[Issue 20844] New: DMD compiler should take care of data alignment, after seeing the 'cas' call

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20844

          Issue ID: 20844
           Summary: DMD compiler should take care of data alignment, after
                    seeing the 'cas' call
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: mingwu at gmail.com

(This is a follow up of: https://issues.dlang.org/show_bug.cgi?id=20838#c12)


I find a segfault with LDC: with this code on https://d.godbolt.org/z/HesA24
i.e. remove the import std.stdio and writeln

--------------------------------------------------------------------------------
$ cat c.d
//import std.stdio;
import core.atomic;

struct N {
  N* prev;
  N* next;
}

shared(N) n;

void main() {
  cas(&n, n, n);
  //writeln(size_t.sizeof*2, N.sizeof);
}

$ ldc2 -m64 c.d
$ ./c
Segmentation fault (core dumped)

$ ldc2 --version
LDC - the LLVM D compiler (1.20.0):
  based on DMD v2.090.1 and LLVM 9.0.1
  built with LDC - the LLVM D compiler (1.20.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: skylake
  http://dlang.org - http://wiki.dlang.org/LDC
--------------------------------------------------------------------------------


With import std.stdio and writeln, the LDC output behave normally (no
segfault):
--------------------------------------------------------------------------------
$ ldc2 -m64 c.d
$ ./c
1616
--------------------------------------------------------------------------------

Currently we need to manually take care of required 16-bytes alignment:

align(16) shared(N) n; // or `align(2 * size_t.sizeof)`


Can the DMD compiler (after seeing the 'cas' call) take care of this alignment?
either silently, or issue an warning message to the programmer?

The current behavior that I just discovered is definitely a puzzle for a D
newbie like me. With a smarter compiler, it will help new users.

The druntime library is supposed to take care of this already, at least with
enabled contracts, see
https://github.com/dlang/druntime/blob/48082ac4e4aa1a3c9f1a1ef87659c941dae0f7f6/src/core/atomic.d#L624-L655.
It only checks for insufficient size_t alignment though, so that needs to be
fixed.

Wrt. original DMD issue here, DMD is supposed to use cmpxchg16b already, see
https://github.com/dlang/druntime/blob/48082ac4e4aa1a3c9f1a1ef87659c941dae0f7f6/src/core/internal/atomic.d#L582.
As it apparently doesn't, I guess the bug is in DMD's codegen.

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