[PATCH] D147789: [CodeGenPrepare][RISCV] Reverse transform in CGP to use zero-compare branch

[Yingwei Zheng via Phabricator via llvm-commits]

dtcxzyw created this revision.
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This patch handles signed icmp cases of Add and Sub based on D101778 <https://reviews.llvm.org/D101778> if it guarantees no overflow after hoisting. For example:
https://alive2.llvm.org/ce/z/wbCO7s

  define i32 @src(i32 %0) {
    %x = icmp sge i32 %0, 0
    br i1 %x, label %nonnegative, label %5
  
  nonnegative:                                      ; preds = %1
    %2 = icmp slt i32 %0, 13
    br i1 %2, label %5, label %3
  
  3:                                                ; preds = %nonnegative
    %4 = add i32 %0, -13
    ret i32 %4
  
  5:                                                ; preds = %nonnegative, %1
    ret i32 0
  }
  
  define i32 @tgt(i32 %0) {
    %x = icmp sge i32 %0, 0
    br i1 %x, label %nonnegative, label %5
  
  nonnegative:                                      ; preds = %1
    %2 = add i32 %0, -13
    %3 = icmp slt i32 %2, 0
    br i1 %3, label %5, label %4
  
  4:                                                ; preds = %nonnegative
    ret i32 %2
  
  5:                                                ; preds = %nonnegative, %1
    ret i32 0
  }

This patch addresses performance regressions in RISC-V benchmarks reported by our CI <https://github.com/dtcxzyw/llvm-ci/issues/88>. I think this patch can protect against regressions from patch D147568 <https://reviews.llvm.org/D147568>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D147789

Files:
  llvm/lib/CodeGen/CodeGenPrepare.cpp
  llvm/test/CodeGen/RISCV/branch-on-zero.ll

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