[X86] Add prefer-128-bit subtarget feature.

Summary:
Similar to the previous prefer-256-bit flag. We might want to
enable this by default some CPUs. This just starts the initial
work to implement and prove that it effects TTI's vector width.

Reviewers: RKSimon, echristo, spatel, atdt

Reviewed By: RKSimon

Subscribers: lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67311

llvm-svn: 371319

4 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86.td b/llvm/lib/Target/X86/X86.td
index da7ec0ab74d..96d24228253 100644
--- a/llvm/lib/Target/X86/X86.td
+++ b/llvm/lib/Target/X86/X86.td
@@ -378,6 +378,10 @@ def FeatureHasFastGather
     : SubtargetFeature<"fast-gather", "HasFastGather", "true",
                        "Indicates if gather is reasonably fast">;
 
+def FeaturePrefer128Bit
+    : SubtargetFeature<"prefer-128-bit", "Prefer128Bit", "true",
+                       "Prefer 128-bit AVX instructions">;
+
 def FeaturePrefer256Bit
     : SubtargetFeature<"prefer-256-bit", "Prefer256Bit", "true",
                        "Prefer 256-bit AVX instructions">;
diff --git a/llvm/lib/Target/X86/X86Subtarget.cpp b/llvm/lib/Target/X86/X86Subtarget.cpp
index c826fdf6964..933d3157522 100644
--- a/llvm/lib/Target/X86/X86Subtarget.cpp
+++ b/llvm/lib/Target/X86/X86Subtarget.cpp
@@ -307,6 +307,8 @@ void X86Subtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
   // Consume the vector width attribute or apply any target specific limit.
   if (PreferVectorWidthOverride)
     PreferVectorWidth = PreferVectorWidthOverride;
+  else if (Prefer128Bit)
+    PreferVectorWidth = 128;
   else if (Prefer256Bit)
     PreferVectorWidth = 256;
 }
diff --git a/llvm/lib/Target/X86/X86Subtarget.h b/llvm/lib/Target/X86/X86Subtarget.h
index 46e777da15f..ce926b40ac3 100644
--- a/llvm/lib/Target/X86/X86Subtarget.h
+++ b/llvm/lib/Target/X86/X86Subtarget.h
@@ -436,6 +436,9 @@ protected:
   // FIXME: this is a known good value for Yonah. How about others?
   unsigned MaxInlineSizeThreshold = 128;
 
+  /// Indicates target prefers 128 bit instructions.
+  bool Prefer128Bit = false;
+
   /// Indicates target prefers 256 bit instructions.
   bool Prefer256Bit = false;
 
diff --git a/llvm/lib/Target/X86/X86TargetTransformInfo.h b/llvm/lib/Target/X86/X86TargetTransformInfo.h
index 7545ef502bb..27d3b65c25b 100644
--- a/llvm/lib/Target/X86/X86TargetTransformInfo.h
+++ b/llvm/lib/Target/X86/X86TargetTransformInfo.h
@@ -83,6 +83,7 @@ class X86TTIImpl : public BasicTTIImplBase<X86TTIImpl> {
       X86::FeatureSlowUAMem32,
 
       // Based on whether user set the -mprefer-vector-width command line.
+      X86::FeaturePrefer128Bit,
       X86::FeaturePrefer256Bit,
 
       // CPU name enums. These just follow CPU string.