Bug 20844 – DMD compiler should take care of data alignment, after seeing the 'cas' call

(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.

@kinke created dlang/druntime pull request #3109 "Fix Issue 20844 - Insufficient alignment check for 16-bytes CAS on x86_64" fixing this issue: - Fix Issue 20844 - Insufficient alignment check for 16-bytes CAS on x86_64 https://github.com/dlang/druntime/pull/3109

dlang/druntime pull request #3109 "Fix Issue 20844 - Insufficient alignment check for 16-bytes CAS on x86_64" was merged into master: - 1e50000f3691b2b0e616dc383d792b1537e072df by Martin Kinkelin: Fix Issue 20844 - Insufficient alignment check for 16-bytes CAS on x86_64 https://github.com/dlang/druntime/pull/3109