[Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value

[Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value

[mans at mansr dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026

             Bug #: 55026
           Summary: [Multiple targets] Inefficient code with structs
                    passed by value
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mans@mansr.com


Multiple targets (including ARM, HPPA, and MIPS) generate inefficient code in
functions taking struct arguments by value.  Consider this code:

struct foo {
    int a;
    int b;
};

int f(struct foo x)
{
    return x.a + x.b;
}

On ARM, this compiles to the following at -O3 optimisation:

f:
        @ args = 0, pretend = 0, frame = 8
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        sub     sp, sp, #8
        add     r3, sp, #8
        stmdb   r3, {r0, r1}
        ldmia   sp, {r0, r3}
        add     r0, r0, r3
        add     sp, sp, #8
        @ sp needed
        bx      lr

Note the entirely unnecessary (and inefficiently done to boot) storing and
loading of the argument registers to/from the stack.

The x86_64 and SH4 targets do not show this behaviour.

[Bug target/55026] [Multiple targets] Inefficient code with structs passed by value

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-10-22 16:41:09 UTC ---
I think I already filed this bug about 4 years ago or so.

[Bug target/55026] Useless stores generated for structures passed by value

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-10-22
                 CC|                            |ebotcazou at gcc dot
                   |                            |gnu.org
            Summary|[Multiple targets]          |Useless stores generated
                   |Inefficient code with       |for structures passed by
                   |structs passed by value     |value
     Ever Confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-10-22 19:08:10 UTC ---
This depends on the internal argument passing mechanism (BLKmode registers vs
PARALLEL) and on the strict-alignment requirements of the architecture.  Try:

struct __attribute__((aligned (8))) foo {
    int a;
    int b;
};

int f(struct foo x)
{
  return x.a + x.b;
}

to watch the effects of the latter.

[Bug target/55026] Useless stores generated for structures passed by value

[mans at mansr dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026

--- Comment #3 from Mans Rullgard <mans at mansr dot com> 2012-10-22 19:34:25 UTC ---
It has actually got worse over time.  With 4.3 I get this:

f:
        sub     sp, sp, #8
        mov     r2, r0
        stmia   sp, {r0, r1}
        add     r0, r1, r2
        add     sp, sp, #8
        bx      lr

Here it's only doing the stores, not the loads, and it does them using sp
directly.

With 4.4 and 4.5 it gets slightly worse:

f:
        sub     sp, sp, #8
        mov     r3, sp
        stmia   r3, {r0, r1}
        mov     r3, r0
        add     r0, r1, r3
        add     sp, sp, #8
        bx      lr

Now it's copying sp to another register used as base address in the store.  The
load is still absent.

4.6 and later produce the code I quoted originally.  I can't say I like the
trend.

[Bug target/55026] Useless stores generated for structures passed by value

[mans at mansr dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026

--- Comment #4 from Mans Rullgard <mans at mansr dot com> 2012-10-22 19:37:58 UTC ---
For the record, clang/llvm gets this right.