[Bug rtl-optimization/49157] New: Unnecessary stack save/restore code generated for a leaf function (arm-elf-gcc)

[Bug rtl-optimization/49157] New: Unnecessary stack save/restore code generated for a leaf function (arm-elf-gcc)

[bmei at broadcom dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49157

           Summary: Unnecessary stack save/restore code generated for a
                    leaf function (arm-elf-gcc)
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bmei@broadcom.com


For the following example:
struct  Complex16{
  short a;
  short b;
};


short foo (struct Complex16 s)
{
  return s.a + s.b;
}


Compile with:

arm-elf-gcc tst.c -O2 -S -mstructure-size-boundary=8

It produces:
foo:
    @ args = 0, pretend = 0, frame = 4
    @ frame_needed = 0, uses_anonymous_args = 0
    @ link register save eliminated.
    mov    r3, r0, asl #16
    mov    r3, r3, lsr #16
    add    r0, r3, r0, lsr #16
    mov    r0, r0, asl #16
    sub    sp, sp, #4
    mov    r0, r0, asr #16
    add    sp, sp, #4
    bx    lr

The problem is with struct-size-boundary=8, the structure has BLKmode and
mapped to memory after RTL expand. However, memory accesses are optimized away
later. But GCC records a stack item anyway and generates stack frame
save/restore code for this leaf function. 

If we compile without -mstructure-size-boundary=8 (default is 32), it generates
much better code.

foo:
    @ args = 0, pretend = 0, frame = 0
    @ frame_needed = 0, uses_anonymous_args = 0
    @ link register save eliminated.
    add    r0, r0, r0, asr #16
    mov    r0, r0, asl #16
    mov    r0, r0, asr #16
    bx    lr

This is not limited to ARM gcc. Our target has the same issue because
STRUCTURE_SIZE_BOUNDARY = 8 to save data memory size.

Though I only tested gcc 4.6, I believe trunk gcc probably has the same
problem.

[Bug rtl-optimization/49157] Unnecessary stack save/restore code generated for a leaf function (arm-elf-gcc)

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49157

Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.06.01 19:56:05
                 CC|                            |ramana at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #1 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-06-01 19:56:05 UTC ---
With an arm-linux-gnueabi toolchain I see the following code - it's pointless
creating space on the stack in this case. 

foo:
    @ args = 0, pretend = 0, frame = 8
    @ frame_needed = 0, uses_anonymous_args = 0
    @ link register save eliminated.
    add    r0, r0, r0, lsr #16
    sub    sp, sp, #8
    sxth    r0, r0
    add    sp, sp, #8
    bx    lr


Possibly related to PR36409.

Ramana

[Bug middle-end/49157] Unnecessary stack save/restore code generated for a leaf function (arm-elf-gcc)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49157

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement