From: Chris Lattner <sabre@nondot.org>
Date: Thu, 2 Feb 2006 19:16:34 +0000 (+0000)
Subject: add a note, I have no idea how important this is.
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=9acddcd07e37ff3c3baaf7654eb4838c8e8377fe;p=oota-llvm.git

add a note, I have no idea how important this is.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25907 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index 36f287bcf92..c8e359fced3 100644
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -350,3 +350,27 @@ much sense (e.g. its an infinite loop). :)
 None of the FPStack instructions are handled in
 X86RegisterInfo::foldMemoryOperand, which prevents the spiller from
 folding spill code into the instructions.
+
+//===---------------------------------------------------------------------===//
+
+In many cases, LLVM generates code like this:
+
+_test:
+        movl 8(%esp), %eax
+        cmpl %eax, 4(%esp)
+        setl %al
+        movzbl %al, %eax
+        ret
+
+on some processors (which ones?), it is more efficient to do this:
+
+_test:
+        movl 8(%esp), %ebx
+	xor %eax, %eax
+        cmpl %ebx, 4(%esp)
+        setl %al
+        ret
+
+Doing this correctly is tricky though, as the xor clobbers the flags.
+
+