[PATCH] D38143: Dynamic stack alignment for Thumb1

[Momchil Velikov via Phabricator via llvm-commits]

chill created this revision.
Herald added subscribers: kristof.beyls, javed.absar, aemerson.

This patch adds dynamic stack alignment for Thumb1.

The motivating issue is micompilation of the following code, when targeting a
CPU, which implements only Thumb-1, like cortex-m0.

  struct foo {
      alignas(16) char buf[12];
      int i;
      int *ip;
      foo() : ip(&i) {}
  };
  
  extern void g(foo &);
  void f() {
      foo myFoo;
      g(myFoo);
  }

When initialising the `ip` member, the address of `i` is calculated using bitwise OR:

  push    {r7, lr}
  .pad    #40
  sub     sp, #40
  movs    r1, #12
  mov     r0, sp
  orrs    r1, r0
  str     r1, [sp, #16]

which is obviously incorrect when the starting address of that object (resp. the
stack pointer) happens to be aligned at 8 or 4 byte boundary.

When compiling for ARM or Thumb2, the stack is realigned and the problem does
not occur.


https://reviews.llvm.org/D38143

Files:
  lib/Target/ARM/ARMBaseRegisterInfo.cpp
  lib/Target/ARM/Thumb1FrameLowering.cpp
  test/CodeGen/ARM/thumb1_return_sequence.ll
  test/CodeGen/Thumb/large-stack.ll
  test/CodeGen/Thumb/long.ll
  test/CodeGen/Thumb/stack-align.ll

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