[PATCH] D45524: Fix incorrect choice of callee-saved registers save/restore points

[Momchil Velikov via Phabricator via llvm-commits]

chill created this revision.
chill added reviewers: thegameg, qcolombet, t.p.northover.
Herald added a subscriber: kristof.beyls.

So, I have this testcase:

  void f(int n, int x[]) {
    if (n < 0)
      return;
  
    int a[n];
  
    for (int i = 0; i < n; i++)
      a[i] = x[n - i - 1];
  
    for (int i = 0; i < n; i++)
      x[i] = a[i] + 1;
  }

that, compiled with -O1/-Os for AArch64 and X86, generates machine code, which
fails to properly restore the stack pointer upon function return.

The testcase allocates a VLA, thus clang generates calls to `llvm.stacksave` and
`llvm.stackrestore`. The latter call is lowered to `mov sp, x2`, however this
move does not affect decisions made by the shrink wrapping pass, as the
instruction does not use or define a callee-saved register.

The end effect is that placement of register save/restore code is such that
along a certain path, the callee-saved registers and the stack pointer are
restored, but then the stack pointer is overwritten with an incorrect value.

This patches fixes the issue this by modifying `ShrinkWrap::useOrDefCSROrFI` to explicitly check
for the stack pointer register (using `TLI.getStackPointerRegisterToSaveRestore`) and also to ignore tall call
instructions (`isCall() && isReturn()`), since they implictly use SP (for AArch{32,64} at least).


Repository:
  rL LLVM

https://reviews.llvm.org/D45524

Files:
  lib/CodeGen/ShrinkWrap.cpp
  test/CodeGen/Generic/shrink-wrapping-vla.ll

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