rustc SIGILL since qsort_r patches

[Adhemerval Zanella Netto]

On 04/11/23 22:10, Andrew Pinski wrote:
> On Sat, Nov 4, 2023 at 5:55 PM Cristian Rodríguez <cristian@rodriguez.im> wrote:
>>
>> Hi:
>>
>> After quite some head scratching I found the recent qsort_r patches triggers a bug "somewhere" that makes rustc LTO end in sigkill (ud2)
>>
>> happens compiling anything in release mode with any toolchain (stable, beta, nightly)
>>
>> #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
>>
>> #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
>>
> 
> I think this is a bug in LLVM's
> BranchFolder::MergePotentialsElt::operator< code:
> See: https://llvm.org/doxygen/BranchFolding_8cpp_source.html
> 
> Specifically it has the following comment:
> // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
> 489 // an object with itself.
> 490#ifndef _GLIBCXX_DEBUG
> 491 llvm_unreachable("Predecessor appears twice");
> 492#else
> 493 return false;
> 494#endif
> 
> Well glibc's qsort implementation will also now compare with itself.
> 
> Thanks,
> Andrew
> 
>> git head == bad
>> 2.38   == good
>>
>> Main interest is to know if I am not alone and if anyone else has observed this problem so far.
>> Thanks..


Just a side note that the quicksort implementation was also used for
size (number of elements times size per element) larger than the
installed system RAM (_SC_PHYS_PAGES / size > _SC_PAGESIZE) or
whether malloc fails.  So it is a latent issue, that did not trigger
before by chance.