[PATCH] D136201: [InstCombine] Handle PHI nodes when eliminating constant memcpy

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

In D136201#3897529 <https://reviews.llvm.org/D136201#3897529>, @gandhi21299 wrote:

> The minor variant compiles correctly actually, no transformation applies so the code stays the same.

I just tried this, and the memcpy does seem to get dropped:

  @g1 = constant [32 x i8] zeroinitializer
  
  define i32 @test1(i1 %c, ptr %ptr) {
  entry:
    %a = alloca [32 x i8]
    br i1 %c, label %if, label %join
  
  if:
    br label %join
  
  join:
    %phi = phi ptr [ %a, %if ], [ %ptr, %entry ]
    call void @llvm.memcpy.p0.p0.i64(ptr %phi, ptr @g1, i64 32, i1 false)
    %v = load i32, ptr %phi
    ret i32 %v
  }
   
  declare void @llvm.memcpy.p0.p0.i64(ptr, ptr, i64, i1)
  
  ; RUN: opt -S -instcombine < %s
  
  @g1 = constant [32 x i8] zeroinitializer
  
  define i32 @test1(i1 %c, ptr %ptr) {
  entry:
    br i1 %c, label %if, label %join
  
  if:                                               ; preds = %entry
    br label %join
  
  join:                                             ; preds = %if, %entry
    %phi = phi ptr [ @g1, %if ], [ %ptr, %entry ]
    %v = load i32, ptr %phi, align 4
    ret i32 %v
  }
  
  ; Function Attrs: argmemonly nocallback nofree nounwind willreturn
  declare void @llvm.memcpy.p0.p0.i64(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i64, i1 immarg) #0


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