target independent notes

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25929 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/README.txt

@@ -0,0 +1,67 @@
+Target Independent Opportunities:
+
+===-------------------------------------------------------------------------===
+
+FreeBench/mason contains code like this:
+
+static p_type m0u(p_type p) {
+  int m[]={0, 8, 1, 2, 16, 5, 13, 7, 14, 9, 3, 4, 11, 12, 15, 10, 17, 6};
+  p_type pu;
+  pu.a = m[p.a];
+  pu.b = m[p.b];
+  pu.c = m[p.c];
+  return pu;
+}
+
+We currently compile this into a memcpy from a static array into 'm', then
+a bunch of loads from m.  It would be better to avoid the memcpy and just do
+loads from the static array.
+
+===-------------------------------------------------------------------------===
+
+Get the C front-end to expand hypot(x,y) -> llvm.sqrt(x*x+y*y) when errno and
+precision don't matter (ffastmath).  Misc/mandel will like this. :)
+
+===-------------------------------------------------------------------------===
+
+For all targets, not just X86:
+When llvm.memcpy, llvm.memset, or llvm.memmove are lowered, they should be 
+optimized to a few store instructions if the source is constant and the length
+is smallish (< 8). This will greatly help some tests like Shootout/strcat.c
+and fldry.
+
+//===---------------------------------------------------------------------===//
+
+Solve this DAG isel folding deficiency:
+
+int X, Y;
+
+void fn1(void)
+{
+  X = X | (Y << 3);
+}
+
+compiles to
+
+fn1:
+	movl Y, %eax
+	shll $3, %eax
+	orl X, %eax
+	movl %eax, X
+	ret
+
+The problem is the store's chain operand is not the load X but rather
+a TokenFactor of the load X and load Y, which prevents the folding.
+
+There are two ways to fix this:
+
+1. The dag combiner can start using alias analysis to realize that y/x
+   don't alias, making the store to X not dependent on the load from Y.
+2. The generated isel could be made smarter in the case it can't
+   disambiguate the pointers.
+
+Number 1 is the preferred solution.
+
+//===---------------------------------------------------------------------===//
+
+