inalloca: Don't remove dead arguments in the presence of inalloca args

It disturbs the layout of the parameters in memory and registers,
leading to problems in the backend.

The plan for optimizing internal inalloca functions going forward is to
essentially SROA the argument memory and demote any captured arguments
(things that aren't trivially written by a load or store) to an indirect
pointer to a static alloca.

llvm-svn: 200717

llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp

@@ -531,6 +531,13 @@ DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
 // well as arguments to functions which have their "address taken".
 //
 void DAE::SurveyFunction(const Function &F) {
+  // Functions with inalloca parameters are expecting args in a particular
+  // register and memory layout.
+  if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
+    MarkLive(F);
+    return;
+  }
+
   unsigned RetCount = NumRetVals(&F);
   // Assume all return values are dead
   typedef SmallVector<Liveness, 5> RetVals;

llvm/test/Transforms/DeadArgElim/keepalive.ll

@@ -28,4 +28,20 @@ define void @caller() {
         ret void
 }
 
+; We can't remove 'this' here, as that would put argmem in ecx instead of
+; memory.
+define internal x86_thiscallcc i32 @unused_this(i32* %this, i32* inalloca %argmem) {
+	%v = load i32* %argmem
+	ret i32 %v
+}
+; CHECK-LABEL: define internal x86_thiscallcc i32 @unused_this(i32* %this, i32* inalloca %argmem)
+
+define i32 @caller2() {
+	%t = alloca i32
+	%m = alloca i32, inalloca
+	store i32 42, i32* %m
+	%v = call x86_thiscallcc i32 @unused_this(i32* %t, i32* inalloca %m)
+	ret i32 %v
+}
+
 ; CHECK: attributes #0 = { nounwind }