<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/104718>104718</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            SelectionDAGBuilder.cpp's `llvm_unreachable` is quite reachable, actually
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          workingjubilee
      </td>
    </tr>
</table>

<pre>
    This IR:

```llvmir
source_filename = "example.1c26dad09674458a-cgu.0"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-none-elf"

@llvm.used = appending global [1 x ptr] [ptr @asan.module_ctor], section "llvm.metadata"
@llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 1, ptr @asan.module_ctor, ptr null }]

define x86_intrcc void @_ZN7example18page_fault_handler17hdb4acb34a86c6a8bE(ptr byval([8 x i8]) align 8 %0, i64 %1) unnamed_addr #0 !dbg !8 {
start:
  %Arg0.byval = alloca [8 x i8], align 8, !dbg !13
  call void @llvm.memcpy.p0.p0.i64(ptr align 8 %Arg0.byval, ptr align 8 %0, i64 8, i1 false), !dbg !13
  call void asm sideeffect alignstack inteldialect "ud2", "~{dirflag},~{fpsr},~{flags},~{memory}"(), !dbg !13, !srcloc !14
 unreachable, !dbg !13
}

declare void @__asan_report_load_n(i64, i64)
declare void @__asan_loadN(i64, i64)
declare void @__asan_report_load1(i64)
declare void @__asan_load1(i64)
declare void @__asan_report_load2(i64)
declare void @__asan_load2(i64)
declare void @__asan_report_load4(i64)
declare void @__asan_load4(i64)
declare void @__asan_report_load8(i64)
declare void @__asan_load8(i64)
declare void @__asan_report_load16(i64)
declare void @__asan_load16(i64)
declare void @__asan_report_store_n(i64, i64)
declare void @__asan_storeN(i64, i64)
declare void @__asan_report_store1(i64)
declare void @__asan_store1(i64)
declare void @__asan_report_store2(i64)
declare void @__asan_store2(i64)
declare void @__asan_report_store4(i64)
declare void @__asan_store4(i64)
declare void @__asan_report_store8(i64)
declare void @__asan_store8(i64)
declare void @__asan_report_store16(i64)
declare void @__asan_store16(i64)
declare void @__asan_report_exp_load_n(i64, i64, i32)
declare void @__asan_exp_loadN(i64, i64, i32)
declare void @__asan_report_exp_load1(i64, i32)
declare void @__asan_exp_load1(i64, i32)
declare void @__asan_report_exp_load2(i64, i32)
declare void @__asan_exp_load2(i64, i32)
declare void @__asan_report_exp_load4(i64, i32)
declare void @__asan_exp_load4(i64, i32)
declare void @__asan_report_exp_load8(i64, i32)
declare void @__asan_exp_load8(i64, i32)
declare void @__asan_report_exp_load16(i64, i32)
declare void @__asan_exp_load16(i64, i32)
declare void @__asan_report_exp_store_n(i64, i64, i32)
declare void @__asan_exp_storeN(i64, i64, i32)
declare void @__asan_report_exp_store1(i64, i32)
declare void @__asan_exp_store1(i64, i32)
declare void @__asan_report_exp_store2(i64, i32)
declare void @__asan_exp_store2(i64, i32)
declare void @__asan_report_exp_store4(i64, i32)
declare void @__asan_exp_store4(i64, i32)
declare void @__asan_report_exp_store8(i64, i32)
declare void @__asan_exp_store8(i64, i32)
