[Bug rtl-optimization/55360] New: [TileGX] Passing structure by value on stack needlessly writes to and reads from memory

[Bug rtl-optimization/55360] New: [TileGX] Passing structure by value on stack needlessly writes to and reads from memory

[colanderman at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55360

             Bug #: 55360
           Summary: [TileGX] Passing structure by value on stack
                    needlessly writes to and reads from memory
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: colanderman@gmail.com


#include <stdint.h>

struct bar { uint8_t a, b, c, d; };
struct bla { unsigned long a:8, b:8, c:8, d:8; };

uint64_t bar(struct bar);
uint64_t bla(struct bla);

uint64_t foo(uint8_t a, uint8_t b, uint8_t c, uint8_t d)
{ return bar((struct bar) { a, b, c, d }); }

uint64_t baz(uint8_t a, uint8_t b, uint8_t c, uint8_t d)
{ return bla((struct bla) { a, b, c, d }); }

when compiled with "gcc -Wall -std=gnu99 -O3 -S pbv.c" produces:

    .file    "pbv.c"
    .text
    .align 8
.global foo
    .type    foo, @function
foo:
.LFB0:
    .cfi_startproc
    {
    st    sp, lr
    .cfi_offset 55, 0
    move    r29, sp
    addi    r28, sp, -16
    }
    addi    sp, sp, -24
    .cfi_def_cfa_offset 24
    {
    st    r28, r29
    addi    r11, sp, 21
    addi    r10, sp, 20
    }
    {
    st1    r11, r1
    addi    r11, sp, 22
    }
    {
    st1    r11, r2
    addi    r11, sp, 23
    }
    {
    st1    r10, r0
    movei    r0, 0
    }
    st1    r11, r3
    ld4u    r10, r10
    {
    bfins    r0, r10, 0, 0+32-1
    jal    bar
    }
    addi    r29, sp, 24
    ld    lr, r29
    {

    addi    sp, sp, 24
    .cfi_restore 54
    .cfi_restore 55
    .cfi_def_cfa_offset 0
    jrp    lr
    }
    .cfi_endproc
.LFE0:
    .size    foo, .-foo
    .align 8
.global baz
    .type    baz, @function
baz:
.LFB1:
    .cfi_startproc
    {
    movei    r10, 0
    st    sp, lr
    .cfi_offset 55, 0
    move    r29, sp
    }
    {
    bfins    r10, r0, 0, 7
    addi    r28, sp, -8
    }
    {
    bfins    r10, r1, 8, 8+8-1
    addi    sp, sp, -16
    }
    .cfi_def_cfa_offset 16
    {
    bfins    r10, r2, 16, 16+8-1
    st    r28, r29
    }
    bfins    r10, r3, 24, 24+8-1
    {
    move    r0, r10
    jal    bla
    }
    addi    r29, sp, 16
    ld    lr, r29
    {

    addi    sp, sp, 16
    .cfi_restore 54
    .cfi_restore 55
    .cfi_def_cfa_offset 0
    jrp    lr
    }
    .cfi_endproc
.LFE1:
    .size    baz, .-baz
    .ident    "GCC: (GNU) 4.7.2"
    .section    .note.GNU-stack,"",@progbits

My expectation is that the foo and baz should compile identically, and should
use the bfins bit-arithmetic functions like baz does, rather than redundant
stores and loads to stack like foo does.

This is with a vanilla GCC 4.7.2 build on a Tilempower system (roughly CentOS
5.7).

The problem does not occur on Debian x86-64 with either GCC 4.4.6 or GCC 4.7.2.

Possibly related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=7061 (however
that case seems to be fixed in 4.7.2).

[Bug rtl-optimization/55360] [TileGX] Passing structure by value on stack needlessly writes to and reads from memory

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55360

--- Comment #1 from H.J. Lu <hjl.tools at gmail dot com> 2012-11-16 23:46:25 UTC ---
Is this related to PR 28831?

[Bug rtl-optimization/55360] [TileGX] Passing structure by value on stack needlessly writes to and reads from memory

[colanderman at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55360

--- Comment #2 from Chris King <colanderman at gmail dot com> 2012-11-19 18:47:39 UTC ---
Possibly, though I doubt it.  PR 28831 has more to do with eliding copies of
the struct in its entirety; the problem I'm having centers around accessing
individual elements.  If PR 28831 were the cause, I would expect both my test
cases (with and without bit-fields) to behave identically, however they do not.

It's possible that fixing PR 28831 may hide this bug in my particular use case
(by avoiding the stack allocation in the first place), but I believe the
difference in handling of normal fields vs. bit fields to be a distinct bug.

[Bug rtl-optimization/55360] [TileGX] Passing structure by value on stack needlessly writes to and reads from memory

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55360

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
   Target Milestone|---                         |13.0
         Resolution|---                         |WONTFIX

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The tile* targets were removed in r13-1267-gfc259b522c0f8b so closing as won't
fix.