CodeGen, Driver: Introduce -fpreferred-function-alignment option.

clang/include/clang/Basic/LangOptions.def

@@ -419,6 +419,7 @@ ENUM_LANGOPT(ClangABICompat, ClangABI, 4, ClangABI::Latest, NotCompatible,
              "with")
 
 VALUE_LANGOPT(FunctionAlignment, 5, 0, Compatible, "Default alignment for functions")
+VALUE_LANGOPT(PreferredFunctionAlignment, 5, 0, Compatible, "Preferred alignment for functions")
 VALUE_LANGOPT(LoopAlignment, 32, 0, Compatible, "Default alignment for loops")
 
 LANGOPT(FixedPoint, 1, 0, NotCompatible, "fixed point types")

clang/include/clang/Driver/Options.td

@@ -1551,6 +1551,8 @@ defm access_control : BoolFOption<"access-control",
   PosFlag<SetTrue>>;
 def falign_functions : Flag<["-"], "falign-functions">, Group<f_Group>;
 def falign_functions_EQ : Joined<["-"], "falign-functions=">, Group<f_Group>;
+def fpreferred_function_alignment_EQ :
+  Joined<["-"], "fpreferred-function-alignment=">, Group<f_Group>;
 def falign_loops_EQ : Joined<["-"], "falign-loops=">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option]>, MetaVarName<"<N>">,
   HelpText<"N must be a power of two. Align loops to the boundary">,
@@ -8505,6 +8507,9 @@ def fencode_extended_block_signature : Flag<["-"], "fencode-extended-block-signa
 def function_alignment : Separate<["-"], "function-alignment">,
     HelpText<"default alignment for functions">,
     MarshallingInfoInt<LangOpts<"FunctionAlignment">>;
+def preferred_function_alignment : Separate<["-"], "preferred-function-alignment">,
+    HelpText<"preferred alignment for functions">,
+    MarshallingInfoInt<LangOpts<"PreferredFunctionAlignment">>;
 def fhalf_no_semantic_interposition : Flag<["-"], "fhalf-no-semantic-interposition">,
   HelpText<"Like -fno-semantic-interposition but don't use local aliases">,
   MarshallingInfoFlag<LangOpts<"HalfNoSemanticInterposition">>;

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2822,13 +2822,19 @@ void CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
 
   F->addFnAttrs(B);
 
-  unsigned alignment = D->getMaxAlignment() / Context.getCharWidth();
-  if (alignment)
-    F->setAlignment(llvm::Align(alignment));
-
-  if (!D->hasAttr<AlignedAttr>())
-    if (LangOpts.FunctionAlignment)
-      F->setAlignment(llvm::Align(1ull << LangOpts.FunctionAlignment));
+  llvm::MaybeAlign ExplicitAlignment;
+  if (unsigned alignment = D->getMaxAlignment() / Context.getCharWidth())
+    ExplicitAlignment = llvm::Align(alignment);
+  else if (LangOpts.FunctionAlignment)
+    ExplicitAlignment = llvm::Align(1ull << LangOpts.FunctionAlignment);
+
+  if (ExplicitAlignment) {
+    F->setAlignment(ExplicitAlignment);
+    F->setPreferredAlignment(ExplicitAlignment);
+  } else if (LangOpts.PreferredFunctionAlignment) {
+    F->setPreferredAlignment(
+        llvm::Align(1ull << LangOpts.PreferredFunctionAlignment));
+  }
 
   // Some C++ ABIs require 2-byte alignment for member functions, in order to
   // reserve a bit for differentiating between virtual and non-virtual member

clang/lib/Driver/ToolChains/Clang.cpp

@@ -47,6 +47,7 @@
 #include "llvm/Support/Compression.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/MathExtras.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/YAMLParser.h"
@@ -5468,6 +5469,18 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back(Args.MakeArgString(std::to_string(FunctionAlignment)));
   }
 
+  if (const Arg *A = Args.getLastArg(options::OPT_fpreferred_function_alignment_EQ)) {
+    unsigned Value = 0;
+    if (StringRef(A->getValue()).getAsInteger(10, Value) || Value > 65536 ||
+        !llvm::isPowerOf2_32(Value))
+      TC.getDriver().Diag(diag::err_drv_invalid_int_value)
+          << A->getAsString(Args) << A->getValue();
+
+    CmdArgs.push_back("-preferred-function-alignment");
+    CmdArgs.push_back(Args.MakeArgString(
+        std::to_string(llvm::Log2_32_Ceil(std::min(Value, 65536u)))));
+  }
+
   // We support -falign-loops=N where N is a power of 2. GCC supports more
   // forms.
   if (const Arg *A = Args.getLastArg(options::OPT_falign_loops_EQ)) {

clang/test/CodeGen/prefalign.c

@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -emit-llvm -triple x86_64-unknown-linux -preferred-function-alignment 4 %s -o - | FileCheck %s
+
+// CHECK: define {{.*}} void @f() {{.*}} prefalign 16
+void f() {}

clang/test/CodeGenCXX/member-alignment.cpp

@@ -31,9 +31,9 @@ class t {
 [[gnu::aligned(16)]]
 void
 t::baz(void) {
-// CHECK-NOEXTRAALIGN: @_ZN1t3bazEv({{.*}}) #0 align 16 {
-// CHECK-EXTRAALIGN: @_ZN1t3bazEv({{.*}}) #0 align 16 {
-// CHECK-MSVC: @"?baz@t@@QEAAXXZ"({{.*}}) #0 align 16 {
+// CHECK-NOEXTRAALIGN: @_ZN1t3bazEv({{.*}}) #0 align 16 prefalign 16 {
+// CHECK-EXTRAALIGN: @_ZN1t3bazEv({{.*}}) #0 align 16 prefalign 16 {
+// CHECK-MSVC: @"?baz@t@@QEAAXXZ"({{.*}}) #0 align 16 prefalign 16 {
 }
 
 void

clang/test/Driver/prefalign.c

@@ -0,0 +1,5 @@
+// RUN: %clang -### -fpreferred-function-alignment=16 %s 2>&1 | FileCheck %s -check-prefix CHECK-16
+// RUN: not %clang -### -fpreferred-function-alignment=3 %s 2>&1 | FileCheck %s -check-prefix CHECK-INVALID
+
+// CHECK-16: "-preferred-function-alignment" "4"
+// CHECK-INVALID: invalid integral value '3' in '-fpreferred-function-alignment=3'