add a missed optimization

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25786 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index ac8f3f1e08f4a1a97bfb1450b267926b3b4b3f6b..0074eb4336933025ac563f386d7a921157228583 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -204,3 +204,13 @@ There are two ways to fix this:
    disambiguate the pointers.
 
 Number 1 is the preferred solution.
+
+//===---------------------------------------------------------------------===//
+
+The instruction selector sometimes misses folding a load into a compare.  The
+pattern is written as (cmp reg, (load p)).  Because the compare isn't 
+commutative, it is not matched with the load on both sides.  The dag combiner
+should be made smart enough to cannonicalize the load into the RHS of a compare
+when it can invert the result of the compare for free.
+
+