[Bug c++/70094] New: Missed optimization when passing a constant struct argument by value

["robryk at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70094

            Bug ID: 70094
           Summary: Missed optimization when passing a constant struct
                    argument by value
           Product: gcc
           Version: 5.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: robryk at gmail dot com
  Target Milestone: ---

Function baz in the code listing below gets compiled into something that writes
to the stack. This is unnecessary: one can just load the argument into rdi with
movabs and get rid of stack adjustments and memory accesses:

---snip---
[robryk@sharya-rana gccbug]$ cat > bug.cc
struct foo {
  int a;
  int b;
  int c;
};

void bar(foo);

void baz() {
  foo f;
  f.a = 1;
  f.b = 2;
  f.c = 3;
  bar(f);
}
[robryk@sharya-rana gccbug]$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/5.3.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /build/gcc-multilib/src/gcc-5.3.0/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --enable-multilib --disable-werror
--enable-checking=release
Thread model: posix
gcc version 5.3.0 (GCC) 
[robryk@sharya-rana gccbug]$ g++ bug.cc -O2 -c -o bug.o
[robryk@sharya-rana gccbug]$ objdump -d bug.o

bug.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <_Z3bazv>:
   0:   48 83 ec 18             sub    $0x18,%rsp
   4:   be 03 00 00 00          mov    $0x3,%esi
   9:   c7 04 24 01 00 00 00    movl   $0x1,(%rsp)
  10:   c7 44 24 04 02 00 00    movl   $0x2,0x4(%rsp)
  17:   00 
  18:   48 8b 3c 24             mov    (%rsp),%rdi
  1c:   e8 00 00 00 00          callq  21 <_Z3bazv+0x21>
  21:   48 83 c4 18             add    $0x18,%rsp
  25:   c3                      retq   
---snip---

I've verified that Clang performs the optimization I was talking about and
that, according to gcc.godbolt.org, a snapshot of gcc 6 misses this
optimization too.

For comparison, the code Clang produces (according to godbolt):

        movabsq $8589934593, %rdi       # imm = 0x200000001
        movl    $3, %esi
        jmp     bar(foo)              # TAILCALL