move PR9408 here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131841 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index ffe3fa477b9eac6b7aeadbce171b6ed3d35b7ad8..0105ae4562a988851d267582c11a459fc17b6aa7 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -2305,4 +2305,44 @@ The two or/and's should be merged into one each.
 
 //===---------------------------------------------------------------------===//
 
+Machine level code hoisting can be useful in some cases.  For example, PR9408
+is about:
+
+typedef union {
+ void (*f1)(int);
+ void (*f2)(long);
+} funcs;
+
+void foo(funcs f, int which) {
+ int a = 5;
+ if (which) {
+   f.f1(a);
+ } else {
+   f.f2(a);
+ }
+}
+
+which we compile to:
+
+foo:                                    # @foo
+# BB#0:                                 # %entry
+       pushq   %rbp
+       movq    %rsp, %rbp
+       testl   %esi, %esi
+       movq    %rdi, %rax
+       je      .LBB0_2
+# BB#1:                                 # %if.then
+       movl    $5, %edi
+       callq   *%rax
+       popq    %rbp
+       ret
+.LBB0_2:                                # %if.else
+       movl    $5, %edi
+       callq   *%rax
+       popq    %rbp
+       ret
+
+Note that bb1 and bb2 are the same.  This doesn't happen at the IR level
+because one call is passing an i32 and the other is passing an i64.
 
+//===---------------------------------------------------------------------===//