[ARM] LLVM: Unknown addressing mode for CP reference! UNREACHABLE executed at ARMConstantIslandPass.cpp:755

[whitequark]

To reproduce in a real-world example, follow these instructions. Note that the bug is somewhat finicky: it's not exhibited in debug builds, and it's not exhibited also until main crosses a certain threshold size (might be related to inliner, not sure). It also does not crash (or AFAICT miscompile) on non-Asserts LLVM builds, so beware.

Reduced example, crashing LLVM from the current nightly:

; RUN: llc crash.ll
target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
target triple = "thumbv7em-none--eabihf"

@__rustc_debug_gdb_scripts_section__ = internal unnamed_addr constant [34 x i8] c"\01gdb_load_rust_pretty_printers.py\00", section ".debug_gdb_scripts", align 1

define i32 @main(i32, i8** nocapture readnone) {
top:
  %2 = load volatile i8, i8* getelementptr inbounds ([34 x i8], [34 x i8]* @__rustc_debug_gdb_scripts_section__, i32 0, i32 0), align 1
  ret i32 0
}

Backtrace:

#2  0x000055555636cbaa in llvm::llvm_unreachable_internal (msg=msg@entry=0x5555567aaec8 "Unknown addressing mode for CP reference!",
    file=file@entry=0x5555567aa908 "/home/whitequark/Work/rust/src/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp", line=line@entry=755)
    at /home/whitequark/Work/rust/src/llvm/lib/Support/ErrorHandling.cpp:118
#3  0x0000555555a5bc48 in (anonymous namespace)::ARMConstantIslands::initializeFunctionInfo (this=this@entry=0x555557402480, CPEMIs=...)
    at /home/whitequark/Work/rust/src/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp:755
#4  0x0000555555a5d23d in (anonymous namespace)::ARMConstantIslands::runOnMachineFunction (this=0x555557402480, mf=...)
    at /home/whitequark/Work/rust/src/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp:373
#5  0x0000555555dacbd5 in llvm::MachineFunctionPass::runOnFunction (this=0x555557402480, F=...)
    at /home/whitequark/Work/rust/src/llvm/lib/CodeGen/MachineFunctionPass.cpp:62
#6  0x000055555606470b in llvm::FPPassManager::runOnFunction (this=0x5555573e5760, F=...)
    at /home/whitequark/Work/rust/src/llvm/lib/IR/LegacyPassManager.cpp:1510
#7  0x00005555560647c4 in llvm::FPPassManager::runOnModule (this=0x5555573e5760, M=...)
    at /home/whitequark/Work/rust/src/llvm/lib/IR/LegacyPassManager.cpp:1531
#8  0x0000555556065321 in (anonymous namespace)::MPPassManager::runOnModule (M=..., this=<optimized out>)
    at /home/whitequark/Work/rust/src/llvm/lib/IR/LegacyPassManager.cpp:1587
#9  llvm::legacy::PassManagerImpl::run (this=0x5555573dcaf0, M=...) at /home/whitequark/Work/rust/src/llvm/lib/IR/LegacyPassManager.cpp:1690
#10 0x000055555583b349 in compileModule (argv=0x7fffffffdb38, Context=...) at /home/whitequark/Work/rust/src/llvm/tools/llc/llc.cpp:532
#11 0x0000555555804e80 in main (argc=2, argv=0x7fffffffdb38) at /home/whitequark/Work/rust/src/llvm/tools/llc/llc.cpp:289

MC dump:

(gdb) p MF->dump()
# Machine code for function main: NoPHIs, NoVRegs
Constant Pool:
  cp#0: , align=4

BB#0: derived from LLVM BB %top
        %R0<def,dead> = t2LDRBpci <cp#0>, pred:14, pred:%noreg; mem:Volatile LD1[getelementptr inbounds ([34 x i8], [34 x i8]* @__rustc_debug_gdb_scripts_section__, i32 0, i32 0)](dereferenceable)
        %R0<def>, %CPSR<def,dead> = tMOVi8 0, pred:14, pred:%noreg
        tBX_RET pred:14, pred:%noreg, %R0<imp-use,kill>

BB#1: Align 2 (4 bytes)
        CONSTPOOL_ENTRY 0, <cp#0>, 36

# End machine code for function main.

This is actually quite baffling because there's a case for ARM::t2LDRBpci in the crashing switch. Not sure what's up.

[arielb1]

Can you report this bug at bugs.llvm.org?

[whitequark]

@arielb1 haven't reproduced it with ToT LLVM yet (I don't have a build handy). Do you?

[arielb1]

I have a "somewhere between 4 and 5" LLVM and it compiles with it. OTOH, it also compiles with the LLVM in build/llvm - maybe I'm missing something Yup I was using an LLVM without debug assertions.

[arielb1]

Looking around, this looks like https://bugs.llvm.org/show_bug.cgi?id=31997 which is fixed on trunk.

[whitequark]

Can someone cherry-pick r295964 into rustc's LLVM?

[arielb1]

You can do it yourself - open a PR against https://github.com./rust-lang/llvm

[whitequark]

See rust-lang/llvm#76.