SIGILL with new libc qsort_r implementation in BranchFolder::MergePotentialsElt

[crrodriguez]

llvm-project/llvm/lib/CodeGen/BranchFolding.cpp

Line 491 in 7c1ad51

llvm_unreachable("Predecessor appears twice");

Now glibc compares an object with itself, just like when _GLIBCXX_DEBUG is enabled therefore explicit unreachable triggers. this breaks at least any rustc compiler available.

#0  0x000015129e40516a in int llvm::array_pod_sort_comparator<llvm::BranchFolder::MergePotentialsElt>(void const*, void const*) () from /home/crrodriguez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/[libLLVM-17-rust-1.75.0-nightly.so](http://libllvm-17-rust-1.75.0-nightly.so/)

#1  0x00001512a0043779 in __GI___qsort_r (pbase=0x15128c90bd80, total_elems=<optimized out>, size=0x10, cmp=<optimized out>, arg=<optimized out>) at qsort.c:335

#2  0x000015129e4df6e6 in llvm::BranchFolder::TryTailMergeBlocks(llvm::MachineBasicBlock*, llvm::MachineBasicBlock*, unsigned int) () from /home/crrodriguez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/[libLLVM-17-rust-1.75.0-nightly.so](http://libllvm-17-rust-1.75.0-nightly.so/)

[llvmbot]

@llvm/issue-subscribers-libc

Author: Cristian Rodríguez (crrodriguez)

https://github.com/llvm/llvm-project/blob/7c1ad51e018c5a1517ca6fb58a4d7027a8bec64e/llvm/lib/CodeGen/BranchFolding.cpp#L491

Now glibc compares an object with itself, just like when _GLIBCXX_DEBUG is enabled therefore explicit unreachable triggers. this breaks at least any rustc compiler available.

#<!-- -->0  0x000015129e40516a in int llvm::array_pod_sort_comparator&lt;llvm::BranchFolder::MergePotentialsElt&gt;(void const*, void const*) () from /home/crrodriguez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/[libLLVM-17-rust-1.75.0-nightly.so](http://libllvm-17-rust-1.75.0-nightly.so/)

#<!-- -->1  0x00001512a0043779 in __GI___qsort_r (pbase=0x15128c90bd80, total_elems=&lt;optimized out&gt;, size=0x10, cmp=&lt;optimized out&gt;, arg=&lt;optimized out&gt;) at qsort.c:335

#<!-- -->2  0x000015129e4df6e6 in llvm::BranchFolder::TryTailMergeBlocks(llvm::MachineBasicBlock*, llvm::MachineBasicBlock*, unsigned int) () from /home/crrodriguez/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/[libLLVM-17-rust-1.75.0-nightly.so](http://libllvm-17-rust-1.75.0-nightly.so/)

[thesamesam]

Original report was at https://inbox.sourceware.org/libc-alpha/CAPBLoAcuhewbFo+6S+fSFb-tjQRtsjW=tAN-XLiWeqi=58z+yg@mail.gmail.com/T/#m94492200e58c27b381e703f1f8ebf43f3c90c54d.