[PATCH] reset iterators when more than one dead code instruction is removed

[Eric Christopher]

Hrm. I guess this seems reasonable.

I'm guessing unlikely, but are there any problems compile time wise
with resetting the iterator to the beginning of the block or
performance wins out of this? I imagine almost everything here is
getting picked up by a later pass normally for deletion and that this
might actually speed things up by not conceivably making the iteration
of the list converge faster.

-eric

On Thu, Apr 24, 2014 at 5:22 PM, Gerolf Hoflehner <ghoflehner at apple.com> wrote:
> Hi
>
> a few places invoke dead instruction removal, but assume only one
> instruction gets removed. This patch
> resets the iterators when dead instructions have been removed.
>
> Gerolf
>
>
> Index: lib/Transforms/Scalar/LoopInstSimplify.cpp
> ===================================================================
> --- lib/Transforms/Scalar/LoopInstSimplify.cpp  (revision 206487)
> +++ lib/Transforms/Scalar/LoopInstSimplify.cpp  (working copy)
> @@ -126,7 +126,15 @@
>              ++NumSimplified;
>            }
>          }
> -        LocalChanged |= RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
> +        bool res = RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
> +        if (res) {
> +          // RecursivelyDeleteTriviallyDeadInstruction can remove
> +          // more than one instruction, so simply incrementing the
> +          // iterator does not work. When instructions get deleted
> +          // re-iterate instead.
> +          BI = BB->begin(); BE = BB->end();
> +          LocalChanged |= res;
> +        }
>
>
>
>          if (IsSubloopHeader && !isa<PHINode>(I))
>            break;
> Index: lib/Transforms/Utils/SimplifyInstructions.cpp
> ===================================================================
> --- lib/Transforms/Utils/SimplifyInstructions.cpp       (revision 206487)
> +++ lib/Transforms/Utils/SimplifyInstructions.cpp       (working copy)
> @@ -75,7 +75,15 @@
>                  ++NumSimplified;
>                  Changed = true;
>                }
> -            Changed |= RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
> +            bool res = RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
> +            if (res)  {
> +              // RecursivelyDeleteTriviallyDeadInstruction can remove
> +              // more than one instruction, so simply incrementing the
> +              // iterator does not work. When instructions get deleted
> +              // re-iterate instead.
> +              BI = BB->begin(); BE = BB->end();
> +              Changed |= res;
> +            }
>            }
>
>
>
>          // Place the list of instructions to simplify on the next loop
> iteration
> Index: test/Transforms/InstSimplify/dead-code-removal.ll
> ===================================================================
> --- test/Transforms/InstSimplify/dead-code-removal.ll   (revision 0)
> +++ test/Transforms/InstSimplify/dead-code-removal.ll   (working copy)
> @@ -0,0 +1,15 @@
> +; RUN: opts -instsimplify -S < %s | FileCheck %s
> +
> +define void @foo() nounwind {
> +  br i1 undef, label %1, label %4
> +
> +; <label>:1                                       ; preds = %1, %0
> +; CHECK-NOT: phi
> +; CHECK-NOT: sub
> +  %2 = phi i32 [ %3, %1 ], [ undef, %0 ]
> +  %3 = sub i32 0, undef
> +  br label %1
> +
> +; <label>:4                                       ; preds = %0
> +  ret void
> +}
>
>
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