[Bug c++/69560] New: x86_64: alignof(uint64_t) produces incorrect results with -m32

[david.merillat at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69560

            Bug ID: 69560
           Summary: x86_64: alignof(uint64_t) produces incorrect results
                    with -m32
           Product: gcc
           Version: 5.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david.merillat at gmail dot com
  Target Milestone: ---

Sample Program (test.cpp):

#######################################################################

#include <cstdint>
#include <cstdio>
#include <cstddef>

using namespace std;

typedef struct {
        int32_t i32;
        int64_t i64;
} DefaultStruct;

typedef struct {
        int32_t i32;
        int64_t i64 __attribute__((aligned(8)));
} AlignedStruct;

int main(void)
{
        DefaultStruct s;
        printf( "DefaultStruct: offset=%d, struct align=%d, member align=%d,
type align=%d\n", 
                offsetof(DefaultStruct,i64), alignof(s), alignof(s.i64),
alignof(int64_t) );


        AlignedStruct s2;
        printf( "AlignedStruct: offset=%d, struct align=%d, member align=%d,
type align=%d\n",
                offsetof(AlignedStruct,i64), alignof(s2), alignof(s2.i64),
alignof(int64_t) );

        return 0;
}


#####################################################

Compile command line:

g++ -m32 --std=c++14 test.cpp -o test

#####################################################

Expected program output:

DefaultStruct: offset=4, struct align=4, member align=4, type align=4
AlignedStruct: offset=8, struct align=8, member align=8, type align=8

#####################################################

Actual program output:

DefaultStruct: offset=4, struct align=4, member align=4, type align=8
AlignedStruct: offset=8, struct align=8, member align=8, type align=8

#####################################################

Comments:

Note that the difference is alignof(uint64_t) returns 8, while the correct
value appears to be 4.

I checked the ISO C++ spec, section 3.11, and it says "The alignment
requirement of a complete type can be queried using an alignof expression", so
if alignof(int64_t) yields 8, then it shouldn't be legal to place an int64_t at
offset 4 in a structure.

Disclaimer:  I checked the draft specification.  I don't have $212 lying around
to purchase the final spec.

This is a possible duplicate with 61053 or 61447, however 61053 doesn't deal
with the output of alignof(), and 61447 has been suspended because the
standards were ambiguous.  Here they seem more clear-cut.