[Bug rtl-optimization/89567] New: [missed-optimization] Should not be initializing unused struct parameter members

[eyalroz at technion dot ac.il]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89567

            Bug ID: 89567
           Summary: [missed-optimization] Should not be initializing
                    unused struct parameter members
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eyalroz at technion dot ac.il
  Target Milestone: ---

The issue is captured in the example here:
https://gcc.godbolt.org/z/_U4X80

The issue was first described in this StackOverflow question:
https://stackoverflow.com/q/54964323/1593077


Consider the following code:

  __attribute__((noinline)) int foo1(int x, int y)
  {
    return x;
  }

  int bar1(int* a)
  {
    int b = foo1(a[5], a[10]);
    return b * b;
  }

GCC (with -O3) optimizes-out the initialization of the y parameter with the
a[10] argument, saving one of the two memory reads. This is good.

Now suppose we put those two int parameters into a struct:

 struct two_ints { int x, y; };

  __attribute__((noinline)) int foo2(struct two_ints s)
  {
    return s.x;
  }

  int bar2(int* a)
  {
    struct two_ints ti = { a[5], a[10] };
    int b = foo2(ti);
    return b * b;
  }

There shouldn't be any difference, right? The parameters (certainly as far as
the assembly, which recognizes no such thing as "structs", is concerned) are
two integers; and the second one is not used. So I would expect to see the same
assembly code. Yet... I don't. Both integers are initialized and two `mov    
eax, DWORD PTR [rdx+something]` instructions are executed.


This behavior also occurs also with "GCC trunk" on GodBolt, i.e. GCC version 
9.0.1.