[Bug c++/39163] New: -mstackalign breaks -O2 optimization using g++

[gyunaev at ulduzsoft dot com]

This is basically a copy of bug report from
http://www.dribin.org/dave/blog/archives/2006/12/05/missing_third_param/

Using the following sample file saved as test.c:

----------------------------------------------------------------------------
#include <stdio.h>

#define NOINLINE __attribute__((noinline))

NOINLINE static void foo1(int i1)
{
    printf("foo1: %d\n", i1);
}

NOINLINE static void foo2(int i1, int i2)
{
    printf("foo2: %d, %d\n", i1, i2);
}

NOINLINE static void foo3(int i1, int i2, int i3)
{
    printf("foo3: %d, %d, %d\n", i1, i2, i3);
}

NOINLINE static void foo4(int i1, int i2, int i3, int i4)
{
    printf("foo4: %d, %d, %d, %d\n", i1, i2, i3, i4);
}

NOINLINE static void foo5(int i1, int i2, int i3, int i4, int i5)
{
    printf("foo5: %d, %d, %d, %d, %d\n", i1, i2, i3, i4, i5);
}

NOINLINE static void foo6(int i1, int i2, int i3, int i4, int i5, int i6)
{
    printf("foo6: %d, %d, %d, %d, %d, %d\n", i1, i2, i3, i4, i5, i6);
}


int main(int argc, char **argv)
{
    foo1(1);
    foo2(1, 2);
    foo3(1, 2, 3);
    foo4(1, 2, 3, 4);
    foo5(1, 2, 3, 4, 5);
    foo6(1, 2, 3, 4, 5, 6);
    return 0;
}
----------------------------------------------------------------------------

produces invalid output (broken 3rd parameter) if compiled using g++ and
-mstackrealign:

> /usr/toolchains/gcc433/bin/g++ -Wall -mstackrealign -O2 test.c
> ./a.out
foo1: 1
foo2: 1, 2
foo3: 1, 2, -1209632279
foo4: 1, 2, 4, 134520820
foo5: 1, 2, 4, 5, -1074446840
foo6: 1, 2, 4, 5, 6, 134514537



However if gcc is used instead of g++, valid output is generated:

> /usr/toolchains/gcc433/bin/gcc -mstackrealign -Wall -O2 test.c
> ./a.out
foo1: 1
foo2: 1, 2
foo3: 1, 2, 3
foo4: 1, 2, 3, 4
foo5: 1, 2, 3, 4, 5
foo6: 1, 2, 3, 4, 5, 6


Analyze of assembly dump revealed that GCC still uses ECX in main() to pass the
parameter. ix86_internal_arg_pointer() in i386.c works fine, and returns same
values for gcc and g++, so the problem is not there.


-- 
           Summary: -mstackalign breaks -O2 optimization using g++
           Product: gcc
           Version: 4.3.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gyunaev at ulduzsoft dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39163