A couple new README entries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127786 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index abd1515cf5d700044fb5ab7333702f6cf58f47f1..77a513de8a75e4c122dccbb39bfad393dd22a2a0 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -1947,3 +1947,39 @@ which is "perfect".
 
 //===---------------------------------------------------------------------===//
 
+For the branch in the following code:
+int a();
+int b(int x, int y) {
+  if (x & (1<<(y&7)))
+    return a();
+  return y;
+}
+
+We currently generate:
+       movb    %sil, %al
+       andb    $7, %al
+       movzbl  %al, %eax
+       btl     %eax, %edi
+       jae     .LBB0_2
+
+movl+andl would be shorter than the movb+andb+movzbl sequence.
+
+//===---------------------------------------------------------------------===//
+
+For the following:
+struct u1 {
+    float x, y;
+};
+float foo(struct u1 u) {
+    return u.x + u.y;
+}
+
+We currently generate:
+       movdqa  %xmm0, %xmm1
+       pshufd  $1, %xmm0, %xmm0        # xmm0 = xmm0[1,0,0,0]
+       addss   %xmm1, %xmm0
+       ret
+
+We could save an instruction here by commuting the addss.
+
+//===---------------------------------------------------------------------===//