Thread overview
[Issue 18232] Union methods fail to initialize local variables to .init
[Issue 18232] string variable in toString method of Union: invalid code (crash/segfault)
January 12
https://issues.dlang.org/show_bug.cgi?id=18232

ag0aep6g@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
                 CC|                            |ag0aep6g@gmail.com

--
January 12
https://issues.dlang.org/show_bug.cgi?id=18232

hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh@quickfur.ath.cx

--- Comment #1 from hsteoh@quickfur.ath.cx ---
The crash is caused by wrong codegen for the toString() method. Here's a
comparison of a toString() method for a struct vs. for a union:

Struct code:
--------
struct Jack
{
    float f;
    int i;
    string toString()
    {
        string s;
        return s;
    }
}
--------

Generated assembly for Jack.toString:
--------
   447dc:       55                      push   %rbp
   447dd:       48 8b ec                mov    %rsp,%rbp
   447e0:       31 c0                   xor    %eax,%eax
   447e2:       31 d2                   xor    %edx,%edx
   447e4:       5d                      pop    %rbp
   447e5:       c3                      retq
--------

As can be easily seen, this basically initializes the string s to have .ptr=null, .length=0, returned by value in %eax and %edx. This is the correct behaviour.

Union code (basically the same as the struct code, just change 'struct' to
'union'):
--------
union Jack
{
    float f;
    int i;
    string toString()
    {
        string s;
        return s;
    }
}
--------

Generated assembly for Jack.toString:
--------
   4471c:       55                      push   %rbp
   4471d:       48 8b ec                mov    %rsp,%rbp
   44720:       48 83 ec 10             sub    $0x10,%rsp
   44724:       48 8b 55 f8             mov    -0x8(%rbp),%rdx
   44728:       48 8b 45 f0             mov    -0x10(%rbp),%rax
   4472c:       c9                      leaveq
   4472d:       c3                      retq
--------

Here, the 'sub' line appears to be allocating space on the stack for a local variable, presumeably s.  But then it fails to initialize s before loading it into the return registers %rdx and %rax.  I'm assuming that this is probably why the allocation of the local variable never got elided, as it was in the struct case -- the optimizer doesn't see the missing initialization of s, so it cannot elide the loads from the stack.

So it looks like the bug is caused by failure to initialize s when generating code for a union vs. a struct.  Why, though, is a mystery to me, since toString never actually references `this`, so in theory whether `this` is a struct or union shouldn't matter.

--
January 12
https://issues.dlang.org/show_bug.cgi?id=18232

--- Comment #2 from hsteoh@quickfur.ath.cx ---
Note also, the name `toString` is irrelevant to this bug.  The same codegen bug appears if you rename the method to something else, like 'member'.

Furthermore, the local variable doesn't have to be a string; any local variable without an initializer would also exhibit the same missing initialization, it doesn't initialize the local to .init, as it should.

--
January 12
https://issues.dlang.org/show_bug.cgi?id=18232

hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|string variable in toString |Union methods fail to
                   |method of Union: invalid    |initialize local variables
                   |code (crash/segfault)       |to .init

--
January 12
https://issues.dlang.org/show_bug.cgi?id=18232

--- Comment #3 from hsteoh@quickfur.ath.cx ---
Minimized code:
------
union U {
    int method() {
        int x;
        return x;
    }
}
------

The disassembly shows that x is never initialized to 0, and a garbage value is returned.

--
January 12
https://issues.dlang.org/show_bug.cgi?id=18232

--- Comment #4 from hsteoh@quickfur.ath.cx ---
More interesting clues: running a union method inside CTFE containing a local variable without an explicit initializer causes a CTFE error "cannot modify variable at compile time", whereas explicitly initializing the local variable works.

------
union U {
    int method() {
        int x; // causes CTFE failure unless explicitly initialized
        return x;
    }
}
enum y = U.init.method();
------

--
January 12
https://issues.dlang.org/show_bug.cgi?id=18232

hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull

--- Comment #5 from hsteoh@quickfur.ath.cx ---
https://github.com/dlang/dmd/pull/7687

--
5 days ago
https://issues.dlang.org/show_bug.cgi?id=18232

--- Comment #6 from github-bugzilla@puremagic.com ---
Commits pushed to master at https://github.com/dlang/dmd

https://github.com/dlang/dmd/commit/29c1e309be359f871d4052c025f7f3144005d092 Fix issue 18232: local vars in union member functions needs initializer.

The `isunion` condition is too broad, because it also excludes local variables declared inside union member functions. Moreoever, it is not necessary, because the `fd` condition later on already excludes union fields, since they would not be inside a function declaration.

https://github.com/dlang/dmd/commit/4cf228eb33ee8e2b4b6856fa7c08990217039ac7 Merge pull request #7687 from quickfur/issue18232

Fix issue 18232: local vars in union member functions needs initializer. merged-on-behalf-of: Sebastian Wilzbach <sebi.wilzbach@gmail.com>

--
5 days ago
https://issues.dlang.org/show_bug.cgi?id=18232

github-bugzilla@puremagic.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

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