LLVM Error on PowerPC64LE: Cannot select ...

[CryZe]

Since the switch to LLVM 6 my builds on PowerPC64LE have been consistently failing with the following error:

LLVM ERROR: Cannot select: 0x7fa027678410: ch = PPCISD::STBRX<ST6[%658](align=1)> 0x7fa02663ce38, 0x7fa0228a63a8, 0x7fa027678ea0, ValueType:ch:i48
  0x7fa0228a63a8: i32 = truncate 0x7fa027938820
    0x7fa027938820: i64 = srl 0x7fa027678750, Constant:i32<16>
      0x7fa027678750: i64 = or 0x7fa0277aebc8, 0x7fa026724680
        0x7fa0277aebc8: i64,ch = CopyFromReg 0x7fa029932440, Register:i64 %200
          0x7fa0279382d8: i64 = Register %200
        0x7fa026724680: i64 = shl nuw nsw 0x7fa027938410, Constant:i32<16>
          0x7fa027938410: i64 = AssertZext 0x7fa027938a28, ValueType:ch:i8
            0x7fa027938a28: i64,ch = CopyFromReg 0x7fa029932440, Register:i64 %202
              0x7fa027938c30: i64 = Register %202
          0x7fa027678958: i32 = Constant<16>
      0x7fa027678958: i32 = Constant<16>
  0x7fa027678ea0: i64,ch = CopyFromReg 0x7fa029932440, Register:i64 %158
    0x7fa0277b0068: i64 = Register %158
In function: _ZN6base646decode13decode_helper17h84289d425683d5b3E.llvm.15558209233772563096

CI Build

This seems to be something that the base64 crate is triggering.

[nagisa]

https://bugs.llvm.org/show_bug.cgi?id=32140.

Minimal rust test case:

#[no_mangle]
pub fn foo(accum: u64, output: &mut [u8;6]) {
    unsafe {
        let bytes = *(&accum.to_be() as *const _ as *const [u8; 8]);
        ::std::ptr::copy_nonoverlapping(bytes.as_ptr(), output.as_mut_ptr(), 6);
    }
}

[cuviper]

We encountered this in Fedora bz1554349 -- sorry I neglected to report it to Rust too.
We linked it to LLVM bz 35402, now RESOLVED FIXED.

[cuviper]

Another PPC64 issue we hit (though big-endian) was LLVM 37039. I can backport both of these to rust-lang/llvm, if nobody else is already.

[cuviper]

Both of those fixes are also in the release_60 branch destined for the imminent LLVM 6.0.1, so we could also just rebase rust-lang/llvm to pick those up.

[nikomatsakis]

cc @cuviper @nagisa -- what is status here, are those backports in progress?

[cuviper]

I merged the release_60 backports from upstream LLVM, and @alexcrichton pulled that into #50514.