[PATCH] D80319: [LICM] Allow movement of non-aliasing calls that at most only write to memory

[Alina Sbirlea via Phabricator via llvm-commits]

asbirlea added a comment.

Let's take the examples below, same scenario.

If `@foo2e` writes 2 to str, and `@foo2f` reads, increments by 1 and writes back, and the loop runs 3 times.
Code below gives result 3. When `@foo2e` is hoisted, the result is 5.
At the very least you need to check `pointerInvalidatedByLoop` for all call args.

  declare void @foo2e(i8*) argmemonly writeonly nounwind
  declare void @foo2f(i8*) nounwind
  
  entry:
    %mem = call noalias i8* @malloc()
    %str = getelementptr inbounds [4 x i8], [4 x i8]* @str, i64 0, i64 0
    br label %loop
  loop:
    call void @foo2e(i8* %mem)
    call void @foo2f(i8* %mem)
    br i1 %c, label %loop, label %out
  out:
    ret void

For inaccessible memory, if two of the calls inside the loop write to that same inaccessible memory location, and one meets the condition to hoist, while the other also reads, then it's legal to hoist one of them?
`@foo2e` always writes 2 to that location.
`@foo2f` reads that location, increments by 1 and writes it back.
With an iteration count of 3, the original code will hold 3 at that location. After hoisting it will hold 5.

  declare void @foo2e() inaccessiblemem writeonly nounwind
  declare void @foo2f() nounwind
  
  entry:
    %mem = call noalias i8* @malloc()
    %str = getelementptr inbounds [4 x i8], [4 x i8]* @str, i64 0, i64 0
    br label %loop
  loop:
    call void @foo2e()
    call void @foo2f()
    br i1 %c, label %loop, label %out
  out:
    ret void


Repository:
  rG LLVM Github Monorepo

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