Subzero: Start a list of SIMD improvement ideas.

src/README.SIMD.rst

+Missing support
+===============
+
+* The PNaCl LLVM backend expands shufflevector operations into
+  sequences of insertelement and extractelement operations. For
+  instance:
+
+    define <4 x i32> @shuffle(<4 x i32> %arg1, <4 x i32> %arg2) {
+    entry:
+      %res = shufflevector <4 x i32> %arg1, <4 x i32> %arg2, <4 x i32> <i32 4, i32 5, i32 0, i32 1>
+      ret <4 x i32> %res
+    }
+
+  gets expanded into:
+
+    define <4 x i32> @shuffle(<4 x i32> %arg1, <4 x i32> %arg2) {
+    entry:
+      %0 = extractelement <4 x i32> %arg2, i32 0
+      %1 = insertelement <4 x i32> undef, i32 %0, i32 0
+      %2 = extractelement <4 x i32> %arg2, i32 1
+      %3 = insertelement <4 x i32> %1, i32 %2, i32 1
+      %4 = extractelement <4 x i32> %arg1, i32 0
+      %5 = insertelement <4 x i32> %3, i32 %4, i32 2
+      %6 = extractelement <4 x i32> %arg1, i32 1
+      %7 = insertelement <4 x i32> %5, i32 %6, i32 3
+      ret <4 x i32> %7
+    }
+
+  Subzero should recognize these sequences and recombine them into
+  shuffle operations where appropriate.
+
+* Add support for vector constants in the backend. The current code
+  materializes the vector constants it needs (eg. for performing icmp
+  on unsigned operands) using register operations, but this should be
+  changed to loading them from a constant pool if the register
+  initialization is too complicated (such as in
+  TargetX8632::makeVectorOfHighOrderBits()).
+
+* [x86 specific] llvm-mc does not allow lea to take a mem128 memory
+  operand when assembling x86-32 code. The current
+  InstX8632Lea::emit() code uses Variable::asType() to convert any
+  mem128 Variables into a compatible memory operand type. However, the
+  emit code does not do any conversions of OperandX8632Mem, so if an
+  OperandX8632Mem is passed to lea as mem128 the resulting code will
+  not assemble.  One way to fix this is by implementing
+  OperandX8632Mem::asType().
+
+* [x86 specific] Lower shl with <4 x i32> using some clever float
+  conversion:
+http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20100726/105087.html
+
+* [x86 specific] Add support for using aligned mov operations
+  (movaps). This will require passing alignment information to loads
+  and stores.
+
+x86 SIMD Diversification
+========================
+
+* Vector "bitwise" operations have several variant instructions: the
+  AND operation can be implemented with pand, andpd, or andps. This
+  pattern also holds for ANDN, OR, and XOR.
+
+* Vector "mov" instructions can be diversified (eg. movdqu instead of
+  movups) at the cost of a possible performance penalty.
+
+* Scalar FP arithmetic can be diversified by performing the operations
+  with the vector version of the instructions.