[Issue 18794] Compiling with -O causes runtime segfault

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18794

ag0aep6g <ag0aep6g at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ag0aep6g at gmail.com
             Blocks|                            |18750

--- Comment #1 from ag0aep6g <ag0aep6g at gmail.com> ---
Reduced:

----
bool method(size_t* p)
{
    int bitIdx = 0;
    func();
    return (*p & (1UL << bitIdx)) != 0;
}

void func() {}

void prep()
{
    asm {}
    ulong[2] x = -1;
}

void main()
{
    prep();
    size_t s;
    method(&s);
}
----

Generated code for `method`:

----
   0:   55                      push   rbp
   1:   48 8b ec                mov    rbp,rsp
   4:   48 83 ec 10             sub    rsp,0x10
   8:   48 89 7d f8             mov    QWORD PTR [rbp-0x8],rdi
   c:   c7 45 f0 00 00 00 00    mov    DWORD PTR [rbp-0x10],0x0
  13:   e8 00 00 00 00          call   18 <_D4test6methodFPmZb+0x18>
                        14: R_X86_64_PLT32      _D4test4funcFZv-0x4
  18:   48 8b 45 f8             mov    rax,QWORD PTR [rbp-0x8]
  1c:   48 8b 4d f0             mov    rcx,QWORD PTR [rbp-0x10]
  20:   48 0f a3 08             bt     QWORD PTR [rax],rcx
  24:   48 0f 92 c0             rex.W setb al
  28:   48 8b e5                mov    rsp,rbp
  2b:   5d                      pop    rbp
  2c:   c3                      ret
----

bitIdx is a DWORD at rbp-0x10. But later a QWORD is read from there and used in
the bt instruction. So that reads garbage from the stack. The garbage can be
controlled by prep.

Looks like this is directly related to the generation of the bt instruction,
which is horribly broken. But it doesn't seem to be a duplicate of the known
issues. Adding to the tracker.


Referenced Issues:

https://issues.dlang.org/show_bug.cgi?id=18750
[Issue 18750] [Tracker] everything wrong with code generation for bt
instruction
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