[PATCH] D37460: [GVN] Prevent LoadPRE from hoisting across instructions that don't pass control flow to successors

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev added inline comments.


================
Comment at: lib/Transforms/Scalar/GVN.cpp:2189
 
+  // Make sure that there are no instructions with implicit control flow that
+  // could prevent us from reaching our instruction.
----------------
reames wrote:
> This is overly conservative for any instruction which can not ever fault.  For instance, PRE a load over a throwing call is perfectly sound.  Note that preserving the nsw/nuw flags is questionable in the same case, but that's a hard discussion.  :)
> 
> I'd prefer to see the scalarPRE part separate into it's own patch.  I know I'm saying the exact opposite of Eli here, but I think this needs more discussion and trying to combine them will delay the loadPRE patch unnecessarily.  
I disagree that PRE over a throwing call is invalid. Imagine the following situation:

  int arr[LEN];

  void foo(index) {
    ...
    call rangeCheck(index, LEN);
    %x = load %arr, %index
    print(%x);
  }

  void rangeCheck(int index, int len) {
    if (index < 0 || index >= len)
      throw exception;
  }

We cannot PRE across the throwing call here.


================
Comment at: lib/Transforms/Scalar/GVN.cpp:2399
+    // TODO: Currently, isGuaranteedToTransferExecutionToSuccessor returns false
+    // for volatime stores and loads because they can trap. The discussion on
+    // whether or not it is correct is still ongoing. We might want to get rid
----------------
typo: `volatile`


================
Comment at: lib/Transforms/Scalar/GVN.cpp:2406
+      if (!isGuaranteedToTransferExecutionToSuccessor(&I) &&
+          !isa<LoadInst>(&I) && !isa<StoreInst>(&I)) {
+        FirstImplicitControlFlowInsts[BB] = &I;
----------------
reames wrote:
> This is scarily permissive.  It (for instance) allows ordered loads and stores which (I think) is valid.  It'd be much safer to add a special matcher for volatile-only loads/stores here.
Will do.


https://reviews.llvm.org/D37460