution for D.2142 is 

(analyze_scalar_evolution 
  (loop_nb = 1)
  (scalar = D.2142_5)
(get_scalar_evolution 
  (scalar = D.2142_5)
  (scalar_evolution = {4, +, 7}_1))
(set_scalar_evolution 
  instantiated_below = 2 
  (scalar = D.2142_5)
  (scalar_evolution = {4, +, 7}_1))
)

Which is correct, because precision of D.2142's type is 3 bits, so the sequence
will wrap: {4,3,2,1}

However for v1.0_7

v1.0_7 = (unsigned int) D.2142_5;

The scev is wrong:

(analyze_scalar_evolution 
  (loop_nb = 1)
  (scalar = v1.0_7)
(get_scalar_evolution 
  (scalar = v1.0_7)
  (scalar_evolution = {4, +, 7}_1))
(set_scalar_evolution 
  instantiated_below = 2 
  (scalar = v1.0_7)
  (scalar_evolution = {4, +, 7}_1))
)

Which will be {4,11,18,....} as there is no wrapping.

The proposed fixed is :

Index: tree-chrec.c
===================================================================
--- tree-chrec.c        (revision 170058)
+++ tree-chrec.c        (working copy)
@@ -1238,8 +1238,13 @@ convert_affine_scev (struct loop *loop, 
      performed by default when CT is signed.  */
   new_step = *step;
   if (TYPE_PRECISION (step_type) > TYPE_PRECISION (ct) && TYPE_UNSIGNED (ct))
-    new_step = chrec_convert_1 (signed_type_for (ct), new_step, at_stmt,
-                               use_overflow_semantics);
+    {
+      tree signed_ct = signed_type_for (ct);
+      if (TYPE_PRECISION (signed_ct) != TYPE_PRECISION (ct))
+        signed_ct = build_nonstandard_integer_type (TYPE_PRECISION (ct), 0);
+      new_step = chrec_convert_1 (signed_ct, new_step, at_stmt,
+                                  use_overflow_semantics);
+    }
   new_step = chrec_convert_1 (step_type, new_step, at_stmt,
use_overflow_semantics);

   if (automatically_generated_chrec_p (new_base)
@@ -1579,4 +1584,3 @@ evolution_function_right_is_integer_cst 
       return false;
     }
 }




David