[Driver][CodeGen] Add -mprefer-vector-width driver option and attribute during CodeGen.

This adds a new command line option -mprefer-vector-width to specify a preferred vector width for the vectorizers. Valid values are 'none' and unsigned integers. The driver will check that it meets those constraints. Specific supported integers will be managed by the targets in the backend.

Clang will take the value and add it as a new function attribute during CodeGen.

This represents the alternate direction proposed by Sanjay in this RFC: http://lists.llvm.org/pipermail/llvm-dev/2017-November/118734.html

The syntax here matches gcc, though gcc treats it as an x86 specific command line argument. gcc only allows values of 128, 256, and 512. I'm not having clang check any values.

Differential Revision: https://reviews.llvm.org/D40230

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320419 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ClangCommandLineReference.rst

@@ -2120,6 +2120,10 @@ Omit frame pointer setup for leaf functions
 
 Use copy relocations support for PIE builds
 
+.. option:: -mprefer-vector-width=<arg>
+
+Specifies preferred vector width for auto-vectorization. Defaults to 'none' which allows target specific decisions.
+
 .. option:: -mqdsp6-compat
 
 Enable hexagon-qdsp6 backward compatibility

include/clang/Driver/Options.td

@@ -1954,6 +1954,8 @@ def mno_implicit_float : Flag<["-"], "mno-implicit-float">, Group<m_Group>,
 def mimplicit_float : Flag<["-"], "mimplicit-float">, Group<m_Group>;
 def mrecip : Flag<["-"], "mrecip">, Group<m_Group>;
 def mrecip_EQ : CommaJoined<["-"], "mrecip=">, Group<m_Group>, Flags<[CC1Option]>;
+def mprefer_vector_width_EQ : Joined<["-"], "mprefer-vector-width=">, Group<m_Group>, Flags<[CC1Option]>,
+  HelpText<"Specifies preferred vector width for auto-vectorization. Defaults to 'none' which allows target specific decisions.">;
 def mpie_copy_relocations : Flag<["-"], "mpie-copy-relocations">, Group<m_Group>,
   Flags<[CC1Option]>,
   HelpText<"Use copy relocations support for PIE builds">;

include/clang/Frontend/CodeGenOptions.h

@@ -253,6 +253,11 @@ public:
 
   std::vector<std::string> Reciprocals;
 
+  /// The preferred width for auto-vectorization transforms. This is intended to
+  /// override default transforms based on the width of the architected vector
+  /// registers.
+  std::string PreferVectorWidth;
+
 public:
   // Define accessors/mutators for code generation options of enumeration type.
 #define CODEGENOPT(Name, Bits, Default)

lib/CodeGen/CGCall.cpp

@@ -1744,6 +1744,11 @@ void CodeGenModule::ConstructDefaultFnAttrList(StringRef Name, bool HasOptnone,
       FuncAttrs.addAttribute("reciprocal-estimates",
                              llvm::join(Recips, ","));
 
+    if (!CodeGenOpts.PreferVectorWidth.empty() &&
+        CodeGenOpts.PreferVectorWidth != "none")
+      FuncAttrs.addAttribute("prefer-vector-width",
+                             CodeGenOpts.PreferVectorWidth);
+
     if (CodeGenOpts.StackRealignment)
       FuncAttrs.addAttribute("stackrealign");
     if (CodeGenOpts.Backchain)

lib/Driver/ToolChains/Clang.cpp

@@ -274,6 +274,27 @@ static void ParseMRecip(const Driver &D, const ArgList &Args,
   OutStrings.push_back(Args.MakeArgString(Out));
 }
 
