Put some of my random notes somewhere public

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23897 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/README.txt

@@ -0,0 +1,70 @@
+//===---------------------------------------------------------------------===//
+// Random ideas for the X86 backend.
+//===---------------------------------------------------------------------===//
+
+Add a MUL2U and MUL2S nodes to represent a multiply that returns both the
+Hi and Lo parts (combination of MUL and MULH[SU] into one node).  Add this to
+X86, & make the dag combiner produce it when needed.  This will eliminate one
+imul from the code generated for:
+
+long long test(long long X, long long Y) { return X*Y; }
+
+by using the EAX result from the mul.  We should add a similar node for
+DIVREM.
+
+//===---------------------------------------------------------------------===//
+
+This should be one DIV/IDIV instruction, not a libcall:
+
+unsigned test(unsigned long long X, unsigned Y) {
+        return X/Y;
+}
+
+This can be done trivially with a custom legalizer.  What about overflow 
+though?  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14224
+
+//===---------------------------------------------------------------------===//
+
+Need to add support for rotate instructions.
+
+//===---------------------------------------------------------------------===//
+
+Some targets (e.g. athlons) prefer freep to fstp ST(0):
+http://gcc.gnu.org/ml/gcc-patches/2004-04/msg00659.html
+
+//===---------------------------------------------------------------------===//
+
+This should use faddi on chips where it is profitable:
+double foo(double P, int *I) { return P+*I; }
+
+//===---------------------------------------------------------------------===//
+
+The FP stackifier needs to be global.  Also, it should handle simple permutates
+to reduce number of shuffle instructions, e.g. turning:
+
+fld P	->		fld Q
+fld Q			fld P
+fxch
+
+or:
+
+fxch	->		fucomi
+fucomi			jl X
+jg X
+
+//===---------------------------------------------------------------------===//
+
+Improvements to the multiply -> shift/add algorithm:
+http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html
+
+//===---------------------------------------------------------------------===//
+
+Improve code like this (occurs fairly frequently, e.g. in LLVM):
+long long foo(int x) { return 1LL << x; }
+
+http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01109.html
+http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01128.html
+http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01136.html
+
+Another useful one would be  ~0ULL >> X and ~0ULL << X.
+