From: Chris Lattner <sabre@nondot.org>
Date: Wed, 9 Jan 2008 00:17:57 +0000 (+0000)
Subject: add a note
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=f61b63ec5b36857f34d3ec93afcd53557369373c;p=oota-llvm.git

add a note


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45766 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index e6178894afe..b331acf0320 100644
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -524,3 +524,24 @@ Investigate lowering of sparse switch statements into perfect hash tables:
 http://burtleburtle.net/bob/hash/perfect.html
 
 //===---------------------------------------------------------------------===//
+
+We should turn things like "load+fabs+store" and "load+fneg+store" into the
+corresponding integer operations.  On a yonah, this loop:
+
+double a[256];
+ for (b = 0; b < 10000000; b++)
+ for (i = 0; i < 256; i++)
+   a[i] = -a[i];
+
+is twice as slow as this loop:
+
+long long a[256];
+ for (b = 0; b < 10000000; b++)
+ for (i = 0; i < 256; i++)
+   a[i] ^= (1ULL << 63);
+
+and I suspect other processors are similar.  On X86 in particular this is a
+big win because doing this with integers allows the use of read/modify/write
+instructions.
+
+//===---------------------------------------------------------------------===//