Author: lattner
Date: Tue Jan 8 18:17:57 2008
New Revision: 45766
URL: http://llvm.org/viewvc/llvm-project?rev=45766&view=rev
Log:
add a note
Modified:
llvm/trunk/lib/Target/README.txt
Modified: llvm/trunk/lib/Target/README.txt
URL:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/README.txt?rev=45766&r1=45765&r2=45766&view=diff
==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Tue Jan 8 18:17:57 2008
@@ -524,3 +524,24 @@
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.
+
+//===---------------------------------------------------------------------===//
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