Potential spiller improvement.

llvm-svn: 35228

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/README.txt b/llvm/lib/CodeGen/README.txt
new file mode 100644
index 00000000000..3f295126fb3
--- /dev/null
+++ b/llvm/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.