[Bug c/53937] New: Pack'ing struct causes gcc to not recognize that an field's address is aligned

[don.delfin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53937

             Bug #: 53937
           Summary: Pack'ing struct causes gcc to not recognize that an
                    field's address is aligned
    Classification: Unclassified
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: don.del...@gmail.com


Created attachment 27779
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27779
Example C code

Hello,

When I pack my structure (use #pragma pack(1)), gcc compiler seems to not
recognize that an field in that structure is aligned.
This causes reading of aligned 32-bit fields, as 4 separate bytes. Apart from
this is very not optimal code, this resets my PowerPC. This is because 32-bit
registers can be accessed only as 32-bit words.

Example code:

================ test.c ================
#pragma pack(1)
typedef struct BasicStructPacked
{
    int field;
}BasicStructPacked;
#pragma pack()

typedef struct BasicStruct
{
    int field;
}BasicStruct;

#define ADDRESS    (0x1000)

#define STRUCT_REGULAR (*((BasicStruct*)(ADDRESS)))
#define STRUCT_PACKED  (*((BasicStructPacked*)(ADDRESS)))

void test_1()
{
    STRUCT_PACKED.field = 0;
}

void test_2()
{
    STRUCT_REGULAR.field = 0;
}
=======================================

This code generates the following assembler:

================ test.s ================
(...)
test_1:
    li 9,4096
    li 0,0
    stb 0,0(9)
    stb 0,1(9)
    stb 0,2(9)
    stb 0,3(9)
    blr
(...)
test_2:
    li 0,0
    li 9,4096
    stw 0,0(9)
    blr
(...)
=======================================

As you can see access to the aligned 32-bit field is done as 4 reads of one
byte (function test_1) instead of reading of one 32-bit word as it is done in
function test_2.


Used compilation command:

...GCC/powerpc-eabispe/4_6_0/bin/powerpc-eabispe-gcc-4.6.0.exe -mstrict-align
-Wall -Wextra -Os -c -S test.c

Regards, Krzysiek