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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66360 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index c4746d0c756189cb63602c53d2aa9ba36dfb779d..5f28579095f10713dd182acd3e90c18399643ac7 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -1840,3 +1840,43 @@ _f:
        ret
 
 //===---------------------------------------------------------------------===//
+
+memcpy/memmove do not lower to SSE copies when possible.  A silly example is:
+define <16 x float> @foo(<16 x float> %A) nounwind {
+       %tmp = alloca <16 x float>, align 16
+       %tmp2 = alloca <16 x float>, align 16
+       store <16 x float> %A, <16 x float>* %tmp
+       %s = bitcast <16 x float>* %tmp to i8*
+       %s2 = bitcast <16 x float>* %tmp2 to i8*
+       call void @llvm.memcpy.i64(i8* %s, i8* %s2, i64 64, i32 16)
+       %R = load <16 x float>* %tmp2
+       ret <16 x float> %R
+}
+
+declare void @llvm.memcpy.i64(i8* nocapture, i8* nocapture, i64, i32) nounwind
+
+which compiles to:
+
+_foo:
+       subl    $140, %esp
+       movaps  %xmm3, 112(%esp)
+       movaps  %xmm2, 96(%esp)
+       movaps  %xmm1, 80(%esp)
+       movaps  %xmm0, 64(%esp)
+       movl    60(%esp), %eax
+       movl    %eax, 124(%esp)
+       movl    56(%esp), %eax
+       movl    %eax, 120(%esp)
+       movl    52(%esp), %eax
+        <many many more 32-bit copies>
+       movaps  (%esp), %xmm0
+       movaps  16(%esp), %xmm1
+       movaps  32(%esp), %xmm2
+       movaps  48(%esp), %xmm3
+       addl    $140, %esp
+       ret
+
+On Nehalem, it may even be cheaper to just use movups when unaligned than to
+fall back to lower-granularity chunks.
+
+//===---------------------------------------------------------------------===//