powerpc64 nightly: discriminant value 0isize already exists #39331

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1. nagisa commented on Jan 27, 2017

   

   nagisa

   on Jan 27, 2017

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   Probably yet another case of i128 fallout. I can’t even begin guessing where the problem is given that the compiler did just fine compiling stage0. May be trans? This for example?
2. cuviper commented on Jan 27, 2017

   

   cuviper

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   I also see this with s390x, but the exact same configuration works fine on powerpc64le and x86_64, so that hints that this may be an endianness issue. I'm not sure how that would come up for enum discriminants though.

   It also works fine on powerpc64 if I use the latest beta (rustc 1.15.0-beta.5 (10893a9a3 2017-01-19)) as the stage0 rustc, so it would seem to be a beta->nightly regression.
3. cuviper commented on Jan 27, 2017

   

   cuviper

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   @nagisa So that's:

   llvm::LLVMConstIntOfArbitraryPrecision(t.to_ref(), 2, &u as *const u128 as *const u64)
   

   Calling:

   LLVMValueRef LLVMConstIntOfArbitraryPrecision(LLVMTypeRef IntTy,
                                                 unsigned NumWords,
                                                 const uint64_t Words[]);
   

   That looks like a pretty good hunch, if LLVM is expecting little-endian Words[].
4. nagisa commented on Jan 27, 2017

   

   nagisa

   on Jan 27, 2017

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   @cuviper Could you try compiling the compiler with the C_big_integral swapped out for code below and see if it works (ugh the pain of not having big endian CI :/):

   pub fn C_big_integral(t: Type, u: u128, sign_extend: bool) -> ValueRef {
       if ::std::mem::size_of::<u128>() == 16 {
           unsafe {
               let words = [u as u64, u.wrapping_shr(64) as u64];
               llvm::LLVMConstIntOfArbitraryPrecision(t.to_ref(), 2, words.as_ptr())
           }
       } else {
           // SNAP: remove after snapshot
           C_integral(t, u as u64, sign_extend)
       }
   }

   ---

   Also if you hop onto IRC, we could organise a “low” latency debugging session :)
5. cuviper commented on Jan 27, 2017

   

   cuviper

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   On IRC, @nagisa suggested running this test command with the patch above:

   python x.py test src/test/run-pass --stage=2 --test-args i128
   

   Now, the patch doesn't directly help me when bootstrapping with the nightly stage0, because that cross-compiled rustc still has the bug. But I can run that test after bootstrapping with the beta stage0, where the build had looked like it succeeded, and indeed run-pass/i128.rs fails:

   thread 'main' panicked at 'assertion failed: `(left == right)` (left: `0`, right: `2338558778202779088322560000000000`)'
   

   on this part in particular:

       let z: i128 = 0xABCD_EF;
       assert_eq!(z * z, 0x734C_C2F2_A521);

   And that makes sense -- if z gets built with its 64-bit halves swapped, then the lower half will be 0 and squaring will overflow all the significant bits resulting in 0 overall. That right in hex also shows it has a reversed value, 0x734c_c2f2_a521_0000_0000_0000_0000.

   With the patched C_big_integral, the same test now succeeds, huzzah!

   (run-pass/i128-ffi.rs fails to link in both cases, because rust_test_helpers.c only defines the necessary functions for defined(__amd64__).)
6. cuviper commented on Jan 27, 2017

   

   cuviper

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   Now, whether that fix will correct the issue with enum discriminants is harder to tell. The run-pass enum tests are passing for me after beta bootstrap with or without the patch. I think the cross-compiled aspect is probably important to the original problem, so I won't know until I can try with a new nightly stage0.
7. cuviper commented on Feb 2, 2017

   

   cuviper

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   Success! Using:

   $ rustc -Vv
   rustc 1.16.0-nightly (24055d0f2 2017-01-31)
   binary: rustc
   commit-hash: 24055d0f2aa8dce5caed7544e6006aa48dceaea5
   commit-date: 2017-01-31
   host: powerpc64-unknown-linux-gnu
   release: 1.16.0-nightly
   LLVM version: 3.9
   

   The native rebuild now completes successfully, and also passes the i128 tests. Thanks!
8. cuviper

   closed this as completedon Feb 2, 2017