declare void @__asan_report_exp_store16(i64, i32)
declare void @__asan_exp_store16(i64, i32)
declare ptr @memmove(ptr, ptr, i64)
declare ptr @memcpy(ptr, ptr, i64)
declare ptr @memset(ptr, i32, i64)
declare void @__asan_handle_no_return()
declare void @__sanitizer_ptr_cmp(i64, i64)
declare void @__sanitizer_ptr_sub(i64, i64)
declare i1 @llvm.amdgcn.is.shared(ptr nocapture) #1
declare i1 @llvm.amdgcn.is.private(ptr nocapture) #1
declare void @llvm.memcpy.p0.p0.i64(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i64, i1 immarg) #2
declare void @__asan_before_dynamic_init(i64)
declare void @__asan_after_dynamic_init()
declare void @__asan_register_globals(i64, i64)
declare void @__asan_unregister_globals(i64, i64)
declare void @__asan_register_image_globals(i64)
declare void @__asan_unregister_image_globals(i64)
declare void @__asan_register_elf_globals(i64, i64, i64)
declare void @__asan_unregister_elf_globals(i64, i64, i64)

define internal void @asan.module_ctor() #3 {
  ret void
}

attributes #0 = { naked nocf_check noinline noredzone nounwind nonlazybind sanitize_address "probe-stack"="inline-asm" "target-cpu"="x86-64" "target-features"="-mmx,-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2,+soft-float" }
attributes #1 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }
attributes #2 = { nocallback nofree nounwind willreturn memory(argmem: readwrite) }
attributes #3 = { nounwind }

!llvm.module.flags = !{!0, !1, !2, !3, !4}
!llvm.ident = !{!5}
!llvm.dbg.cu = !{!6}

!0 = !{i32 8, !"PIC Level", i32 2}
!1 = !{i32 1, !"Code Model", i32 2}
!2 = !{i32 2, !"RtLibUseGOT", i32 1}
!3 = !{i32 2, !"Dwarf Version", i32 4}
!4 = !{i32 2, !"Debug Info Version", i32 3}
!5 = !{!"rustc version 1.82.0-nightly (2c93fabd9 2024-08-15)"}
!6 = distinct !DICompileUnit(language: DW_LANG_Rust, file: !7, producer: "clang LLVM (rustc version 1.82.0-nightly (2c93fabd9 2024-08-15))", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
!7 = !DIFile(filename: "/app/example.rs/@/example.1c26dad09674458a-cgu.0", directory: "/app")
!8 = distinct !DISubprogram(name: "page_fault_handler", linkageName: "_ZN7example18page_fault_handler17hdb4acb34a86c6a8bE", scope: !10, file: !9, line: 6, type: !11, scopeLine: 6, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !6, templateParams: !12)
!9 = !DIFile(filename: "example.rs", directory: "/app", checksumkind: CSK_MD5, checksum: "85bf207046f646941a20021898b6c9b6")
!10 = !DINamespace(name: "example", scope: null)
!11 = !DISubroutineType(types: !12)
!12 = !{}
!13 = !DILocation(line: 8, column: 9, scope: !8)
!14 = !{i32 187}
```

Hits this `llvm_unreachable`: https://github.com/llvm/llvm-project/blob/bf5cd4220d20d0ee5533d55f463612fbe2980071/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp#L1957

If you don't enable trap on `llvm_unreachable` in the CMake, we get this instead:
```
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.      Program arguments: /opt/compiler-explorer/clang-trunk/bin/llc -o /app/output.s -x86-asm-syntax=intel <source>
