[llvm.noalias] add llvm.noalias to pointer use visitor.

[amehsan]

Summary

This lets SROA (and pontentially other users of PtrUseVisitor) see through llvm.noalias intrinsic. Without this, SROA does misses some optimization opportunities because the pointers escapes through the intrinsic

include/llvm/Analysis/PtrUseVisitor.h

protected:
   void visitIntrinsicInst(IntrinsicInst &II) {
     switch (II.getIntrinsicID()) {
     default:
       return Base::visitIntrinsicInst(II);
 
     case Intrinsic::lifetime_start:
     case Intrinsic::lifetime_end:
       return; // No-op intrinsics.
+    case Intrinsic::noalias:
+      enqueueUsers(II);
+      return;
     }
   }
 
   // Generically, arguments to calls and invokes escape the pointer to some
   // other function. Mark that.
   void visitCallSite(CallSite CS) {
     PI.setEscaped(CS.getInstruction());
     Base::visitCallSite(CS);
   }
 };
 
 }
 
 #endif

test/Transforms/SROA/noalias.ll

+; RUN: opt < %s -sroa -S | FileCheck %s
+define void @func1(i32 %a, i32* nocapture %c) local_unnamed_addr #0 {
+entry:
+  %a.addr = alloca i32, align 4
+  store i32 %a, i32* %a.addr, align 4, !tbaa !1
+  %0 = call i32* @llvm.noalias.p0i32(i32* %a.addr, metadata !5)
+  %1 = call i32* @llvm.noalias.p0i32(i32* %c, metadata !8)
+  %2 = load i32, i32* %0, align 4, !tbaa !1, !noalias !10
+  %sub.i = add nsw i32 %2, -1
+  store i32 %sub.i, i32* %1, align 4, !tbaa !1, !noalias !10
+  ret void
+; CHECK-LABEL: func1
+; CHECK-NOT: load
+}
+
+; Function Attrs: argmemonly nounwind
+declare i32* @llvm.noalias.p0i32(i32* returned, metadata) #1
+
+attributes #0 = { nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="hidspv100" "target-features"="+dsp-align,+support-FP,+support-VT" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { argmemonly nounwind }
+
+!llvm.ident = !{!0}
+
+!0 = !{!"clang version 4.0.1 ([e-mail]:llvm-dsp/clang.git 9668a848677344729757118551803787ec9b2ddb) ([e-mail]:llvm-dsp/llvm-4.0.0.git 7e5546a857c742dc551afc4d294df7f285befcef)"}
+!1 = !{!2, !2, i64 0}
+!2 = !{!"int", !3, i64 0}
+!3 = !{!"omnipotent char", !4, i64 0}
+!4 = !{!"Simple C/C++ TBAA"}
+!5 = !{!6}
+!6 = distinct !{!6, !7, !"foo: %a"}
+!7 = distinct !{!7, !"foo"}
+!8 = !{!9}
+!9 = distinct !{!9, !7, !"foo: %b"}
+!10 = !{!6, !9}