add a optimization note

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22732 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/PowerPC/README.txt b/lib/Target/PowerPC/README.txt
index be6a571..057a574 100644 (file)
--- a/lib/Target/PowerPC/README.txt
+++ b/lib/Target/PowerPC/README.txt
@@ -11,6 +11,24 @@ TODO:
 * Support 'update' load/store instructions.  These are cracked on the G5, but
   are still a codesize win.
 
+* Add a custom legalizer for the GlobalAddress node, to move the funky darwin
+  stub stuff from the instruction selector to the legalizer (exposing low-level
+  operations to the dag for optzn.  For example, we want to codegen this:
+
+        int A = 0;
+        void B() { A++; }
+  as:
+        lis r9,ha16(_A)
+        lwz r2,lo16(_A)(r9)
+        addi r2,r2,1
+        stw r2,lo16(_A)(r9)
+  not:
+        lis r2, ha16(_A)
+        lwz r2, lo16(_A)(r2)
+        addi r2, r2, 1
+        lis r3, ha16(_A)
+        stw r2, lo16(_A)(r3)
+
 * should hint to the branch select pass that it doesn't need to print the
   second unconditional branch, so we don't end up with things like:
        b .LBBl42__2E_expand_function_8_674     ; loopentry.24