1.      Running pass 'Function Pass Manager' on module '<source>'.
2.      Running pass 'X86 Assembly Printer' on function '@_ZN7example18page_fault_handler17hdb4acb34a86c6a8bE'
 #0 0x00000000037ec688 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-trunk/bin/llc+0x37ec688)
 #1 0x00000000037ea02c SignalHandler(int) Signals.cpp:0:0
 #2 0x00007f26b2442520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #3 0x0000000002306054 (anonymous namespace)::X86MCCodeEmitter::emitPrefixImpl(unsigned int&, llvm::MCInst const&, llvm::MCSubtargetInfo const&, llvm::SmallVectorImpl<char>&) const X86MCCodeEmitter.cpp:0:0
 #4 0x00000000023093e8 (anonymous namespace)::X86MCCodeEmitter::encodeInstruction(llvm::MCInst const&, llvm::SmallVectorImpl<char>&, llvm::SmallVectorImpl<llvm::MCFixup>&, llvm::MCSubtargetInfo const&) const X86MCCodeEmitter.cpp:0:0
 #5 0x0000000001b8b846 llvm::X86AsmPrinter::emitInstruction(llvm::MachineInstr const*) (/opt/compiler-explorer/clang-trunk/bin/llc+0x1b8b846)
 #6 0x000000000261e787 llvm::AsmPrinter::emitFunctionBody() (/opt/compiler-explorer/clang-trunk/bin/llc+0x261e787)
 #7 0x0000000001b7dedb llvm::X86AsmPrinter::runOnMachineFunction(llvm::MachineFunction&) (/opt/compiler-explorer/clang-trunk/bin/llc+0x1b7dedb)
 #8 0x00000000028aac6b llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (.part.0) MachineFunctionPass.cpp:0:0
 #9 0x0000000002df8272 llvm::FPPassManager::runOnFunction(llvm::Function&) (/opt/compiler-explorer/clang-trunk/bin/llc+0x2df8272)
#10 0x0000000002df8531 llvm::FPPassManager::runOnModule(llvm::Module&) (/opt/compiler-explorer/clang-trunk/bin/llc+0x2df8531)
#11 0x0000000002df77cb llvm::legacy::PassManagerImpl::run(llvm::Module&) (/opt/compiler-explorer/clang-trunk/bin/llc+0x2df77cb)
#12 0x0000000000859e94 compileModule(char**, llvm::LLVMContext&) llc.cpp:0:0
#13 0x000000000074c296 main (/opt/compiler-explorer/clang-trunk/bin/llc+0x74c296)
#14 0x00007f26b2429d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#15 0x00007f26b2429e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#16 0x0000000000851f1e _start (/opt/compiler-explorer/clang-trunk/bin/llc+0x851f1e)
Program terminated with signal: SIGSEGV
Compiler returned: 139
```

