[all-commits] [llvm/llvm-project] 2f1e6e: [BPF] Report an warning if certain insn imm operan...

[yonghong-song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2f1e6eb6c3e731266052536c3f98cce3a71a316e
      https://github.com/llvm/llvm-project/commit/2f1e6eb6c3e731266052536c3f98cce3a71a316e
  Author: yonghong-song <yhs at fb.com>
  Date:   2025-06-13 (Fri, 13 Jun 2025)

  Changed paths:
    M llvm/lib/Target/BPF/MCTargetDesc/BPFMCCodeEmitter.cpp
    A llvm/test/CodeGen/BPF/warn-cmp.ll

  Log Message:
  -----------
  [BPF] Report an warning if certain insn imm operand cannot fit in 32bit (#142989)

Ihor Solodrai reported a case ([1]) where gcc reports an error but clang
ignores that error and proceeds to generate incorrect code. More
specifically, the problematic code looks like:
   if r1 == 0xcafefeeddeadbeef goto <label>

Here, 0xcafefeeddeadbeef needs to be encoded in a 32-bit imm field
of the insns and the 32-bit imm allows sign extenstion to 64-bit imm.
Obviously, 0xcafefeeddeadbeef cannot encode properly.

The compilation failed for gcc with the following error:
  Error: immediate out of range, shall fit in 32 bits

Given a 64-bit imm value, converting to the proper 32-bit imm value
must satisfy the following 64-bit patterns:
  00000000 00000000 00000000 00000000 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
  11111111 11111111 11111111 11111111 1xxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx

So if the top 32-bits is 0 or the top 33-bits is 0x1ffffffff, then the 64-bit imm
value can be truncated into proper 32-bit imm. Otherwise, a warning
message, the same as gcc, will be issued. If -Werror is enabled during
compilation, the warning will turn into an error.

[1] https://lore.kernel.org/bpf/70affb12-327b-4882-bd1d-afda8b8c6f56@linux.dev/



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