Index: llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -9862,14 +9862,14 @@
     if (*Info != StaticAllocaInfo::Unknown)
       continue;
 
-    // Check if the stored value is an argument, and that this store fully
-    // initializes the alloca. Don't elide copies from the same argument twice.
+    // If the argument type has padding bits we can't directly forward a pointer
+    // as the upper bits may contain garbage.
+    // Don't elide copies from the same argument twice.
     const Value *Val = SI->getValueOperand()->stripPointerCasts();
     const auto *Arg = dyn_cast<Argument>(Val);
     if (!Arg || Arg->hasPassPointeeByValueCopyAttr() ||
         Arg->getType()->isEmptyTy() ||
-        DL.getTypeStoreSize(Arg->getType()) !=
-            DL.getTypeAllocSize(AI->getAllocatedType()) ||
+        !DL.typeSizeEqualsStoreSize(Arg->getType()) ||
         ArgCopyElisionCandidates.count(Arg)) {
       *Info = StaticAllocaInfo::Clobbered;
       continue;
Index: llvm/test/CodeGen/X86/arg-copy-elide.ll
===================================================================
--- llvm/test/CodeGen/X86/arg-copy-elide.ll
+++ llvm/test/CodeGen/X86/arg-copy-elide.ll
@@ -73,12 +73,17 @@
 ; CHECK-LABEL: i1_arg:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    pushl %ebx
+; CHECK-NEXT:    pushl %eax
 ; CHECK-NEXT:    movb {{[0-9]+}}(%esp), %bl
+; CHECK-NEXT:    movl %ebx, %eax
+; CHECK-NEXT:    andb $1, %al
+; CHECK-NEXT:    movb %al, {{[0-9]+}}(%esp)
 ; CHECK-NEXT:    leal {{[0-9]+}}(%esp), %eax
 ; CHECK-NEXT:    pushl %eax
 ; CHECK-NEXT:    calll _addrof_i1
 ; CHECK-NEXT:    addl $4, %esp
 ; CHECK-NEXT:    movl %ebx, %eax
+; CHECK-NEXT:    addl $4, %esp
 ; CHECK-NEXT:    popl %ebx
 ; CHECK-NEXT:    retl
   %x.addr = alloca i1
@@ -369,20 +374,49 @@
   ret void
 }
 
-; FIXME: The argument elision is not legal in presence of irregular types.
+define void @avoid_partially_initialized_alloca(i32 %x) {
+; CHECK-LABEL: avoid_partially_initialized_alloca:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    pushl %ebp
+; CHECK-NEXT:    movl %esp, %ebp
+; CHECK-NEXT:    andl $-8, %esp
+; CHECK-NEXT:    subl $8, %esp
+; CHECK-NEXT:    movl 8(%ebp), %eax
+; CHECK-NEXT:    movl %eax, (%esp)
+; CHECK-NEXT:    movl %esp, %eax
+; CHECK-NEXT:    pushl %eax
+; CHECK-NEXT:    calll _addrof_i32
+; CHECK-NEXT:    addl $4, %esp
+; CHECK-NEXT:    movl %ebp, %esp
+; CHECK-NEXT:    popl %ebp
+; CHECK-NEXT:    retl
+  %a = alloca i64
+  %p = bitcast i64* %a to i32*
+  store i32 %x, i32* %p
+  call void @addrof_i32(i32* %p)
+  ret void
+}
+
+; Ensure no copy elision happens as the two i3 values fed into icmp may have
+; garbage in the upper bits, a truncation is needed.
 
 define i1 @use_i3(i3 %a1, i3 %a2) {
 ; CHECK-LABEL: use_i3:
 ; CHECK:       # %bb.0:
+; CHECK-NEXT:    pushl %eax
 ; CHECK-NEXT:    movb {{[0-9]+}}(%esp), %al
 ; CHECK-NEXT:    andb $7, %al
-; CHECK-NEXT:    cmpb {{[0-9]+}}(%esp), %al
+; CHECK-NEXT:    movb {{[0-9]+}}(%esp), %cl
+; CHECK-NEXT:    andb $7, %cl
+; CHECK-NEXT:    movb %cl, {{[0-9]+}}(%esp)
+; CHECK-NEXT:    cmpb %cl, %al
 ; CHECK-NEXT:    sete %al
+; CHECK-NEXT:    popl %ecx
 ; CHECK-NEXT:    retl
-  %tmp = alloca i3, align 4
-  store i3 %a2, i3* %tmp, align 4
-  %l = load i3, i3* %tmp
-  %res = icmp eq i3 %a1, %l
+  %tmp = alloca i3
+  store i3 %a2, i3* %tmp
+  %val = load i3, i3* %tmp
+  %res = icmp eq i3 %a1, %val
   ret i1 %res
 }