add a note

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122559 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 91 insertions, 0 deletions

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index d199a3712d..6b83d47bce 100644
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -96,6 +96,97 @@ It appears icc use push for parameter passing. Need to investigate.
 
 //===---------------------------------------------------------------------===//
 
+This:
+
+void foo(void);
+void bar(int x, int *P) { 
+  x >>= 2;
+  if (x) 
+    foo();
+  *P = x;
+}
+
+compiles into:
+
+	movq	%rsi, %rbx
+	movl	%edi, %r14d
+	sarl	$2, %r14d
+	testl	%r14d, %r14d
+	je	LBB0_2
+
+Instead of doing an explicit test, we can use the flags off the sar.  This
+occurs in a bigger testcase like this, which is pretty common:
+
+#include <vector>
+
+
+int test1(std::vector<int> &X) {
+  int Sum = 0;
+  for (long i = 0, e = X.size(); i != e; ++i)
+    X[i] = 0;
+  return Sum;
+}
+compiles into:
+
+	movq	%rsi, %rbx
+	movl	%edi, %r14d
+	sarl	$2, %r14d
+	testl	%r14d, %r14d
+	je	LBB0_2
+
+Instead of doing an explicit test, we can use the flags off the sar.  This
+occurs in a bigger testcase like this, which is pretty common:
+
+#include <vector>
+
+
+int test1(std::vector<int> &X) {
+  int Sum = 0;
+  for (long i = 0, e = X.size(); i != e; ++i)
+    X[i] = 0;
+  return Sum;
+}
+compiles into:
+
+	movq	%rsi, %rbx
+	movl	%edi, %r14d
+	sarl	$2, %r14d
+	testl	%r14d, %r14d
+	je	LBB0_2
+
+Instead of doing an explicit test, we can use the flags off the sar.  This
+occurs in a bigger testcase like this, which is pretty common:
+
+#include <vector>
+
+
+int test1(std::vector<int> &X) {
+  int Sum = 0;
+  for (long i = 0, e = X.size(); i != e; ++i)
+    X[i] = 0;
+  return Sum;
+}
+compiles into:
+
+	movq	%rsi, %rbx
+	movl	%edi, %r14d
+	sarl	$2, %r14d
+	testl	%r14d, %r14d
+	je	LBB0_2
+
+Instead of doing an explicit test, we can use the flags off the sar.  This
+occurs in a bigger testcase like this, which is pretty common in bootstrap:
+
+#include <vector>
+int test1(std::vector<int> &X) {
+  int Sum = 0;
+  for (long i = 0, e = X.size(); i != e; ++i)
+    X[i] = 0;
+  return Sum;
+}
+
+//===---------------------------------------------------------------------===//
+
 Only use inc/neg/not instructions on processors where they are faster than
 add/sub/xor.  They are slower on the P4 due to only updating some processor
 flags.