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From: "olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/64299] New: [SH] improve FPSCR.PR mode switching by reordering insns
Date: Sat, 13 Dec 2014 17:50:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64299

            Bug ID: 64299
           Summary: [SH] improve FPSCR.PR mode switching by reordering
                    insns
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: olegendo at gcc dot gnu.org
            Target: sh*-*-*

Compiling the following function with -O2 results in 4 FPSCR.PR mode switches:

double test_0 (const float* a, const float* b, const double* c, float x)
{
  float aa = a[0] * b[0];  // single
  double cc = c[0] + c[1];  // double

  aa += b[1] * b[2];  // single
  cc += c[2] + c[3];  // double
  aa += b[3] * b[4];  // single

  return aa / cc; // double
}

Since the calculations are independent and FPSCR flags are not read between the
operations, the fp insns can be reordered to reduce the amount of mode
switches.  The resulting code should be the same as when doing the reordering
manually:

double test_1 (const float* a, const float* b, const double* c, float x)
{
  float aa = a[0] * b[0] + b[1] * b[2] + b[3] * b[4];  // single
  double cc = c[0] + c[1] + c[2] + c[3];  // double

  return aa / cc; // double
}

which results in only 2 FPSCR.PR mode switches.


Moreover, the following example

double test_2 (const float* x, const double* y, unsigned int c)
{
  float var0 = 0;
  double var1 = 0;

  while (c--)
  {
    float xx = x[0] * x[1] + x[2] + 123.0f;
    x += 3;

    double yy = y[0] + y[1];
    y += 2;

    var0 += xx;
    var1 += yy;
  }

  return var0 + var1;
}

is a good candidate for doing loop distribution.  Since var0 and var1 are
independent the loop can be replaced with a single precision loop and a double
precision loop, eliminating the FPSCR.PR mode switches inside the loop.