more notes

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25890 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/README.txt

@@ -313,3 +313,57 @@ bool %X(int %X) {
 
 We need to lower switch statements to tablejumps when appropriate instead of
 always into binary branch trees.
+
+//===---------------------------------------------------------------------===//
+
+SSE doesn't have [mem] op= reg instructions.  If we have an SSE instruction
+like this:
+
+  X += y
+
+and the register allocator decides to spill X, it is cheaper to emit this as:
+
+Y += [xslot]
+store Y -> [xslot]
+
+than as:
+
+tmp = [xslot]
+tmp += y
+store tmp -> [xslot]
+
+..and this uses one fewer register (so this should be done at load folding
+time, not at spiller time).  *Note* however that this can only be done
+if Y is dead.  Here's a testcase:
+
+%.str_3 = external global [15 x sbyte]          ; <[15 x sbyte]*> [#uses=0]
+implementation   ; Functions:
+declare void %printf(int, ...)
+void %main() {
+build_tree.exit:
+        br label %no_exit.i7
+no_exit.i7:             ; preds = %no_exit.i7, %build_tree.exit
+        %tmp.0.1.0.i9 = phi double [ 0.000000e+00, %build_tree.exit ], [ %tmp.34.i18, %no_exit.i7 ]      ; <double> [#uses=1]
+        %tmp.0.0.0.i10 = phi double [ 0.000000e+00, %build_tree.exit ], [ %tmp.28.i16, %no_exit.i7 ]     ; <double> [#uses=1]
+        %tmp.28.i16 = add double %tmp.0.0.0.i10, 0.000000e+00
+        %tmp.34.i18 = add double %tmp.0.1.0.i9, 0.000000e+00
+        br bool false, label %Compute_Tree.exit23, label %no_exit.i7
+Compute_Tree.exit23:            ; preds = %no_exit.i7
+        tail call void (int, ...)* %printf( int 0 )
+        store double %tmp.34.i18, double* null
+        ret void
+}
+
+We currently emit:
+
+.BBmain_1:
+        xorpd %XMM1, %XMM1
+        addsd %XMM0, %XMM1
+***     movsd %XMM2, QWORD PTR [%ESP + 8]
+***     addsd %XMM2, %XMM1
+***     movsd QWORD PTR [%ESP + 8], %XMM2
+        jmp .BBmain_1   # no_exit.i7
+
+This is a bugpoint reduced testcase, which is why the testcase doesn't make
+much sense (e.g. its an infinite loop). :)
+