Split FP-stack notes out of the main readme. Next up: splitting out SSE.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28399 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 99 insertions, 0 deletions

diff --git a/lib/Target/X86/README-FPStack.txt b/lib/Target/X86/README-FPStack.txt
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+//===---------------------------------------------------------------------===//
+// Random ideas for the X86 backend: FP stack related stuff
+//===---------------------------------------------------------------------===//
+
+//===---------------------------------------------------------------------===//
+
+Some targets (e.g. athlons) prefer freep to fstp ST(0):
+http://gcc.gnu.org/ml/gcc-patches/2004-04/msg00659.html
+
+//===---------------------------------------------------------------------===//
+
+On darwin/x86, we should codegen:
+
+        ret double 0.000000e+00
+
+as fld0/ret, not as:
+
+        movl $0, 4(%esp)
+        movl $0, (%esp)
+        fldl (%esp)
+	...
+        ret
+
+//===---------------------------------------------------------------------===//
+
+This should use fiadd on chips where it is profitable:
+double foo(double P, int *I) { return P+*I; }
+
+We have fiadd patterns now but the followings have the same cost and
+complexity. We need a way to specify the later is more profitable.
+
+def FpADD32m  : FpI<(ops RFP:$dst, RFP:$src1, f32mem:$src2), OneArgFPRW,
+                    [(set RFP:$dst, (fadd RFP:$src1,
+                                     (extloadf64f32 addr:$src2)))]>;
+                // ST(0) = ST(0) + [mem32]
+
+def FpIADD32m : FpI<(ops RFP:$dst, RFP:$src1, i32mem:$src2), OneArgFPRW,
+                    [(set RFP:$dst, (fadd RFP:$src1,
+                                     (X86fild addr:$src2, i32)))]>;
+                // ST(0) = ST(0) + [mem32int]
+
+//===---------------------------------------------------------------------===//
+
+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
+
+Ideas:
+http://gcc.gnu.org/ml/gcc-patches/2004-11/msg02410.html
+
+
+//===---------------------------------------------------------------------===//
+
+Add a target specific hook to DAG combiner to handle SINT_TO_FP and
+FP_TO_SINT when the source operand is already in memory.
+
+//===---------------------------------------------------------------------===//
+
+Open code rint,floor,ceil,trunc:
+http://gcc.gnu.org/ml/gcc-patches/2004-08/msg02006.html
+http://gcc.gnu.org/ml/gcc-patches/2004-08/msg02011.html
+
+Opencode the sincos[f] libcall.
+
+//===---------------------------------------------------------------------===//
+
+None of the FPStack instructions are handled in
+X86RegisterInfo::foldMemoryOperand, which prevents the spiller from
+folding spill code into the instructions.
+
+//===---------------------------------------------------------------------===//
+
+Currently the x86 codegen isn't very good at mixing SSE and FPStack
+code:
+
+unsigned int foo(double x) { return x; }
+
+foo:
+	subl $20, %esp
+	movsd 24(%esp), %xmm0
+	movsd %xmm0, 8(%esp)
+	fldl 8(%esp)
+	fisttpll (%esp)
+	movl (%esp), %eax
+	addl $20, %esp
+	ret
+
+This will be solved when we go to a dynamic programming based isel.
+
+//===---------------------------------------------------------------------===//