--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/Target/SystemZ/README.txt	Thu Dec 12 13:56:28 2013 +0900
@@ -0,0 +1,181 @@
+//===---------------------------------------------------------------------===//
+// Random notes about and ideas for the SystemZ backend.
+//===---------------------------------------------------------------------===//
+
+The initial backend is deliberately restricted to z10.  We should add support
+for later architectures at some point.
+
+--
+
+SystemZDAGToDAGISel::SelectInlineAsmMemoryOperand() is passed "m" for all
+inline asm memory constraints; it doesn't get to see the original constraint.
+This means that it must conservatively treat all inline asm constraints
+as the most restricted type, "R".
+
+--
+
+If an inline asm ties an i32 "r" result to an i64 input, the input
+will be treated as an i32, leaving the upper bits uninitialised.
+For example:
+
+define void @f4(i32 *%dst) {
+  %val = call i32 asm "blah $0", "=r,0" (i64 103)
+  store i32 %val, i32 *%dst
+  ret void
+}
+
+from CodeGen/SystemZ/asm-09.ll will use LHI rather than LGHI.
+to load 103.  This seems to be a general target-independent problem.
+
+--
+
+The tuning of the choice between LOAD ADDRESS (LA) and addition in
+SystemZISelDAGToDAG.cpp is suspect.  It should be tweaked based on
+performance measurements.
+
+--
+
+There is no scheduling support.
+
+--
+
+We don't use the BRANCH ON INDEX instructions.
+
+--
+
+We might want to use BRANCH ON CONDITION for conditional indirect calls
+and conditional returns.
+
+--
+
+We don't use the TEST DATA CLASS instructions.
+
+--
+
+We could use the generic floating-point forms of LOAD COMPLEMENT,
+LOAD NEGATIVE and LOAD POSITIVE in cases where we don't need the
+condition codes.  For example, we could use LCDFR instead of LCDBR.
+
+--
+
+We only use MVC, XC and CLC for constant-length block operations.
+We could extend them to variable-length operations too,
+using EXECUTE RELATIVE LONG.
+
+MVCIN, MVCLE and CLCLE may be worthwhile too.
+
+--
+
+We don't use CUSE or the TRANSLATE family of instructions for string
+operations.  The TRANSLATE ones are probably more difficult to exploit.
+
+--
+
+We don't take full advantage of builtins like fabsl because the calling
+conventions require f128s to be returned by invisible reference.
+
+--
+
+ADD LOGICAL WITH SIGNED IMMEDIATE could be useful when we need to
+produce a carry.  SUBTRACT LOGICAL IMMEDIATE could be useful when we
+need to produce a borrow.  (Note that there are no memory forms of
+ADD LOGICAL WITH CARRY and SUBTRACT LOGICAL WITH BORROW, so the high
+part of 128-bit memory operations would probably need to be done
+via a register.)
+
+--
+
+We don't use the halfword forms of LOAD REVERSED and STORE REVERSED
+(LRVH and STRVH).
+
+--
+
+We don't use ICM or STCM.
+
+--
+
+DAGCombiner doesn't yet fold truncations of extended loads.  Functions like:
+
+    unsigned long f (unsigned long x, unsigned short *y)
+    {
+      return (x << 32) | *y;
+    }
+
+therefore end up as:
+
+        sllg    %r2, %r2, 32
+        llgh    %r0, 0(%r3)
+        lr      %r2, %r0
+        br      %r14
+
+but truncating the load would give:
+
+        sllg    %r2, %r2, 32
+        lh      %r2, 0(%r3)
+        br      %r14
+
+--
+
+Functions like:
+
+define i64 @f1(i64 %a) {
+  %and = and i64 %a, 1
+  ret i64 %and
+}
+
+ought to be implemented as:
+
+        lhi     %r0, 1
+        ngr     %r2, %r0
+        br      %r14
+
+but two-address optimisations reverse the order of the AND and force:
+
+        lhi     %r0, 1
+        ngr     %r0, %r2
+        lgr     %r2, %r0
+        br      %r14
+
+CodeGen/SystemZ/and-04.ll has several examples of this.
+
+--
+
+Out-of-range displacements are usually handled by loading the full
+address into a register.  In many cases it would be better to create
+an anchor point instead.  E.g. for:
+
+define void @f4a(i128 *%aptr, i64 %base) {
+  %addr = add i64 %base, 524288
+  %bptr = inttoptr i64 %addr to i128 *
+  %a = load volatile i128 *%aptr
+  %b = load i128 *%bptr
+  %add = add i128 %a, %b
+  store i128 %add, i128 *%aptr
+  ret void
+}
+
+(from CodeGen/SystemZ/int-add-08.ll) we load %base+524288 and %base+524296
+into separate registers, rather than using %base+524288 as a base for both.
+
+--
+
+Dynamic stack allocations round the size to 8 bytes and then allocate
+that rounded amount.  It would be simpler to subtract the unrounded
+size from the copy of the stack pointer and then align the result.
+See CodeGen/SystemZ/alloca-01.ll for an example.
+
+--
+
+Atomic loads and stores use the default compare-and-swap based implementation.
+This is much too conservative in practice, since the architecture guarantees
+that 1-, 2-, 4- and 8-byte loads and stores to aligned addresses are
+inherently atomic.
+
+--
+
+If needed, we can support 16-byte atomics using LPQ, STPQ and CSDG.
+
+--
+
+We might want to model all access registers and use them to spill
+32-bit values.