[Bug tree-optimization/71016] [6/7 Regression] Redundant sign extension with conditional __builtin_clzl

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71016

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-05-09
   Target Milestone|---                         |6.2
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
So PRE still does the desired tranform:

  <bb 2>:
  if (i_4(D) != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  i.1_1 = (long unsigned int) i_4(D);
  _2 = __builtin_clzl (i.1_1);
  _5 = (long int) _2;

  <bb 4>:
  # _7 = PHI <_2(3), 17(2)>
  # prephitmp_8 = PHI <_5(3), 17(2)>
  return prephitmp_8;

but then later phiopt comes along and un-does it:

--- t.c.172t.forwprop4  2016-05-09 13:15:20.830707963 +0200
+++ t.c.173t.phiopt3    2016-05-09 13:15:20.830707963 +0200
@@ -6,7 +6,7 @@
 {
   long unsigned int i.1_1;
   int _2;
-  long int _5;
+  int _7;
   long int prephitmp_8;

   <bb 2>:
@@ -18,10 +18,10 @@
   <bb 3>:
   i.1_1 = (long unsigned int) i_4(D);
   _2 = __builtin_clzl (i.1_1);
-  _5 = (long int) _2;

   <bb 4>:
-  # prephitmp_8 = PHI <_5(3), 17(2)>
+  # _7 = PHI <_2(3), 17(2)>
+  prephitmp_8 = (long int) _7;
   return prephitmp_8;

 }

I think this was intended to be an enablement transform of some sort -
obviously
here it mis-fires.