`slice::as_chunks` lowered to poison

[xdoardo]

This test generates poison values when targeting CHERIoT:

define dso_local { ptr addrspace(200), i32 } @chunks4(ptr addrspace(200) align 1 %x.0, i32 %x.1) unnamed_addr addrspace(200) #2 {
start:
  %_2 = alloca [32 x i8], align 8, addrspace(200)
; call <[u8]>::as_chunks::<4>
  call void @_RINvMNtCseGUygyxNKUp_4core5sliceSh9as_chunksKj4_ECs9We5GNEBTqK_15slice_as_chunks(ptr addrspace(200) sret([32 x i8]) align 8 %_2, ptr addrspace(200) align 1 %x.0, i32 %x.1, ptr addrspace(200) align 8 @alloc_009ef07d224df88a1692249518be7cbf) #6
  %_0.0 = load ptr addrspace(200), ptr addrspace(200) %_2, align 8
  %0 = getelementptr inbounds i8, ptr addrspace(200) %_2, i32 8
  %_0.1 = load i32, ptr addrspace(200) %0, align 8
  %1 = insertvalue { ptr addrspace(200), i32 } poison, ptr addrspace(200) %_0.0, 0
  %2 = insertvalue { ptr addrspace(200), i32 } %1, i32 %_0.1, 1
  ret { ptr addrspace(200), i32 } %2
}

...


; <[u8]>::as_chunks_unchecked::<4>
; Function Attrs: inlinehint nounwind
define dso_local { ptr addrspace(200), i32 } @_RINvMNtCseGUygyxNKUp_4core5sliceSh19as_chunks_uncheckedKj4_ECs9We5GNEBTqK_15slice_as_chunks(ptr addrspace(200) align 1 %self.0, i32 %self.1, ptr addrspace(200) align 8 %0) unnamed_addr addrspace(200) #0 {
...

bb4:                                              ; preds = %bb5
  %_13.0 = load ptr addrspace(200), ptr addrspace(200) %data, align 8
  %_13.1 = load i32, ptr addrspace(200) %new_len, align 4
  %2 = insertvalue { ptr addrspace(200), i32 } poison, ptr addrspace(200) %_13.0, 0
  %3 = insertvalue { ptr addrspace(200), i32 } %2, i32 %_13.1, 1
  ret { ptr addrspace(200), i32 } %3
...

I think it might be due to some checks done in MIR.

[xdoardo]

False alarm, the same poison is generated on other platforms. In the examples above it is used to invalidate further reads of the pointers. The failure on this test stems from the use of a 64 bits mask in the second function in the test, unrelated to poisons.

[xdoardo]

#69 patches this test to make it work on CHERIoT.