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[oota-llvm.git] / lib / Target / X86 / README.txt

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index 4dd7238b6f5b3bfb479f3d5c89251c667121630c..c3bcfffd09a6587a8d23d15b0afd8015f84f2cf8 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -1144,3 +1144,47 @@ It should be
        ret
 
 since paddw is commutative.
+
+//===---------------------------------------------------------------------===//
+
+This testcase:
+
+%G1 = weak global <4 x float> zeroinitializer           ; <<4 x float>*> [#uses=1]
+%G2 = weak global <4 x float> zeroinitializer           ; <<4 x float>*> [#uses=1]
+%G3 = weak global <4 x float> zeroinitializer           ; <<4 x float>*> [#uses=1]
+%G4 = weak global <4 x float> zeroinitializer           ; <<4 x float>*> [#uses=1]
+
+implementation   ; Functions:
+
+void %test() {
+        %tmp = load <4 x float>* %G1            ; <<4 x float>> [#uses=2]
+        %tmp2 = load <4 x float>* %G2           ; <<4 x float>> [#uses=2]
+        %tmp135 = shufflevector <4 x float> %tmp, <4 x float> %tmp2, <4 x uint> < uint 0, uint 4, uint 1, uint 5 >            ; <<4 x float>> [#uses=1]
+        store <4 x float> %tmp135, <4 x float>* %G3
+        %tmp293 = shufflevector <4 x float> %tmp, <4 x float> %tmp2, <4 x uint> < uint 1, uint undef, uint 3, uint 4 >        ; <<4 x float>> [#uses=1]
+        store <4 x float> %tmp293, <4 x float>* %G4
+        ret void
+}
+
+Compiles (llc -march=x86 -mcpu=yonah -relocation-model=static) to:
+
+_test:
+        movaps _G2, %xmm0
+        movaps _G1, %xmm1
+        movaps %xmm1, %xmm2
+2)      shufps $3, %xmm0, %xmm2
+        movaps %xmm1, %xmm3
+2)      shufps $1, %xmm0, %xmm3
+1)      unpcklps %xmm0, %xmm1
+2)      shufps $128, %xmm2, %xmm3
+1)      movaps %xmm1, _G3
+        movaps %xmm3, _G4
+        ret
+
+The 1) marked instructions could be scheduled better for reduced register 
+pressure.  The scheduling issue is more pronounced without -static.
+
+The 2) marked instructions are the lowered form of the 1,undef,3,4 
+shufflevector.  It seems that there should be a better way to do it :)
+
+