It seems to me, given the frequency with which code hits this, that LLVM prefers to put this macro in reachable branches of code. May I suggest renaming it, or perhaps removing the feature that it causes undefined behavior if reached at runtime?
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy0Wt9zo7iW_mvIi8ouEBjwQx4cp903dZOe1KRv79S-uIQ4YN0IiSuJxJ6H_du3JMAGt5O23btTPeGXvu98Ojoc6QgTrVkpAG692Z03u78hjdlIdfsu1SsT5b-bjHGAm0zmu9vvG6bRw59euPD8e8_v_8Z--4_zt4qp9qaWjaKwLhgHQSpAXniPPIxhS6qawzSgOM5J7s_jJIpmKZnQspn6HsYt2hBVgkE5MYSTnWzMHj-pvHABkxonvhcuQuz-2MtgfGnP4sj9mbD9SYBTL1wEOJ0Uqd-dCXcr7uFxNHmxzY6kGMVqvu_GNo3XcTRpxKuQ72IipIAJ8GIP6v5GziXTRkPukKSuQeRMlKjkMiMcebO7AG1RbZQ3u7dXtVHIi3yiiZhWMm84rKmR9qmHl0gDNUwKq8ARV2CI9dHBbmexpXdQ_allL7lDLMSW3IpoD8hL7js9Hz7vkSjY3zwhu3skGs470qF_ciiYAGSdyYRRlKI3yXJLtP7vb0kXKkFakxLWBWm4WW-IyDmoINnkWURoFkYkjWlM0uyLh1NrKtu9Ee7h1JvdpWiLWOpcN0eEs1KgFHl45ltZLI7seWCfNcKGaL4mea6Qh0MfeTjIs9IeUtvRLqQNUWYf-8jCF6r0p85k62XOJSVobHvZ27anB-Ig7Hko4Xzf825cK1rvprVv_9nobfs26MPBcO_jUx10JlmACsI1eHj-awVEV0izHKAogJqWVBtCXxETBnjOCLf3PYybHNuwc4z4f7zkLmeq4KS0w4yX9kZRazW44qTUh8sKKql27hrb4Tohrr3WinJJ3a2o09sIBYRuSMbhdI8s7SjOKCcKDtG1toG6VlBLZdZcknwtPJw6RzvHWTWfIS3k20WIga2gB_7axplNB-T4fPIzmw7Io_PJz2w6IE_PJz-z6dDn8QVOP7NtR6-NVHBh_DjMVQHkkGeGxSVth_RnBsYlbYf0Z4bGJW2H9GcGxyVtR74_MzouatwZgG19Ohst2zn4c5IefRxXZ4GPFAQDjvMtX4Y6MomvMnkZ6shkdJXJy1BHJtOrTF6GOh7L-LrBvAw2MHo6I55t_GRyvErD5VF8BezY6OVxfAXs2OjlkXwF7Njo5bF8BeynMb08ms_CdRVMBVUl36BdcA-qnlNJ_ACh9e5ChAZzQHQF1hkTRVv_rIVcKzCNEt3K-QOIJoIZ9jeodW3Umlb1uSuOMVA32edAFuwrF1LlJRVTpqd6QxTkXeUiJCW1aZQtQmyFFZxBUCv2Rgycx3BO-SQk4YzoAxV6V8yAFHy3L1R_aqKA5H2L3gUBYlVFVNlJwZ8OWQaFTYf5TpCK0TUTzJy3KCCFAXWM-_XrUjJtcW2Zry9aY9q66jfgezCrbMV-RHG26SvQeyzw4gPplzrgXKbRToatkZUgh2r-5w2RtAua8LCvgJAC4yAnq1dijGJZY0B3exNhu_kiyCvkNlKLNd0AfUVCMsGtDCEV5H9Ld9aIdyZsM8HJ37vMnvdvt9vvAG1pca1kBhNX6duCPLRlecs2IbryMLaN2q2wCa2bfZttGk-sN4bPCyD21dH7RpOq2np4OdEaugPujmF73J9ANA32Z64Redt2B3vp4TstCzMpuCTGWe19NfZScPCSpITzjDj_FAqsT_RO0INrdA204cSQjAN6Z5y3qRV1OxQ4FVK0Wee0Lfyprc7IKV6iygoqL1y4JONy0SdmwoGZjvQoUDwctOnPxdvUbbh0G5aBDTYc-N12SdAdu-2b_V5LdGDsuFgOwoxJZj81yrNySptxq_hncf6gBQvxfkPMw_j5YYke4Q14t6VkH-OhneAIGxywS5kDepL5x2B8BMYH8J_mkWX_0vD1j-8DdDBEhx-j79-JKtAPUJpJMcCP_Bh9goesKdGDKOQJknBIMht718NYNdpQ9NbCUDBN8dSfCFZuDN8hD6eYzsOCZPkcYR9HEz-dBDO3yzZyTex4c6YNE25fL7h_WMqqZhz-1c44nIiyISXYOL3_r_Xj4tvX9Z-