new note

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30286 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index 5e2ab199ad25b50b9dce89d5c64da6d66247461a..14ea97d6710669e63f6afe0a702ebbdac12c01e9 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -643,3 +643,35 @@ This saves a movzbl, and saves a truncate if it doesn't get coallesced right.
 This is a simple DAGCombine to propagate the zext through the and.
 
 //===---------------------------------------------------------------------===//
+
+GCC's ix86_expand_int_movcc function (in i386.c) has a ton of interesting
+simplifications for integer "x cmp y ? a : b".  For example, instead of:
+
+int G;
+void f(int X, int Y) {
+  G = X < 0 ? 14 : 13;
+}
+
+compiling to:
+
+_f:
+        movl $14, %eax
+        movl $13, %ecx
+        movl 4(%esp), %edx
+        testl %edx, %edx
+        cmovl %eax, %ecx
+        movl %ecx, _G
+        ret
+
+it could be:
+_f:
+        movl    4(%esp), %eax
+        sarl    $31, %eax
+        notl    %eax
+        addl    $14, %eax
+        movl    %eax, _G
+        ret
+
+etc.
+
+//===---------------------------------------------------------------------===//