[PATCH] D85473: [Clang] Add option to allow marking pass-by-value args as noalias.

[Florian Hahn via Phabricator via cfe-commits]

fhahn updated this revision to Diff 283837.
fhahn added a comment.

Thanks for taking a look!

I adjusted the LangOpts description and added the flag to f_Group, as suggested.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D85473/new/

https://reviews.llvm.org/D85473

Files:
  clang/include/clang/Basic/LangOptions.def
  clang/include/clang/Driver/Options.td
  clang/lib/CodeGen/CGCall.cpp
  clang/lib/Frontend/CompilerInvocation.cpp
  clang/test/CodeGen/pass-by-value-noalias.c


Index: clang/test/CodeGen/pass-by-value-noalias.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/pass-by-value-noalias.c
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fpass-by-value-is-noalias -triple arm64-apple-iphoneos -emit-llvm -disable-llvm-optzns %s -o - 2>&1 | FileCheck --check-prefix=WITH_NOALIAS %s
+// RUN: %clang_cc1 -triple arm64-apple-iphoneos -emit-llvm -disable-llvm-optzns %s -o - 2>&1 | FileCheck --check-prefix=NO_NOALIAS %s
+
+// A struct large enough so it is not passed in registers on ARM64.
+struct Foo {
+  int a;
+  int b;
+  int c;
+  int d;
+  int e;
+  int f;
+};
+
+// WITH_NOALIAS: define void @take(%struct.Foo* noalias %arg)
+// NO_NOALIAS: define void @take(%struct.Foo* %arg)
+void take(struct Foo arg) {}
Index: clang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- clang/lib/Frontend/CompilerInvocation.cpp
+++ clang/lib/Frontend/CompilerInvocation.cpp
@@ -3419,6 +3419,7 @@
   Opts.PCHInstantiateTemplates = Args.hasArg(OPT_fpch_instantiate_templates);
 
   Opts.MatrixTypes = Args.hasArg(OPT_fenable_matrix);
+  Opts.PassByValueIsNoAlias = Args.hasArg(OPT_fpass_by_value_is_noalias);
 
   Opts.MaxTokens = getLastArgIntValue(Args, OPT_fmax_tokens_EQ, 0, Diags);
 
Index: clang/lib/CodeGen/CGCall.cpp
===================================================================
--- clang/lib/CodeGen/CGCall.cpp
+++ clang/lib/CodeGen/CGCall.cpp
@@ -2192,6 +2192,11 @@
       if (AI.getIndirectByVal())
         Attrs.addByValAttr(getTypes().ConvertTypeForMem(ParamType));
 
+      if (getLangOpts().PassByValueIsNoAlias)
+        // When calling the function, the pointer passed in will be the only
+        // reference to the underlying object. Mark it accordingly.
+        Attrs.addAttribute(llvm::Attribute::NoAlias);
+
       // TODO: We could add the byref attribute if not byval, but it would
       // require updating many testcases.
 
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -4289,6 +4289,9 @@
 def fcompatibility_qualified_id_block_param_type_checking : Flag<["-"], "fcompatibility-qualified-id-block-type-checking">,
   HelpText<"Allow using blocks with parameters of more specific type than "
            "the type system guarantees when a parameter is qualified id">;
+def fpass_by_value_is_noalias: Flag<["-"], "fpass-by-value-is-noalias">,
+  HelpText<"Allows assuming by-value parameters do not alias any other value."
+           "Note that this may not hold for C++.">, Group<f_Group>;
 
 // FIXME: Remove these entirely once functionality/tests have been excised.
 def fobjc_gc_only : Flag<["-"], "fobjc-gc-only">, Group<f_Group>,
Index: clang/include/clang/Basic/LangOptions.def
===================================================================
--- clang/include/clang/Basic/LangOptions.def
+++ clang/include/clang/Basic/LangOptions.def
@@ -368,6 +368,8 @@
 LANGOPT(RegisterStaticDestructors, 1, 1, "Register C++ static destructors")
 
 LANGOPT(MatrixTypes, 1, 0, "Enable or disable the builtin matrix type")
+LANGOPT(PassByValueIsNoAlias, 1, 0, "assumption that by-value parameters do "
+                                    "not alias any other values")
 
 COMPATIBLE_VALUE_LANGOPT(MaxTokens, 32, 0, "Max number of tokens per TU or 0")
 


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