add some late optimizations that GCC does. It thinks these are a win

even on Core2, not just AMD processors which was a surprise to me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72396 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index 191288889755a19c0a173d7d76b91cd0b5a1f5ca..710bd0357433ed80749fa7198aedcdfb60ff8d05 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -1883,3 +1883,17 @@ On Nehalem, it may even be cheaper to just use movups when unaligned than to
 fall back to lower-granularity chunks.
 
 //===---------------------------------------------------------------------===//
+
+Implement processor-specific optimizations for parity with GCC on these
+processors.  GCC does two optimizations:
+
+1. ix86_pad_returns inserts a noop before ret instructions if immediately
+   preceeded by a conditional branch or is the target of a jump.
+2. ix86_avoid_jump_misspredicts inserts noops in cases where a 16-byte block of
+   code contains more than 3 branches.
+   
+The first one is done for all AMDs, Core2, and "Generic"
+The second one is done for: Atom, Pentium Pro, all AMDs, Pentium 4, Nocona,
+  Core 2, and "Generic"
+
+//===---------------------------------------------------------------------===//