[Bug rtl-optimization/108318] Floating point calculation moved out of loop despite fesetround

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108318

Andrew Pinski  changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution|--- |DUPLICATE

--- Comment #2 from Andrew Pinski  ---
Dup of bug 34678.

Techincally you need to use "#pragma STDC FENV_ACCESS ON" also to get this
correct but GCC does not implement that pragma which is what PR 34678 is about.

*** This bug has been marked as a duplicate of bug 34678 ***

[Bug rtl-optimization/108318] Floating point calculation moved out of loop despite fesetround

[amonakov at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108318

Alexander Monakov  changed:

   What|Removed |Added

 CC||amonakov at gcc dot gnu.org

--- Comment #1 from Alexander Monakov  ---
Please see documentation for the -frounding-math option, but even with that
option added, your testcase still has the faux-invariant moved by RTL PRE
(-fno-gcse).

Interestingly, if your testcase is modified to compute the sum before the call:

#include 
void
foo (double res[4], double a, double b, double x[])
{
  a = x[0];
  b = x[1];
  static const int rm[4]
  = { FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD };
  for (int i = 0; i < 4; ++i)
{
  double t = a + b;
  fesetround (rm[i]);
  res[i] = t;
}
  fesetround (FE_TONEAREST); // restore default
}

Then it demonstrates how a few *other* optimizations also perform unwanted
motion:

* SSA PRE (-fno-tree-pre)
* TER (-fno-tree-ter)
* RTL LIM (-fno-move-loop-invariants)
* and finally the register allocator (unavoidable)