[llvm-commits] [llvm] r47939 - /llvm/trunk/lib/Target/X86/README-SSE.txt
Chris Lattner
sabre at nondot.org
Tue Mar 4 23:22:40 PST 2008
Author: lattner
Date: Wed Mar 5 01:22:39 2008
New Revision: 47939
URL: http://llvm.org/viewvc/llvm-project?rev=47939&view=rev
Log:
add a note
Modified:
llvm/trunk/lib/Target/X86/README-SSE.txt
Modified: llvm/trunk/lib/Target/X86/README-SSE.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/README-SSE.txt?rev=47939&r1=47938&r2=47939&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/README-SSE.txt (original)
+++ llvm/trunk/lib/Target/X86/README-SSE.txt Wed Mar 5 01:22:39 2008
@@ -748,3 +748,33 @@
right, but we shouldn't have to custom lower anything. This is probably related
to <2 x i64> ops being so bad.
+//===---------------------------------------------------------------------===//
+
+'select' on vectors and scalars could be a whole lot better. We currently
+lower them to conditional branches. On x86-64 for example, we compile this:
+
+double test(double a, double b, double c, double d) { return a<b ? c : d; }
+
+to:
+
+_test:
+ ucomisd %xmm0, %xmm1
+ ja LBB1_2 # entry
+LBB1_1: # entry
+ movapd %xmm3, %xmm2
+LBB1_2: # entry
+ movapd %xmm2, %xmm0
+ ret
+
+instead of:
+
+_test:
+ cmpltsd %xmm1, %xmm0
+ andpd %xmm0, %xmm2
+ andnpd %xmm3, %xmm0
+ orpd %xmm2, %xmm0
+ ret
+
+For unpredictable branches, the later is much more efficient. This should
+just be a matter of having scalar sse map to SELECT_CC and custom expanding
+or iseling it.
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