+/// The -mprefer-vector-width option accepts either a positive integer
+/// or the string "none".
+static void ParseMPreferVectorWidth(const Driver &D, const ArgList &Args,
+                                    ArgStringList &CmdArgs) {
+  Arg *A = Args.getLastArg(options::OPT_mprefer_vector_width_EQ);
+  if (!A)
+    return;
+
+  StringRef Value = A->getValue();
+  if (Value == "none") {
+    CmdArgs.push_back("-mprefer-vector-width=none");
+  } else {
+    unsigned Width;
+    if (Value.getAsInteger(10, Width)) {
+      D.Diag(diag::err_drv_invalid_value) << A->getOption().getName() << Value;
+      return;
+    }
+    CmdArgs.push_back(Args.MakeArgString("-mprefer-vector-width=" + Value));
+  }
+}
+
 static void getWebAssemblyTargetFeatures(const ArgList &Args,
                                          std::vector<StringRef> &Features) {
   handleTargetFeaturesGroup(Args, Features, options::OPT_m_wasm_Features_Group);
@@ -4333,6 +4354,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                    options::OPT_fno_slp_vectorize, EnableSLPVec))
     CmdArgs.push_back("-vectorize-slp");
 
+  ParseMPreferVectorWidth(D, Args, CmdArgs);
+
   if (Arg *A = Args.getLastArg(options::OPT_fshow_overloads_EQ))
     A->render(Args, CmdArgs);
 

lib/Frontend/CompilerInvocation.cpp

@@ -713,6 +713,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.VectorizeLoop = Args.hasArg(OPT_vectorize_loops);
   Opts.VectorizeSLP = Args.hasArg(OPT_vectorize_slp);
 
+  Opts.PreferVectorWidth = Args.getLastArgValue(OPT_mprefer_vector_width_EQ);
+
   Opts.MainFileName = Args.getLastArgValue(OPT_main_file_name);
   Opts.VerifyModule = !Args.hasArg(OPT_disable_llvm_verifier);
 

test/CodeGen/attr-mprefer-vector-width.c

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -mprefer-vector-width=128 -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECK128
+// RUN: %clang_cc1 -mprefer-vector-width=256 -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECK256
+// RUN: %clang_cc1 -mprefer-vector-width=none -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECKNONE
+
+int baz(int a) { return 4; }
+
+// CHECK128: baz{{.*}} #0
+// CHECK128: #0 = {{.*}}"prefer-vector-width"="128"
+
+// CHECK256: baz{{.*}} #0
+// CHECK256: #0 = {{.*}}"prefer-vector-width"="256"
+
+// CHECKNONE: baz{{.*}} #0
+// CHECKNONE-NOT: #0 = {{.*}}"prefer-vector-width"="none"

test/Driver/mprefer-vector-width.c

@@ -0,0 +1,24 @@
+////
+//// Verify that valid options for the -mprefer-vector-width flag are passed through and invalid options cause an error.
+////
+
+//// If there are no options, convert to 'all'.
+
+// RUN: %clang -### -S %s -mprefer-vector-width=none  2>&1 | FileCheck --check-prefix=WIDTHNONE %s
+// WIDTHNONE: "-mprefer-vector-width=none"
+
+//// Check options that cover all types.
+
+// RUN: %clang -### -S %s -mprefer-vector-width=128  2>&1 | FileCheck --check-prefix=WIDTH128 %s
+// WIDTH128: "-mprefer-vector-width=128"
+
+//// Check invalid parameters.
+
+// RUN: %clang -### -S %s -mprefer-vector-width=one  2>&1 | FileCheck --check-prefix=WIDTHONE %s
+// WIDTHONE: invalid value 'one' in 'mprefer-vector-width='
+
+// RUN: %clang -### -S %s -mprefer-vector-width=128.5  2>&1 | FileCheck --check-prefix=WIDTH128p5 %s
+// WIDTH128p5: invalid value '128.5' in 'mprefer-vector-width='
+
+// RUN: %clang -### -S %s -mprefer-vector-width=-128  2>&1 | FileCheck --check-prefix=WIDTHNEG128 %s
+// WIDTHNEG128: invalid value '-128' in 'mprefer-vector-width='