[LLVMbugs] [Bug 23056] New: Clang emits calls to libcompiler-rt for complex numbers, even if they are trivial

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23056

            Bug ID: 23056
           Summary: Clang emits calls to libcompiler-rt for complex
                    numbers, even if they are trivial
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ed at 80386.nl
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Consider the following piece of code:

#include <complex.h>

double complex times_two(double complex x) {
  return x * 2;
}

double complex rotate(double complex x) {
  return x * I;
}

These functions are trivial:

- The first function multiplies both the real and imaginary part of the complex
number with a simple scalar value.
- The second function rotates the number on the complex plane.

It seems that Clang creates a call to __muldc3(). While correct, I'm not sure
this is really needed. Clang could be optimized to omit a call to __muldc3()
and friends if one of the parts of any of the two numbers can be proven to be
zero.

Another example:

double mul(double a, double b) {
  return (double complex)a * (double complex)b;
}

Clang generates horrible code for this function, even though it can completely
ignore the complex keyword.

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