Workaround Clang v15 AArch64 miscompile that affects parallel collection | Hacker News Reader

Workaround Clang v15 AArch64 miscompile that affects parallel collection

github.com

70 pointsgus_massa9 comments

The bug probably wasn’t reproducing on Linux because the clang shipped with Xcode is from Apple’s fork. The version may say “15”, but it was closer to upstream LLVM’s clang 16. There are also changes that haven’t been upstreamed yet and probably some that may not ever be (if my experience updating libtapi in nixpkgs was anything to go by).

i tried reproducing the zipped example on macOS 14.4.1 (23E224) and Apple clang version 15.0.0 (clang-1500.3.9.4) and it looks like the issue is not present on that version.

here are the instructions marked with '<<<<' from `disassem.txt` and my version:

    0x1000026a8 <+3300>: adrp   x14, 6152a                ; <<<<<< address used for `in_parallel_sweepers`?
      1000026a8: 9000c04e     adrp x14, 0x10180a000 <_tgc+0x6c8>

    0x100002bb8 <+4596>: ldr    w22, [x14]               ; <<<<<< load for `in_par` value, offset missing?
      100002bb8: b94961d6     ldr     w22, [x14, #2400]

    0x100002c60 <+4764>: cbnz   w22, 0x100002bec          ; <+4648> at gc.c:264:34  <<<<<<< branch on `in_par` value?
      100002c60: 35fffc76     cbnz    w22, 0x100002bec <_sweep_thread+0x1228>

    0x100002c64 <+4768>: ldr    w11, [x14, #0x960]        ;                         <<<<<<< addition ot `in_parallel_sweepers`, has offset
      100002c64: b94961cb     ldr     w11, [x14, #2400]

on my version it uses the offset for the first load #2400 = #0x960. so i guess it must have been fixed somewhere between `clang-1500.0.40.1` and `clang-1500.3.9.4`. but that is an insane bug. also, `sweep_thread` seems to have the exact same number of instructions under both versions of clang and i'm guessing the only difference is this offset which is kind of wild.

Apple clang versions don't coincide with public clang versions. Apple clang's major versions follow Xcode's. For instance, Xcode 16.1's clang is 16.0.0 (clang-1600.0.26.4).

I dont get how the C macro with the endless C loop can be called from within scheme, just asis. Without any registration. Is this a chez speciality, or am I missing something?

If I had to guess, they’re probably just expanding the Scheme code into C code during compilation. Also: it isn’t an endless C loop; do{/statements/}while(0) is a common macro idiom for a block of code that requires a trailing semicolon, i.e. one that behaves syntactically like a normal (void) function call. It only runs once because the condition is false.

I'm somewhat lost in the details, but IIUC, you are confused for the "use" of the macro in the file mkgs.ss The fist part of that file is a transpiler, that transpile it's second half from pseudo-scheme to c, and then the c version includes the definition of the macro and uses it https://github.com/cisco/ChezScheme/blob/fc577f234171af80859...

It's a very weird bug and a very weird debugging story. The description of the change is short, but don't miss the first comment that explains all the details.

A bit disappointing that it just ends at "we have this magic compiler bug that was magically fixed in the next version" without any attempt to find the cause (e.g. by bisecting Clang). How can you know that it was really fixed and isn't just hiding until something unrelated in the codebase moves around again?

I agree, but I guess the original quest went already too deep into the rabbit hole and he wanted to escape as soon as possible (after 2 years!). It looks like an interesting project, and if someone can track the exact commit, then can farm some karma here with a nice blog post.

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