[Bug rtl-optimization/44194] struct returned by value generates useless stores

[jhaberman at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44194

--- Comment #8 from Josh Haberman <jhaberman at gmail dot com> 2011-02-24 
03:27:04 UTC ---
I found another test case for this.  I thought I'd post it since it's extremely
different than the original one.

--

class Foo {
 public:
  virtual ~Foo() {}
  virtual void DoSomething() = 0;
};

void foo(Foo *f, void (Foo::*member)()) {
  (f->*member)();
}

--

$ g++ -c -O3 -fomit-frame-pointer test.cc
$ objdump -M intel -d test.o

test.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <_Z3fooP3FooMS_FvvE>:
   0:    40 f6 c6 01              test   sil,0x1
   4:    48 89 74 24 e8           mov    QWORD PTR [rsp-0x18],rsi
   9:    48 89 54 24 f0           mov    QWORD PTR [rsp-0x10],rdx
   e:    74 10                    je     20 <_Z3fooP3FooMS_FvvE+0x20>
  10:    48 01 d7                 add    rdi,rdx
  13:    48 8b 07                 mov    rax,QWORD PTR [rdi]
  16:    48 8b 74 30 ff           mov    rsi,QWORD PTR [rax+rsi*1-0x1]
  1b:    ff e6                    jmp    rsi
  1d:    0f 1f 00                 nop    DWORD PTR [rax]
  20:    48 01 d7                 add    rdi,rdx
  23:    ff e6                    jmp    rsi

--

We spilled rsi and rdx to the stack (in the red zone, it appears) for no reason
(AFAICS).