A memcpy out of an fresh alloca is a no-op, delete it. Patch by Patrick Walton!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200907 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index 3c24e6d3643a0bed74473a612f1484198cd5e699..6619d542885990369bdf556f2997ed9a1d8084bc 100644 (file)
--- a/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -843,9 +843,12 @@ bool MemCpyOpt::processMemCpy(MemCpyInst *M) {
   ConstantInt *CopySize = dyn_cast<ConstantInt>(M->getLength());
   if (CopySize == 0) return false;
 
-  // The are two possible optimizations we can do for memcpy:
+  // The are three possible optimizations we can do for memcpy:
   //   a) memcpy-memcpy xform which exposes redundance for DSE.
   //   b) call-memcpy xform for return slot optimization.
+  //   c) memcpy from freshly alloca'd space copies undefined data, and we can
+  //      therefore eliminate the memcpy in favor of the data that was already
+  //      at the destination.
   MemDepResult DepInfo = MD->getDependency(M);
   if (DepInfo.isClobber()) {
     if (CallInst *C = dyn_cast<CallInst>(DepInfo.getInst())) {
@@ -865,6 +868,13 @@ bool MemCpyOpt::processMemCpy(MemCpyInst *M) {
   if (SrcDepInfo.isClobber()) {
     if (MemCpyInst *MDep = dyn_cast<MemCpyInst>(SrcDepInfo.getInst()))
       return processMemCpyMemCpyDependence(M, MDep, CopySize->getZExtValue());
+  } else if (SrcDepInfo.isDef()) {
+    if (isa<AllocaInst>(SrcDepInfo.getInst())) {
+      MD->removeInstruction(M);
+      M->eraseFromParent();
+      ++NumMemCpyInstr;
+      return true;
+    }
   }
 
   return false;

diff --git a/test/Transforms/MemCpyOpt/memcpy-undef.ll b/test/Transforms/MemCpyOpt/memcpy-undef.ll
new file mode 100644 (file)
index 0000000..fd4965f
--- /dev/null
+++ b/test/Transforms/MemCpyOpt/memcpy-undef.ll
@@ -0,0 +1,25 @@
+; RUN: opt < %s -basicaa -memcpyopt -S | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.8.0"
+
+%struct.foo = type { i8, [7 x i8], i32 }
+
+define i32 @test1(%struct.foo* nocapture %foobie) nounwind noinline ssp uwtable {
+  %bletch.sroa.1 = alloca [7 x i8], align 1
+  %1 = getelementptr inbounds %struct.foo* %foobie, i64 0, i32 0
+  store i8 98, i8* %1, align 4
+  %2 = getelementptr inbounds %struct.foo* %foobie, i64 0, i32 1, i64 0
+  %3 = getelementptr inbounds [7 x i8]* %bletch.sroa.1, i64 0, i64 0
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %2, i8* %3, i64 7, i32 1, i1 false)
+  %4 = getelementptr inbounds %struct.foo* %foobie, i64 0, i32 2
+  store i32 20, i32* %4, align 4
+  ret i32 undef
+
+; Check that the memcpy is removed.
+; CHECK-LABEL: @test1(
+; CHECK-NOT: call void @llvm.memcpy
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) nounwind
+