[Bug tree-optimization/47059] New: compiler fails to coalesce loads/stores

[Bug tree-optimization/47059] New: compiler fails to coalesce loads/stores

[rahul at icerasemi dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47059

           Summary: compiler fails to coalesce loads/stores
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rahul@icerasemi.com
                CC: sdkteam-gnu@icerasemi.com
              Host: i686-pc-linux-gnu
            Target: i686-pc-linux-gnu
             Build: i686-pc-linux-gnu


Consider the following test case compiled with GCC4.5.1 (x86) and the following
command:

gcc -S -Os test.c

struct struct1
{
  void *data;
  unsigned short f1;
  unsigned short f2;
};
typedef struct struct1 S1;

struct struct2
{
  int f3;
  S1 f4;
};
typedef struct struct2 S2;


extern void foo (S1 *ptr);
extern S2 gstruct2_var;
extern S1 gstruct1_var;

static S1 bar (const S1 *ptr) __attribute__ ((always_inline));

static S1
bar (const S1 *ptr)
{
  S1 ls_var = *ptr;
  foo (&ls_var);
  return ls_var;
}

int
main ()
{
  S2 *ps_var;

  ps_var = &gstruct2_var;
  ps_var->f4 = bar (&gstruct1_var);

  return 0;
}

We get:

main:
        leal    4(%esp), %ecx
        andl    $-16, %esp
        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ecx
        subl    $32, %esp
        movl    gstruct1_var, %eax
        movl    gstruct1_var+4, %edx
        movl    %eax, -16(%ebp)
        leal    -16(%ebp), %eax
        pushl   %eax
        movl    %edx, -12(%ebp)
        call    foo
        movl    -16(%ebp), %eax
        movl    -4(%ebp), %ecx
        movl    %eax, gstruct2_var+4
        movl    -12(%ebp), %eax        <-- load1   [ebp - 12] @ 4 bytes
        movw    %ax, gstruct2_var+8    <-- store1  [gstruct2_var + 8] @ 2 bytes
        movw    -10(%ebp), %ax         <-- load2   [ebp - 10] @ 2 bytes
        movw    %ax, gstruct2_var+10   <-- store2  [gstruct2_var + 10] @ 2
bytes
        xorl    %eax, %eax
        leave
        leal    -4(%ecx), %esp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.5.1"
        .section        .note.GNU-stack,"",@progbits


With GCC4.4.1 we get:

main:
        leal    4(%esp), %ecx
        andl    $-16, %esp
        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ecx
        subl    $32, %esp
        movl    gstruct1_var, %eax
        movl    gstruct1_var+4, %edx
        movl    %eax, -16(%ebp)
        leal    -16(%ebp), %eax
        movl    %edx, -12(%ebp)
        pushl   %eax
        call    foo
        movl    -12(%ebp), %eax       <-- Load1 [ebp - 12] @ 4 bytes
        movl    -4(%ebp), %ecx
        movl    %eax, gstruct2_var+8  <-- Store1 [gstruct2_var + 8] @ 4 bytes
        movl    -16(%ebp), %eax
        movl    %eax, gstruct2_var+4
        xorl    %eax, %eax
        leave
        leal    -4(%ecx), %esp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.4.1"
        .section        .note.GNU-stack,"",@progbits


The extra load stores appear to be the result of change to SRA fully
scalarizing structure members f1 and f2. With GCC4.4.1 the access to these
fields is done using a BIT_FIELD_REF which combines the two loads and stores.

Talking to MartinJ on IRC I was told the changes to SRA make aggressive
scalarization of aggregates. In the past there was some functionality to try
and combine appropriate components into BIT_FIELD_REFs so as to reduce the
number of loads/stores. This has been removed from 4.5 in favour of simplicity
of the Gimple IR and working towards generic MEM_REFs. The plan is to introduce
new IR constructs to load/store individual bits and in a separate gimple pass
decide how to combine them together. But, this will only be available in 4.7+.

We also have the exact same issue on our port and causes a significant
performance regression on our software.

[Bug tree-optimization/47059] compiler fails to coalesce loads/stores

[rahul at icerasemi dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47059

--- Comment #1 from Rahul Kharche <rahul at icerasemi dot com> 2011-01-15 12:32:01 UTC ---
Created attachment 22974
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22974
Patch Vs 4.5.2 Rev 167088

[Bug tree-optimization/47059] compiler fails to coalesce loads/stores

[rahul at icerasemi dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47059

--- Comment #2 from Rahul Kharche <rahul at icerasemi dot com> 2011-01-15 12:43:27 UTC ---
This issue also exists on the trunk. I am in the process of bootstrap testing
this for i686-pc-linux-gnu. I will send out this patch once it checks out.
The attached patch is Vs 4.5.2 Rev 167088.

[Bug tree-optimization/47059] compiler fails to coalesce loads/stores

[vda.linux at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47059

Denis Vlasenko <vda.linux at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vda.linux at googlemail dot com

--- Comment #3 from Denis Vlasenko <vda.linux at googlemail dot com> ---
I encountered this behavior with 4.8.0:

                struct pollfd pfd[3];
                ...
                pfd[2].events = POLLOUT;
                pfd[2].revents = 0;

This compiled to:

        movw    $4, 44(%rsp)    #, pfd[2].events
        movw    $0, 46(%rsp)    #, pfd[2].revents

[Bug tree-optimization/47059] compiler fails to coalesce loads/stores

[jamborm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47059

Martin Jambor <jamborm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jamborm at gcc dot gnu.org

--- Comment #4 from Martin Jambor <jamborm at gcc dot gnu.org> ---
Store merging currently merges the two loads and stores of short ints
into a normal int, so this can be closed.  I have proposed to add this
testcase to the testsuite:
https://gcc.gnu.org/pipermail/gcc-patches/2021-January/563096.html

[Bug tree-optimization/47059] compiler fails to coalesce loads/stores

[jamborm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47059

Martin Jambor <jamborm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #5 from Martin Jambor <jamborm at gcc dot gnu.org> ---
Eh, really closing.

[Bug tree-optimization/47059] compiler fails to coalesce loads/stores

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47059

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Jambor <jamborm@gcc.gnu.org>:

https://gcc.gnu.org/g:d7e681fc3afff24a6279058cbb0b0dc4cd96be8c

commit r11-6862-gd7e681fc3afff24a6279058cbb0b0dc4cd96be8c
Author: Martin Jambor <mjambor@suse.cz>
Date:   Fri Jan 22 18:09:38 2021 +0100

    Testcase for old PR 47059

    I stumbled across PR 47059 from 2010 which has been addressed by
    store-merging.  I am going to close it but would like to add its
    testcase too.

    gcc/testsuite/ChangeLog:

    2021-01-08  Martin Jambor  <mjambor@suse.cz>

            PR tree-optimization/47059
            * gcc.dg/tree-ssa/pr47059.c: New test.