MemorySSA: Revert r269678 and r268068; replace with special casing in MemorySSA.

It turns out that too many passes are relying on alias analysis results
for control dependencies. Until we fix that by introducing a more accurate
modelling of control dependencies, special case assume in MemorySSA instead.

Also introduce tests to ensure we don't regress the FunctionAttrs or LICM
passes.

Differential Revision: http://reviews.llvm.org/D20658

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270823 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/BasicAliasAnalysis.cpp

@@ -580,20 +580,11 @@ FunctionModRefBehavior BasicAAResult::getModRefBehavior(ImmutableCallSite CS) {
 
 /// Returns the behavior when calling the given function. For use when the call
 /// site is not known.
-/// NOTE: Because of the special case handling of llvm.assume below, the result
-/// of this function may not match similar results derived from function
-/// attributes (e.g. "readnone").
 FunctionModRefBehavior BasicAAResult::getModRefBehavior(const Function *F) {
   // If the function declares it doesn't access memory, we can't do better.
   if (F->doesNotAccessMemory())
     return FMRB_DoesNotAccessMemory;
 
-  // While the assume intrinsic is marked as arbitrarily writing so that
-  // proper control dependencies will be maintained, it never aliases any
-  // actual memory locations.
-  if (F->getIntrinsicID() == Intrinsic::assume)
-    return FMRB_DoesNotAccessMemory;
-
   FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
 
   // If the function declares it only reads memory, go with that.

lib/Transforms/Utils/MemorySSA.cpp

@@ -354,6 +354,14 @@ MemorySSAWalker *MemorySSA::buildMemorySSA(AliasAnalysis *AA,
 
 /// \brief Helper function to create new memory accesses
 MemoryUseOrDef *MemorySSA::createNewAccess(Instruction *I) {
+  // The assume intrinsic has a control dependency which we model by claiming
+  // that it writes arbitrarily. Ignore that fake memory dependency here.
+  // FIXME: Replace this special casing with a more accurate modelling of
+  // assume's control dependency.
+  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
+    if (II->getIntrinsicID() == Intrinsic::assume)
+      return nullptr;
+
   // Find out what affect this instruction has on memory.
   ModRefInfo ModRef = AA->getModRefInfo(I);
   bool Def = bool(ModRef & MRI_Mod);

test/Transforms/FunctionAttrs/assume.ll

@@ -0,0 +1,4 @@
+; RUN: opt -S -o - -functionattrs %s | FileCheck %s
+
+; CHECK-NOT: readnone
+declare void @llvm.assume(i1)

test/Transforms/LICM/assume.ll

@@ -0,0 +1,34 @@
+; RUN: opt -licm -basicaa < %s -S | FileCheck %s
+
+define void @f(i1 %p) nounwind ssp {
+entry:
+  br label %for.body
+
+for.body:
+  br i1 undef, label %if.then, label %for.cond.backedge
+
+for.cond.backedge:
+  br i1 undef, label %for.end104, label %for.body
+
+if.then:
+  br i1 undef, label %if.then27, label %if.end.if.end.split_crit_edge.critedge
+
+if.then27:
+; CHECK: tail call void @llvm.assume
+  tail call void @llvm.assume(i1 %p)
+  br label %for.body61.us
+
+if.end.if.end.split_crit_edge.critedge:
+  br label %for.body61
+
+for.body61.us:
+  br i1 undef, label %for.cond.backedge, label %for.body61.us
+
+for.body61:
+  br i1 undef, label %for.cond.backedge, label %for.body61
+
+for.end104:
+  ret void
+}
+
+declare void @llvm.assume(i1)

test/Transforms/Util/MemorySSA/assume.ll

@@ -8,6 +8,7 @@ define i32 @foo(i32* %a, i32* %b, i1 %c) {
 ; CHECK: 1 = MemoryDef(liveOnEntry)
 ; CHECK-NEXT: store i32 4
   store i32 4, i32* %a, align 4
+; CHECK-NOT: MemoryDef
 ; CHECK: call void @llvm.assume
   call void @llvm.assume(i1 %c)
 ; CHECK: MemoryUse(1)