[PATCH] D89189: [ARM][MachineOutliner] Do not overestimate LR liveness in return block

[Momchil Velikov via Phabricator via llvm-commits]

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The `LiveRegUnits` utility (as well as `LivePhysRegs`) considers callee-saved registers to be alive at the point after the return instruction in a block. In the ARM backend, the `LR` register is classified as callee-saved, which is not really correct (from an ARM eABI or just common sense point of view).
These two conditions cause the `MachineOutliner` to overestimate the liveness of `LR`, which results in unnecessary saves/restores of `LR` around calls to outlined sequences.
It also causes the MachineVerifer to crash in some cases, because the save instruction reads a dead `LR`, for example when the following program:

  int h(int, int);
  
  int f(int a, int b, int c, int d) {
    a = h(a + 1, b - 1);
    b = b + c;
    return 1 + (2 * a + b) * (c - d) / (a - b) * (c + d);
  }
  
  int g(int a, int b, int c, int d) {
    a = h(a - 1, b + 1);
    b = b + c;
    return 2 + (2 * a + b) * (c - d) / (a - b) * (c + d);
  }

is compiled with `-target arm-eabi -march=armv7-m -Oz`.

This patch computes the liveness of `LR` in return blocks only, while taking into account the few ARM instructions, which 
read `LR`, but nevertheless the register is not mentioned (explicitly or implicitly) in the instruction operands.


https://reviews.llvm.org/D89189

Files:
  llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
  llvm/test/CodeGen/ARM/machine-outliner-return-1.ll
  llvm/test/CodeGen/ARM/machine-outliner-return-2.ll

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