PNaCl: Add NoAlias attributes in ExpandByVal and ExpandVarArgs passes

This could help prevent these expansion passes from inhibiting
optimisations than run after the expansion.  e.g. It gives the
optimiser more freedom to move around reads from the varargs buffer
because they will not alias writes to other locations.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3400
TEST=PNaCl toolchain trybots + GCC torture tests + LLVM test suite + Spec2k

Review URL: https://codereview.chromium.org/14060026

Author: Mark Seaborn

lib/Transforms/NaCl/ExpandByVal.cpp

@@ -82,6 +82,14 @@ AttributeSet RemoveAttrs(LLVMContext &Context, AttributeSet Attrs) {
           Attr->getKindAsEnum() != Attribute::StructRet) {
         AB.addAttribute(*Attr);
       }
+      // IR semantics require that ByVal implies NoAlias.  However, IR
+      // semantics do not require StructRet to imply NoAlias.  For
+      // example, a global variable address can be passed as a
+      // StructRet argument, although Clang does not do so and Clang
+      // explicitly adds NoAlias to StructRet arguments.
+      if (Attr->getKindAsEnum() == Attribute::ByVal) {
+        AB.addAttribute(Attribute::get(Context, Attribute::NoAlias));
+      }
     }
     AttrList.push_back(AttributeSet::get(Context, Index, AB));
   }

lib/Transforms/NaCl/ExpandVarArgs.cpp

@@ -87,6 +87,11 @@ static void ExpandVarArgFunc(Function *Func) {
   NewFunc->getBasicBlockList().splice(NewFunc->begin(),
                                       Func->getBasicBlockList());
 
+  // Declare the new argument as "noalias".
+  NewFunc->setAttributes(
+      Func->getAttributes().addAttribute(
+          Func->getContext(), FTy->getNumParams() + 1, Attribute::NoAlias));
+
   // Move the arguments across to the new function.
   for (Function::arg_iterator Arg = Func->arg_begin(), E = Func->arg_end(),
          NewArg = NewFunc->arg_begin();

test/Transforms/NaCl/expand-byval.ll

@@ -15,12 +15,12 @@ define void @byval_receiver(%MyStruct* byval align 32 %ptr) {
   ret void
 }
 ; Strip the "byval" and "align" attributes.
-; CHECK: define void @byval_receiver(%MyStruct* %ptr) {
+; CHECK: define void @byval_receiver(%MyStruct* noalias %ptr) {
 ; CHECK-NEXT: call void @ext_func(%MyStruct* %ptr)
 
 
 declare void @ext_byval_func(%MyStruct* byval)
-; CHECK: declare void @ext_byval_func(%MyStruct*)
+; CHECK: declare void @ext_byval_func(%MyStruct* noalias)
 
 define void @byval_caller(%MyStruct* %ptr) {
   call void @ext_byval_func(%MyStruct* byval %ptr)
@@ -30,15 +30,15 @@ define void @byval_caller(%MyStruct* %ptr) {
 ; CHECK-NEXT: %ptr.byval_copy = alloca %MyStruct, align 4
 ; CHECK: call void @llvm.lifetime.start(i64 12, i8* %{{.*}})
 ; CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* %{{.*}}, i8* %{{.*}}, i64 12, i32 0, i1 false)
-; CHECK-NEXT: call void @ext_byval_func(%MyStruct* %ptr.byval_copy)
+; CHECK-NEXT: call void @ext_byval_func(%MyStruct* noalias %ptr.byval_copy)
 
 
 define void @byval_tail_caller(%MyStruct* %ptr) {
   tail call void @ext_byval_func(%MyStruct* byval %ptr)
   ret void
 }
 ; CHECK: define void @byval_tail_caller(%MyStruct* %ptr) {
-; CHECK: {{^}} call void @ext_byval_func(%MyStruct* %ptr.byval_copy)
+; CHECK: {{^}} call void @ext_byval_func(%MyStruct* noalias %ptr.byval_copy)
 
 
 define void @byval_invoke(%MyStruct* %ptr) {
@@ -53,7 +53,7 @@ lpad:
 ; CHECK: define void @byval_invoke(%MyStruct* %ptr) {
 ; CHECK: %ptr.byval_copy = alloca %MyStruct, align 32
 ; CHECK: call void @llvm.lifetime.start(i64 12, i8* %{{.*}})
-; CHECK: invoke void @ext_byval_func(%MyStruct* %ptr.byval_copy)
+; CHECK: invoke void @ext_byval_func(%MyStruct* noalias %ptr.byval_copy)
 ; CHECK: cont:
 ; CHECK: call void @llvm.lifetime.end(i64 12, i8* %{{.*}})
 ; CHECK: lpad:
@@ -64,7 +64,7 @@ lpad:
 
 ; Check that "align" is stripped for declarations too.
 declare void @ext_byval_func_align(%MyStruct* byval align 32)
-; CHECK: declare void @ext_byval_func_align(%MyStruct*)
+; CHECK: declare void @ext_byval_func_align(%MyStruct* noalias)
 
 define void @byval_caller_align_via_attr(%MyStruct* %ptr) {
   call void @ext_byval_func(%MyStruct* byval align 32 %ptr)

test/Transforms/NaCl/expand-varargs-attrs.ll

@@ -10,7 +10,7 @@ declare i32 @varargs_func(i32 %arg, ...)
 define void @func_with_arg_attrs(%MyStruct* byval, ...) {
   ret void
 }
-; CHECK: define void @func_with_arg_attrs(%MyStruct* byval, i8* %varargs) {
+; CHECK: define void @func_with_arg_attrs(%MyStruct* byval, i8* noalias %varargs) {
 
 
 declare void @take_struct_arg(%MyStruct* byval %s, ...)

test/Transforms/NaCl/expand-varargs.ll

@@ -17,7 +17,7 @@ define i32 @varargs_func(i32 %arg, ...) {
   call void @llvm.va_end(i8* %arglist)
   ret i32 %result
 }
-; CHECK: define i32 @varargs_func(i32 %arg, i8* %varargs) {
+; CHECK: define i32 @varargs_func(i32 %arg, i8* noalias %varargs) {
 ; CHECK-NEXT: %arglist_alloc = alloca i8*
 ; CHECK-NEXT: %arglist = bitcast i8** %arglist_alloc to i8*
 ; CHECK-NEXT: %arglist1 = bitcast i8* %arglist to i8**