[all-commits] [llvm/llvm-project] 9a9848: [RISCVInsertVSETVLI] Remove an unsound optimization

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 9a9848f4b95895ad97d3dc117f0a94773dc1607f
      https://github.com/llvm/llvm-project/commit/9a9848f4b95895ad97d3dc117f0a94773dc1607f
  Author: Philip Reames <preames at rivosinc.com>
  Date:   2022-08-05 (Fri, 05 Aug 2022)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
    M llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
    M llvm/test/CodeGen/RISCV/rvv/vsetvli-insert.ll

  Log Message:
  -----------
  [RISCVInsertVSETVLI] Remove an unsound optimization

This fixes a bug reported privately by @craig.topper. Here's an example which illustrates the problem:

vsetivli a1, a0, e32, m1, ta, mu # both DefInfo and PrevInfo
vsetivli a2, a1, e32, m4, ta, mu

With the unsound result being:

vsetivli a1, a0, e32, m1, ta, mu
vsetivli a2, a0, e32, m4, ta, mu

Consider the case where this is running on a machine with VLEN=512,. For this case, the VLMAXs are 16 and 64 respectively.

Consider for a0 = 33. The correct result is: a1 = 16, and a2 = 16

After the unsound optimization: a1 = 16 and a2 = 33

This particular example used VLMAXs which differed by more than a power of two. With a difference of only one power of two, there's another form of this bug which involves the AVL < 2 x VLMAX special case, but that ones more complicated to construct as many examples turn out accidentally sound.

This patch takes the approach of simply removing the unsound optimization, but there are multiple sound sub-cases of it. I plan to return to at least a couple of them, but figured it was cleaner to remove the unsound optimization (for ease of backporting), and then review the new optimizations on their own.

Differential Revision: https://reviews.llvm.org/D131264