[X86] Reduce Store Forward Block issues in HW - Recommit after fixing…

… Bug 36346

If a load follows a store and reloads data that the store has written to memory, Intel microarchitectures can in many cases forward the data directly from the store to the load, This "store forwarding" saves cycles by enabling the load to directly obtain the data instead of accessing the data from cache or memory.
A "store forward block" occurs in cases that a store cannot be forwarded to the load. The most typical case of store forward block on Intel Core microarchiticutre that a small store cannot be forwarded to a large load.
The estimated penalty for a store forward block is ~13 cycles.

This pass tries to recognize and handle cases where "store forward block" is created by the compiler when lowering memcpy calls to a sequence
of a load and a store.

The pass currently only handles cases where memcpy is lowered to XMM/YMM registers, it tries to break the memcpy into smaller copies.
breaking the memcpy should be possible since there is no atomicity guarantee for loads and stores to XMM/YMM.

Differential revision: https://reviews.llvm.org/D41330

Change-Id: Ib48836ccdf6005989f7d4466fa2035b7b04415d9
llvm-svn: 328973

llvm/lib/Target/X86/CMakeLists.txt

@@ -31,6 +31,7 @@ set(sources
   X86FastISel.cpp
   X86FixupBWInsts.cpp
   X86FixupLEAs.cpp
+  X86AvoidStoreForwardingBlocks.cpp
   X86FixupSetCC.cpp
   X86FloatingPoint.cpp
   X86FrameLowering.cpp

llvm/lib/Target/X86/X86.h

@@ -70,6 +70,9 @@ FunctionPass *createX86OptimizeLEAs();
 /// Return a pass that transforms setcc + movzx pairs into xor + setcc.
 FunctionPass *createX86FixupSetCC();
 
+/// Return a pass that avoids creating store forward block issues in the hardware.
+FunctionPass *createX86AvoidStoreForwardingBlocks();
+
 /// Return a pass that expands WinAlloca pseudo-instructions.
 FunctionPass *createX86WinAllocaExpander();