[Bug rtl-optimization/54585] New: stack space allocated but never used when calling functions that return structs in registers

[Bug rtl-optimization/54585] New: stack space allocated but never used when calling functions that return structs in registers

[chip at pobox dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54585

             Bug #: 54585
           Summary: stack space allocated but never used when calling
                    functions that return structs in registers
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: chip@pobox.com


Now that bug #44194 is fixed, and a returned structure used as a parameter is
no longer stored unnecessarily, a new bug is visible: a stack frame is being
allocated that is entirely unused.  On x86_64 target with the fix for 44194
backported to the 4.7 branch, this code:


#include <stdint.h>

struct blargh { uint32_t a, b, c; } foo();
void bar(uint32_t a, uint32_t b, uint32_t c);

void func() {
  struct blargh s = foo();
  bar(s.a, s.b, s.c);
}


no longer uses any stack memory at all, but still the function call reserves 24
bytes with "subq $24,%rsp" and promptly returns it with "addq $24,%rsp".   The
generated code looks like this:

 func:
        .cfi_startproc
        xorl    %eax, %eax
        subq    $24, %rsp
        .cfi_def_cfa_offset 32
        call    foo
        movq    %rax, %rsi
        movl    %eax, %edi
        addq    $24, %rsp
        .cfi_def_cfa_offset 8
        shrq    $32, %rsi
        jmp     bar
        .cfi_endproc

[Bug rtl-optimization/54585] stack space allocated but never used when calling functions that return structs in registers

[chip at pobox dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54585

--- Comment #1 from Chip Salzenberg <chip at pobox dot com> ---
I'd like to suggest this ticket be at least CONFIRMED what with the code
samples in the ticket.

What will it take to fix this?

[Bug rtl-optimization/54585] stack space allocated but never used when calling functions that return structs in registers

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54585

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-09-05
                 CC|                            |ebotcazou at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
It's quite hard because the frame is laid out way before all the memory
accesses to the stack slots are optimized away.  My personal take is that it's
not worth the hassle, but I'm ready to be proved wrong.

[Bug rtl-optimization/54585] stack space allocated but never used when calling functions that return structs in registers

[chip at pobox dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54585

--- Comment #3 from Chip Salzenberg <chip at pobox dot com> ---
It's worth it for cache reasons I believe.  The data cache works better you
don't spread out the stack data unnecessarily.  More concretely, if the stack
frame can entirely disappear then you also reduce the instruction count. 
That's fewer instructions to dispatch and less icache pressure.

[Bug middle-end/54585] stack space allocated but never used when calling functions that return structs in registers

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54585

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|rtl-optimization            |middle-end
           Severity|normal                      |enhancement