LLVM 14: optimizer regression wrt. noalias attribute for return value

[kinke]

Input IR: current3.txt

With LLVM 13 (opt -O3), the _Dmain function is perfectly optimized to:

define i32 @_Dmain({ i64, { i64, i8* }* } %unnamed) #5 {
  %.frame.i = tail call noalias i8* @_d_allocmemory(i64 4) #1
  %x.i = bitcast i8* %.frame.i to i32*
  store i32 0, i32* %x.i, align 4
  %.gc_struct.i.i = tail call noalias i8* @_d_newitemT(%object.TypeInfo* bitcast (%object.TypeInfo_Struct* @_D28TypeInfo_S3mod9test14846FZ1S6__initZ to %object.TypeInfo*)) #1
  %.vthis.i.i = bitcast i8* %.gc_struct.i.i to i8**
  store i8* %.frame.i, i8** %.vthis.i.i, align 8
  store i32 1, i32* %x.i, align 4
  ret i32 0
}

With LLVM 14.0.0, it's a meager and invalid:

define i32 @_Dmain({ i64, { i64, i8* }* } %unnamed) #5 {
  %.frame.i = tail call noalias i8* @_d_allocmemory(i64 4) #1
  %.gc_struct.i.i = tail call noalias i8* @_d_newitemT(%object.TypeInfo* bitcast (%object.TypeInfo_Struct* @_D28TypeInfo_S3mod9test14846FZ1S6__initZ to %object.TypeInfo*)) #1
  ret i32 0
}

The thing is that _d_newitemT allocates garbage-collected memory, which is cleaned up at program termination, incl. destructing %.gc_struct.i.i, so the store to %.vthis.i.i etc. cannot be elided.

Removing the noalias function attribute from _d_newitemT 'fixes' the testcase. I haven't found anything related in the LLVM 14 changelog.

[nikic]

Dropping the noalias attribute is indeed the right thing to do here. _d_newitemT does not satisfy the noalias return contract, because the object is also accessible to whatever is invoking the destructor on the object storage.

[kinke]

Thx @nikic. _d_newitemT keeps track of its allocations, so the returned pointer escapes from the callee. From the spec:

On function return values, the noalias attribute indicates that the function acts like a system memory allocation function, returning a pointer to allocated storage disjoint from the storage for any other object accessible to the caller.

I guess there are hardly any allocators satisfying noalias return then?

[nikic]

Normal allocators like malloc can and do use noreturn return values.