[PATCH] D73997: [BPF] disable ReduceLoadWidth during SelectionDag phase

[Yonghong Song via Phabricator via llvm-commits]

yonghong-song created this revision.
yonghong-song added reviewers: ast, anakryiko.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

The compiler may transform the following code
(e.g., the test case in this commit)

  ctx = ctx + reloc_offset
  ... (*(u32 *)ctx) & 0x8000 ... 

to

  ctx = ctx + reloc_offset
  ... (*(u8 *)(ctx + 1)) & 0x80 ... 

where reloc_offset will be replaced with a constant during
AsmPrinter phase.

The above transformed code will be rejected the kernel verifier
as it does not allow

  *(type *)((ctx + non_zero_offset1) + non_zero_offset2)

style access pattern.

It is hard at SelectionDag phase to identify whether a load
is related to context or not. Sometime, interprocedure analysis
may be needed. So let us simply prevent such optimization
from happening.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D73997

Files:
  llvm/lib/Target/BPF/BPFISelLowering.h
  llvm/test/CodeGen/BPF/CORE/no-narrow-load.ll

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