add a deadargelim note.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@90009 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 267f60a204728ff685a8dcd07f1ddadb1773802e..4029134f13961906a1967587e3d26a1bf5da7131 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -2,6 +2,29 @@ Target Independent Opportunities:
 
 //===---------------------------------------------------------------------===//
 
+Dead argument elimination should be enhanced to handle cases when an argument is
+dead to an externally visible function.  Though the argument can't be removed
+from the externally visible function, the caller doesn't need to pass it in.
+For example in this testcase:
+
+  void foo(int X) __attribute__((noinline));
+  void foo(int X) { sideeffect(); }
+  void bar(int A) { foo(A+1); }
+
+We compile bar to:
+
+define void @bar(i32 %A) nounwind ssp {
+  %0 = add nsw i32 %A, 1                          ; <i32> [#uses=1]
+  tail call void @foo(i32 %0) nounwind noinline ssp
+  ret void
+}
+
+The add is dead, we could pass in 'i32 undef' instead.  This occurs for C++
+templates etc, which usually have linkonce_odr/weak_odr linkage, not internal
+linkage.
+
+//===---------------------------------------------------------------------===//
+
 With the recent changes to make the implicit def/use set explicit in
 machineinstrs, we should change the target descriptions for 'call' instructions
 so that the .td files don't list all the call-clobbered registers as implicit