enhance the "alloca is just a memcpy from constant global"

to ignore calls that obviously can't modify the alloca
because they are readonly/readnone.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119683 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/ScalarReplAggregates.cpp

@@ -34,6 +34,7 @@
 #include "llvm/Target/TargetData.h"
 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
 #include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Support/CallSite.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/GetElementPtrTypeIterator.h"
@@ -1810,6 +1811,12 @@ static bool isOnlyCopiedFromConstantGlobal(Value *V, MemTransferInst *&TheCopy,
       continue;
     }
     
+    // If this is a readonly/readnone call site, then we know it is just a load
+    // and we can ignore it.
+    if (CallSite CS = U)
+      if (CS.onlyReadsMemory())
+        continue;
+    
     // If this is isn't our memcpy/memmove, reject it as something we can't
     // handle.
     MemTransferInst *MI = dyn_cast<MemTransferInst>(U);

test/Transforms/ScalarRepl/memcpy-from-global.ll

@@ -68,3 +68,16 @@ define void @test2() {
 }
 
 declare void @bar(i8*)
+
+
+;; Should be able to eliminate the alloca.
+define void @test3() {
+  %A = alloca %T
+  %a = bitcast %T* %A to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* bitcast (%T* @G to i8*), i64 124, i32 4, i1 false)
+  call void @bar(i8* %a) readonly
+; CHECK: @test3
+; CHECK-NEXT: %a = bitcast %T* @G to i8*
+; CHECK-NEXT: call void @bar(i8* %a)
+  ret void
+}