[Lldb-commits] [PATCH] D53435: [x86] Fix issues with a realigned stack in MSVC compiled applications

Aleksandr Urakov via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Fri Oct 19 07:10:48 PDT 2018

aleksandr.urakov created this revision.
aleksandr.urakov added reviewers: labath, zturner, jasonmolenda, stella.stamenova.
aleksandr.urakov added a project: LLDB.
Herald added subscribers: lldb-commits, abidh.

This patch fixes issues with a stack realignment.

MSVC maintains two frame pointers (`ebx` and `ebp`) for a realigned stack - one is used for access to function parameters, while another is used for access to locals. To support this the patch:

- adds an alternative frame pointer (`ebx`);
- considers stack realignment instructions (e.g. `and esp, -32`);
- along with CFA (Canonical Frame Address) which point to the position next to the saved return address (or to the first parameter on the stack) introduces AFA (Aligned Frame Address) which points to the position of the stack pointer right after realignment. AFA is used for access to registers saved after the realignment (see the test);

Here is an example of the code with the realignment:

  struct __declspec(align(256)) OverAligned {
    char c;
  void foo(int foo_arg) {
    OverAligned oa_foo = { 1 };
    auto aaa_foo = 1234;
  void bar(int bar_arg) {
    OverAligned oa_bar = { 2 };
    auto aaa_bar = 5678;
  int main() {
    return 0;

and here is the `bar` disassembly:

  push    ebx
  mov     ebx, esp
  sub     esp, 8
  and     esp, -100h
  add     esp, 4
  push    ebp
  mov     ebp, [ebx+4]
  mov     [esp+4], ebp
  mov     ebp, esp
  sub     esp, 200h
  mov     byte ptr [ebp-200h], 2
  mov     dword ptr [ebp-4], 5678
  push    1111            ; foo_arg
  call    j_?foo@@YAXH at Z  ; foo(int)
  add     esp, 4
  mov     esp, ebp
  pop     ebp
  mov     esp, ebx
  pop     ebx

Btw, it seems that the code of `x86AssemblyInspectionEngine` has overgrown. I have some ideas how to refactor this, if you don't mind I can do it in the future?

https://reviews.llvm.org/D53086 also contains some discussion on the topic.




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