[Bug middle-end/29158] New: store merge optimization not done

[Bug middle-end/29158] New: store merge optimization not done

[acahalan at gmail dot com]

In the example below, define T as "char" or "short". (probably "int" too on a
64-bit machine, with minor adjustment to the example code) Compile the code
with "-Os -S" and observe the assembly. Even if there are no aliasing or
alignment issues, the compiler fails to merge multiple stores into larger
stores.

This is tested with three gcc 4.1 compilers:

i386 from Fedora Core 6 test2
x86-64 from Fedora Core 6 test2
ppc32 from Debian-unstable

BTW, I hope I guessed right in filing against "middle-end". I didn't know if it
was tree-optimization or rtl-optimization, and there still isn't an obvious
place to file bugs which are common to both C and C++.

-------------------------------------
tmp 0 $ cat e.c
#ifndef T
#define T short
#endif

typedef struct S{
        T a;
        T b;
        T c;
        T d;
}S;

S *structbug(S *s){
        s->a = 1;
        s->b = 2;
        s->c = 3;
        s->d = 4;
        return s;
}

T *arraybug(T *s){
        s[0] = 1;
        s[1] = 2;
        s[2] = 3;
        s[3] = 4;
        return s;
}

tmp 0 $ gcc -W -Wall -m32 -Os -fomit-frame-pointer -S e.c
tmp 0 $ cat e.s
        .file   "e.c"
        .text
.globl structbug
        .type   structbug, @function
structbug:
        movl    4(%esp), %eax
        movw    $1, (%eax)
        movw    $2, 2(%eax)
        movw    $3, 4(%eax)
        movw    $4, 6(%eax)
        ret
        .size   structbug, .-structbug
.globl arraybug
        .type   arraybug, @function
arraybug:
        movl    4(%esp), %eax
        movw    $1, (%eax)
        movw    $2, 2(%eax)
        movw    $3, 4(%eax)
        movw    $4, 6(%eax)
        ret
        .size   arraybug, .-arraybug
        .ident  "GCC: (GNU) 4.1.1 20060828 (Red Hat 4.1.1-20)"
        .section        .note.GNU-stack,"",@progbits
tmp 0 $


-- 
           Summary: store merge optimization not done
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: acahalan at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29158

[Bug middle-end/29158] store merge optimization not done

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29158

[Bug middle-end/29158] store merge optimization not done

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-09-21 04:39 -------


*** This bug has been marked as a duplicate of 23684 ***


-- 

pinskia at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29158

[Bug middle-end/29158] store merge optimization not done

[acahalan at gmail dot com]

------- Comment #2 from acahalan at gmail dot com  2006-11-29 05:09 -------
This hits it too. (example is PowerPC)

///////////////////
#include <string.h>

char *foo(char *buf)
{
 short temp;
 int temp1;

 *buf++=42;

 temp = 0xfeed;
 memcpy(buf, &temp, sizeof(temp));
 buf+=sizeof(temp);

 temp1 = 0x12345678;
 memcpy(buf, &temp1, sizeof(temp1));
 buf+=sizeof(temp1);

 temp1 = 0x12345678;
 memcpy(buf, &temp1, sizeof(temp1));
 buf+=sizeof(temp1);

 *buf++=42;

 return buf;
}
///////////////
foo:
        stwu 1,-32(1)
        lis 9,0x1234
        li 11,42
        li 0,-275
        ori 9,9,22136
        stb 11,11(3)
        stb 11,0(3)
        sth 0,1(3)
        stw 9,7(3)
        stw 9,3(3)
        addi 3,3,12
        addi 1,1,32
        blr


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29158

[Bug middle-end/29158] store merge optimization not done

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2006-11-29 05:26 -------
(In reply to comment #2)
> This hits it too. (example is PowerPC)
No it does not, look at the what it gives:
        stb 11,11(3)
        stb 11,0(3)
        sth 0,1(3)
        stw 9,7(3)
        stw 9,3(3)

Reordering the stores to be in a natural order:

        stb 11,0(3)
        sth 0,1(3)
        stw 9,3(3)
        stw 9,7(3)
        stb 11,11(3)

we do one byte store and then half word store and then two word stores and
finally one byte store, yes we could reagrange the stores to get three word
stores but is that really useful in this case, almost no but because of the
constant generation is going to hurt.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29158