X86 memcpy: use REPMOVSB instead of REPMOVS{Q,D,W} for inline copies

when the subtarget has fast strings.

This has two advantages:
  - Speed is improved. For example, on Haswell thoughput improvements increase
    linearly with size from 256 to 512 bytes, after which they plateau:
    (e.g. 1% for 260 bytes, 25% for 400 bytes, 40% for 508 bytes).
  - Code is much smaller (no need to handle boundaries).

llvm-svn: 300957

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86Subtarget.cpp b/llvm/lib/Target/X86/X86Subtarget.cpp
index 92a68759195..0bd29a5a27d 100644
--- a/llvm/lib/Target/X86/X86Subtarget.cpp
+++ b/llvm/lib/Target/X86/X86Subtarget.cpp
@@ -303,6 +303,7 @@ void X86Subtarget::initializeEnvironment() {
   HasFastVectorFSQRT = false;
   HasFastLZCNT = false;
   HasFastSHLDRotate = false;
+  HasFastString = false;
   HasSlowDivide32 = false;
   HasSlowDivide64 = false;
   PadShortFunctions = false;