[llvm-bugs] [Bug 31930] New: Optimization of loop via loop peeling does not occur for float and double

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31930

            Bug ID: 31930
           Summary: Optimization of loop via loop peeling does not occur
                    for float and double
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: drraph at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Consider:

float f(float x[]) {
  float p = 1.0;
  for (int i = 0; i < 960; i++)
    p += 1;
  return p;
}

When compiled with  -march=core-avx2 -O3 -ffast-math the assembly loops round
adding until it gets to 961.

However:

int f(int x[]) {
  int p = 1;
  for (int i = 0; i < 960; i++)
    p += 1;
  return p;
}

gives:

f:                                      # @f
        mov     eax, 961
        ret

I don't know how hard it would be to add the same optimization for floats and
double.

As a side note, there are in fact a number of interesting details with the
first (float) loop. First, if we reduce the i < 960 limit to i < 959 the loop
is optimized out. Second if we change the type to 'double' this upper limit
goes down to i < 479.  My guess is that this corresponds to an unpeeling cost
model that is incorporated into the compiler.

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