NNlZmwbi77-EgcVOoknlDQbX3MLVY9Pj448kKulrwvHeK_qM2rGJ_Q24NdJuaS6QaYVgFvQfDBXJxDhXT9sY_mXDtVw3nzuFuJ73mzLirB5vlmChHlIJU8N0mhB78rc0Be78l_XjcP6ysE3Daf-no-u7hFalrD6_6Lx5Ke3jlRf7g1scfQfAS5UyBnTJ2R4R4KCM9MXwvTVYrWSpS2dx1UPTzNnpniTPxSkr4dmh61fa749JU1n1ItOlmECPzzpq7ju2F2R1aB3v847BJu29sg-9hxUn5rKSRFpa3w7g6PH55thf3dipmxr1LS9TYIG4NtAahqjkx8EwUqXRveuTT-a-GdjiivxqqJXJzs26q1y6Sli__XD_dz4aPOlg6ywrsJ34UF3EUz6OAYN_HQTpPs5jOs_ho7A_59P7Bjp2uCYXxkHdKj8dGNJyPmIID00uTKdkYJuC7HRucWl-fdtQor46SdXjge5SUtIOR9iPv8j6VvKncyzo_jpt0ZOQ4fQZpcrDVf3kczjT_YEYjs2EadZ8k18PvBbFvrWyMqW2n7CuJVyUzmyabUll5eGUR3WFSK_lvoMbDq4zLzB6KGc0jjP0c-7kPMJuFYT6bFVEcxgEuMsDz1PeTYMDDLM7OUV9BeHj1Arz9iHe_-Hp0edcwnoOaUhs74WMwnyXDfj0UaCcblFtfJgaBcAsWo0iNpPigr4gJZDaAlk_k1SW2d0Dua6Z1DxPaAMkPX3PH3nx-_LJ4-YJ0k1XMIILsVNVW48jIyz3ItG7smnCFiMgRE5Q3OThxVBG9QXbZZBShMG3Nv7ivTnlT1XuB_tTz589tbkNElU0FwnSxuZK1tULbuUtNYFtzqWyOW7lpaGJUI17tEDLhxFE0kWifpWVj6sZMNZrYFS3R1UTvhCFbL7x3H76QFy7bT9pe-KVVE1g1fzbCTh6oJm4dnawa0X6ifbY3noggpdWQ2CFq12W21YgMJ12P8SnCv9IYLbSGKuM79KxchdHxFb0ty3jlN9MuwtrKwt_6_X9hAjROU-TGMlx44ULvdHviRLjR-a7ajHNopMj7WmqjwE4-LuEyYdqKJ71mjDx85287Lfus0C7wx2KJjyl6YaUg_B_95JZ2ttvb2r1X4cJ3_--JcEeUFDjOcBThGfY7te2r233q50w020kpmvYBnWo5jZ08BxmJC4ficOjH_iyynERIsatko936osvX89Zzf6Xx09KmiS8VM8YtobxwARUzzwoKtn2oau7htBHuxxp561fn4YP3n5YPQhtEpdCnHr40WVuhuQXn6VYvFeH8h5vNnMVwSTdEuTCNrS8dCh2LPe3a6MgN8xDSK90gqMzBdk41tJ9Nzuv25x36vOXQxIptm_oU7GO_Xuat2dBbQZZmaRQPzPyVxgtd9QlgHxwf-oTQDROty3pJi999ETtVo1iPR4McB5CkyUD2Kc19jryT-W6_IXK9qs7oSFUydmaSQ5597kzViD9E57Ve4CmPHp7Fv-9Op2skPB25MyWExkPhRyLsJDOQf1L3z4KnNVHG1hlzdILvdHTOR7ryIsUJHuhaPVtoP91drOj6sW-VDBaMYeAfS52FwRlSn9zsPB7y7tb_icxZGIxkBkcyk4QOR5pDSeium3EPgtvM1In-fxNrtYzE4qFYP53NYR6hjnjvOJdYbZZZjDPk4-OPp6UUBrZ9XuSc_hxm1s5o5vSTiOJ5jCrCxG_1qOUZdSgaT_t4ns8vnPYdZMQ5O-aEyEfrtYWt3W_H1oOenG0ForGV-GgogiIA1PL_lpNapr2pfpltQFVMEAM5emdmg7RbTNk198vD15cvX3-0zbs9I4XaTc52hyYI55_UaQ8GaYBK22qicsVJyd6grVcKBf9pQNBda_R9w-gG2TUA2vTVnavmN8S0-0u1ggKU46qbrr6pCFXSVkD7ighligi6AY1k4eim6Ins0APSTVmCNkjZOt8uv5nb35IK1aA2pNZIQSXf7BMnr93hbu0zgyhpNGjUiPY7QI4y2JA3JhViRWsdckRMv1Plhaub_DbM5-Gc3MBtkODI95Nk5t9sbrM8TOdpSsk8jLKCZj4EgZ9keZIWKY3m_g27xT6O_DRIg7ldVU2TOQ3TBHJCZ0GKE9-LfKgI41O3NytVeeNKr9vAj5IgveEkA67dj30xFvCO3FMPY292f6NuXdWWNaX2Ip8zbfSBxjDD4fbDijX5qOBGTKP_NMwAGv1sj1DTEM53N43it9eXk1233m7x_wYAAP__V8GKSg">