[all-commits] [llvm/llvm-project] a02ef6: [llubi] Add support for undef values (#205602)

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: a02ef60ff46c1a9c99cce6ca159229dc2f6c5ff2
      https://github.com/llvm/llvm-project/commit/a02ef60ff46c1a9c99cce6ca159229dc2f6c5ff2
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2026-06-28 (Sun, 28 Jun 2026)

  Changed paths:
    M llvm/include/llvm/Support/Allocator.h
    A llvm/test/tools/llubi/undef.ll
    R llvm/test/tools/llubi/unsupported_constant.ll
    M llvm/tools/llubi/lib/Context.cpp
    M llvm/tools/llubi/lib/Context.h
    M llvm/tools/llubi/lib/Interpreter.cpp

  Log Message:
  -----------
  [llubi] Add support for undef values (#205602)

Although we are planning to deprecate the undef value, it is still
widely used in the intermediate results of the pipeline, which blocks
the pass bisection. This patch uses `freeze poison` as a refinement of
undef.

Note that the undef value evaluates to different values each time the
user is executed. So it cannot be cached like other constants. A
temporary buffer is introduced to take ownership of these values and
avoid breaking the interface (although this is a bit ugly...). This will
also be used by a follow-up patch for ptrtoint/inttoptr.

>From my experience, it is enough for test case reduction of middle-end
miscompilation bugs (there are still counterexamples like
https://github.com/dtcxzyw/llvm-autoreduce/issues/61). However, when
processing backend miscompilation bugs, lli typically uses a garbage
value, so that llvm-reduce may produce an invalid result. I think we may
need to introduce two flags to migrate this issue: one for poisoning
uninitialized memory, and another for diagnosing immediately when
non-deterministic behaviour occurs. This would prevent llvm-reduce from
converting a deterministic repro into a non-deterministic one.

`llvm/test/tools/llubi/unsupported_constant.ll` is also dropped because
it is annoying to pick another unimplemented constant value...



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