Potential spiller improvement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35228 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/README.txt

@@ -0,0 +1,27 @@
+Common register allocation / spilling problem:
+
+	mul lr, r4, lr
+	str lr, [sp, #+52]
+	ldr lr, [r1, #+32]
+	sxth r3, r3
+	ldr r4, [sp, #+52]
+	mla r4, r3, lr, r4
+
+can be:
+
+	mul lr, r4, lr
+        mov r4, lr
+	str lr, [sp, #+52]
+	ldr lr, [r1, #+32]
+	sxth r3, r3
+	mla r4, r3, lr, r4
+
+and then "merge" mul and mov:
+
+	mul r4, r4, lr
+	str lr, [sp, #+52]
+	ldr lr, [r1, #+32]
+	sxth r3, r3
+	mla r4, r3, lr, r4
+
+It also increase the likelyhood the store may become dead.