[all-commits] [llvm/llvm-project] 6e071a: [RISCV] Rewrite ADD to SUB to reuse a materialized...

[Pengcheng Wang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 6e071ab17caa0058a83cdd1580026259fdaecb4f
      https://github.com/llvm/llvm-project/commit/6e071ab17caa0058a83cdd1580026259fdaecb4f
  Author: Pengcheng Wang <wangpengcheng.pp at bytedance.com>
  Date:   2026-09-14 (Mon, 14 Sep 2026)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
    M llvm/test/CodeGen/RISCV/add-imm64-to-sub.ll

  Log Message:
  -----------
  [RISCV] Rewrite ADD to SUB to reuse a materialized negated constant (#221027)

When an ADD needs a large constant whose negation is already
present in the DAG, rewrite (add X, C) into (sub X, -C) so both
share one materialization of -C instead of building it twice.

selectNegImm already does a related rewrite, but only when -C is
strictly cheaper than C and during instruction selection, where the
sibling constant may already have been selected into instructions.

Do it in PreprocessISelDAG instead, before selection, while both are
still ConstantSDNodes that getConstant can unique together. The
rewrite only fires when -C is no more expensive than C and a node
holding -C already exists, so it never adds a materialization.

Fixes https://github.com/llvm/llvm-project/issues/220555

Assisted-by: TRAE CLI (Opus 4.8)



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