make fast unaligned memory accesses implicit with SSE4.2 or SSE4a

This is a follow-on from the discussion in http://reviews.llvm.org/D12154.

This change allows memset/memcpy to use SSE or AVX memory accesses for any chip that has
generally fast unaligned memory ops.

A motivating use case for this change is a clang invocation that doesn't explicitly set
the CPU, but does target a feature that we know only exists on a CPU that supports fast
unaligned memops. For example:
$ clang -O1 foo.c -mavx

This resolves a difference in lowering noted in PR24449:
https://llvm.org/bugs/show_bug.cgi?id=24449

Before this patch, we used different store types depending on whether the example can be
lowered as a memset or not.

Differential Revision: http://reviews.llvm.org/D12288

llvm-svn: 245950

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86Subtarget.cpp b/llvm/lib/Target/X86/X86Subtarget.cpp
index 565ba1ded7e..b23b3c0e99a 100644
--- a/llvm/lib/Target/X86/X86Subtarget.cpp
+++ b/llvm/lib/Target/X86/X86Subtarget.cpp
@@ -192,6 +192,13 @@ void X86Subtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
   // Parse features string and set the CPU.
   ParseSubtargetFeatures(CPUName, FullFS);
 
+  // All CPUs that implement SSE4.2 or SSE4A support unaligned accesses of
+  // 16-bytes and under that are reasonably fast. These features were
+  // introduced with Intel's Nehalem/Silvermont and AMD's Family10h
+  // micro-architectures respectively.
+  if (hasSSE42() || hasSSE4A())
+    IsUAMemUnder32Slow = false;
+  
   InstrItins = getInstrItineraryForCPU(CPUName);
 
   // It's important to keep the MCSubtargetInfo feature bits in sync with