[PATCH] Allow call-slop optzn for destinations with a suitable dereferenceable attribute

[hfinkel at anl.gov]

LGTM, thanks!

================
Comment at: lib/Transforms/Scalar/MemCpyOptimizer.cpp:637
@@ -636,5 +636,3 @@
   } else if (Argument *A = dyn_cast<Argument>(cpyDest)) {
-    // If the destination is an sret parameter then only accesses that are
-    // outside of the returned struct type can trap.
-    if (!A->hasStructRetAttr())
-      return false;
+    if (A->getDereferenceableBytes() < srcSize) {
+      // If the destination is an sret parameter then only accesses that are
----------------
dotdash wrote:
> hfinkel wrote:
> > But if you do it this way you're now *requiring* the dereferenceable attribute, and we don't want to do that.
> > 
> > What you want to do is keep the existing logic for sret, and just add additional support for dereferenceable.
> How so? The new check just skips the sret check if we already know that we have enough deferenceable bytes.
> 
> If there is no dereferenceable attribute, getDereferenceableBytes() returns 0, which is < srcSize and so we do the sret check, and only if that fails, false is returned.
> 
> 
You're right; sorry about that.

http://reviews.llvm.org/D5832