c++/10332: Template classes are not instantiated correctly in presense of #pragma pack()

[igusarov@mail.ru]

>Number:         10332
>Category:       c++
>Synopsis:       Template classes are not instantiated correctly in presense of #pragma pack()
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Apr 07 10:46:02 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Akella corp.
>Release:        3.2
>Organization:
>Environment:
System: FreeBSD gate.studio1001.akella.ru 4.4-RELEASE FreeBSD 4.4-RELEASE #11: Thu Jun 6 12:58:54
host: i386-unknown-freebsd4.4
build: i386-unknown-freebsd4.4
target: i386-unknown-freebsd4.4
configured with: /usr/src/gcc-3.2/configure --prefix=/usr
>Description:
The packing size used for class template instantiation is set by the #pragma pack directive which is in effect at the point of template instantiation (whether implicit or explicit), rather then by the #pragma pack directive which was in effect at the point of class template definition.

This is not a correct behaviour, because in a complex program it's hard to control the point of implicit instantiation of any given template class. And the wrong packing size is leading to incorrect code generation if a given template class was instantiadet with different packing sizes in different translation units. Consider the code examples in Howtorepeat section.

Related problem reports: other/4957
>How-To-Repeat:
1. Try to compile and run the following example (all code examples in this report could be compiled with "g++ file.cpp", no special command-line options are required)

//---------------------------- begin example
#include <stdio.h>

#pragma pack(4)

template <typename X >
struct T
{
    char      x1;   /* Usually 3 padding bytes are added after x1 member. */
    int       x2;
};

#pragma pack(1)
template T<int>;   /* T<int> is instantiated here */

#pragma pack(4)
template T<float>; /* T<float> is instantiated here */

int main()
{
 printf("sizeof T<int>   = %d\n", sizeof(T<int>));
 printf("sizeof T<float> = %d\n", sizeof(T<float>));
}
//---------------------------- end example

(parameter types 'int' and 'float' are not used in any way, they are required just to separate two template instances)

The output is:
sizeof T<int>   = 5
sizeof T<float> = 8
Which shows that the size (and member offsets) of the template class depends on the #pragma pack at place of instantiation.

2. There is a more complicated example which shows how to trigger this bug accidently.
Unpack the attached archive and compile two sample translation units (tu01.cpp and tu02.cpp) in alignment_bug/case_02 directory. The only difference between these translation units is the order of #include directives which causes different implicit instantiation of the same template class (include guards are skipped in such simple case). This test case repeats the behaviour reported in other/4957.
>Fix:
Do not rely on implicit instantiation. Instantiate all desired template classes explicitly (sic!).
>Release-Note:
>Audit-Trail:
>Unformatted:
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