[llvm-commits] [llvm] r123405 - /llvm/trunk/lib/Target/README.txt
Chris Lattner
sabre at nondot.org
Thu Jan 13 14:08:15 PST 2011
Author: lattner
Date: Thu Jan 13 16:08:15 2011
New Revision: 123405
URL: http://llvm.org/viewvc/llvm-project?rev=123405&view=rev
Log:
memcpy + metadata = bliss :)
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=123405&r1=123404&r2=123405&view=diff
==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Thu Jan 13 16:08:15 2011
@@ -2259,3 +2259,51 @@
transform the fmul to 0.0, and then the fadd to 2.0.
//===---------------------------------------------------------------------===//
+
+We should enhance memcpy/memcpy/memset to allow a metadata node on them
+indicating that some bytes of the transfer are undefined. This is useful for
+frontends like clang when lowering struct lowering, when some elements of the
+struct are undefined. Consider something like this:
+
+struct x {
+ char a;
+ int b[4];
+};
+void foo(struct x*P);
+struct x testfunc() {
+ struct x V1, V2;
+ foo(&V1);
+ V2 = V1;
+
+ return V2;
+}
+
+We currently compile this to:
+$ clang t.c -S -o - -O0 -emit-llvm | opt -scalarrepl -S
+
+
+%struct.x = type { i8, [4 x i32] }
+
+define void @testfunc(%struct.x* sret %agg.result) nounwind ssp {
+entry:
+ %V1 = alloca %struct.x, align 4
+ call void @foo(%struct.x* %V1)
+ %tmp1 = bitcast %struct.x* %V1 to i8*
+ %0 = bitcast %struct.x* %V1 to i160*
+ %srcval1 = load i160* %0, align 4
+ %tmp2 = bitcast %struct.x* %agg.result to i8*
+ %1 = bitcast %struct.x* %agg.result to i160*
+ store i160 %srcval1, i160* %1, align 4
+ ret void
+}
+
+This happens because SRoA sees that the temp alloca has is being memcpy'd into
+and out of and it has holes and it has to be conservative. If we knew about the
+holes, then this could be much much better.
+
+Having information about these holes would also improve memcpy (etc) lowering at
+llc time when it gets inlined, because we can use smaller transfers. This also
+avoids partial register stalls in some important cases.
+
+//===---------------------------------------------------------------------===//
+
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