[all-commits] [llvm/llvm-project] 6564e0: BPF: Fix a bug in peephole TRUNC elimination optim...

[yonghong-song via All-commits]

  Branch: refs/heads/release/12.x
  Home:   https://github.com/llvm/llvm-project
  Commit: 6564e0cf7e61518cb15443fca42bc2206a6123e2
      https://github.com/llvm/llvm-project/commit/6564e0cf7e61518cb15443fca42bc2206a6123e2
  Author: Yonghong Song <yhs at fb.com>
  Date:   2021-05-03 (Mon, 03 May 2021)

  Changed paths:
    M llvm/lib/Target/BPF/BPFMIPeephole.cpp
    A llvm/test/CodeGen/BPF/remove_truncate_8.ll

  Log Message:
  -----------
  BPF: Fix a bug in peephole TRUNC elimination optimization

Andrei Matei reported a llvm11 core dump for his bpf program
   https://bugs.llvm.org/show_bug.cgi?id=48578
The core dump happens in LiveVariables analysis phase.
  #4 0x00007fce54356bb0 __restore_rt
  #5 0x00007fce4d51785e llvm::LiveVariables::HandleVirtRegUse(unsigned int,
      llvm::MachineBasicBlock*, llvm::MachineInstr&)
  #6 0x00007fce4d519abe llvm::LiveVariables::runOnInstr(llvm::MachineInstr&,
      llvm::SmallVectorImpl<unsigned int>&)
  #7 0x00007fce4d519ec6 llvm::LiveVariables::runOnBlock(llvm::MachineBasicBlock*, unsigned int)
  #8 0x00007fce4d51a4bf llvm::LiveVariables::runOnMachineFunction(llvm::MachineFunction&)
The bug can be reproduced with llvm12 and latest trunk as well.

Futher analysis shows that there is a bug in BPF peephole
TRUNC elimination optimization, which tries to remove
unnecessary TRUNC operations (a <<= 32; a >>= 32).
Specifically, the compiler did wrong transformation for the
following patterns:
   %1 = LDW ...
   %2 = SLL_ri %1, 32
   %3 = SRL_ri %2, 32
   ... %3 ...
   %4 = SRA_ri %2, 32
   ... %4 ...

The current transformation did not check how many uses of %2
and did transformation like
   %1 = LDW ...
   ... %1 ...
   %4 = SRL_ri %2, 32
   ... %4 ...
and pseudo register %2 is used by not defined and
caused LiveVariables analysis core dump.

To fix the issue, when traversing back from SRL_ri to SLL_ri,
check to ensure SLL_ri has only one use. Otherwise, don't
do transformation.

Differential Revision: https://reviews.llvm.org/D97792

(cherry picked from commit 51cdb780db3b9b46c783efcec672c4da272e9992)