[LLVMbugs] [Bug 22728] New: Many allocations in clang aren't correctly aligned.

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Fri Feb 27 09:57:26 PST 2015


            Bug ID: 22728
           Summary: Many allocations in clang aren't correctly aligned.
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ahmed.bougacha at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Consider "DeclRefExpr::Create(.., NameInfo, ..)".  It allocates itself
and a few other structs in a single allocation, using the well-known

  std::size_t Size = sizeof(DeclRefExpr);
  if (...)
    Size += sizeof(...);
  void *Mem = Context.Allocate(Size, llvm::alignOf<DeclRefExpr>());

The alignment should also take into account the trailing classes,
really.  In every Allocate call that has a non-trivial size, the
alignment should be correspondingly non-trivial.  Some calls just use
ASTContext's default, 8 (enough for most purposes, at least; better
than the explicit alignOf, which might be 4).

[skip forward if you don't care about the investigation]

Currently, I think we get away with this because 1) X86 is lenient, 2)
most people don't self-host on another architecture, and 3) at least
on ARM, alignment checking has to be explicitly enabled, statically or

On ARM, if you try running clang with either:
- SCTRL.A==1 (alignment checking on non-strict-aligned instructions)
(I should say I couldn't get this to work); or
- different LLVM CodeGen picking stricter instructions that actually
enforce alignment (what made me investigate),

it should crash, at some point.  On the LIT-tests, the first (of many)
to fail was
Indeed, on trunk clang, the "this" pointer in
  ASTTemplateArgumentListInfo::initializeFrom(Info, b, b, b)
is sometimes dynamically 4-aligned, whereas it is supposed to be
8-aligned (according to alignof and the IR we generate.)

This pattern is pervasive throughout clang, and even though a few
classes try to get it right (with a trailing AlignedCharArray), most

UBSan's alignment sanitizer would be *very* useful here.  On ARM,
self-hosting clang using clang, with the alignment sanitizer:
  -fsanitize=alignment -fsanitize-undefined-trap-on-error

Should exhibit the problems.

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