Asm block causes compiler crash

[tyler569]

Compiling the following source file:

const std = @import("std");

pub fn main() void {
    const foo: i64 = undefined;

    asm volatile (
        \\ mov $12, %[foo]
        :
        : [foo] "=m" (foo)
    );

    std.debug.warn("{}\n", foo);
}

with zig build-exe asm.zig causes the compiler to segfault.

This seems very similar to #1689, I get a similar crash deep in the LLVM weeds:

(gdb) bt
#0  0x00007ffff54c342c in llvm::SelectionDAGBuilder::visitInlineAsm(llvm::ImmutableCallSite) ()                                                                                      
#1  0x00007ffff54d4b51 in llvm::SelectionDAGBuilder::visitCall(llvm::CallInst const&) ()
#2  0x00007ffff54dc1f9 in llvm::SelectionDAGBuilder::visit(llvm::Instruction const&) ()
#3  0x00007ffff551ede2 in llvm::SelectionDAGISel::SelectBasicBlock(llvm::ilist_iterator<llvm::ilist_detail::node_options<llvm::Instruction, false, false, void>, false, true>, llvm::ilist_iterator<llvm::ilist_detail::node_options<llvm::Instruction, false, false, void>, false, true>, bool&) ()                                                                        
#4  0x00007ffff5525703 in llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) ()                                                                                      
#5  0x00007ffff5526a0e in llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) ()                                                                                    
#6  0x00007ffff45cf3c0 in (anonymous namespace)::X86DAGToDAGISel::runOnMachineFunction(llvm::MachineFunction&) ()                                                                    
#7  0x00007ffff5822ee5 in llvm::MachineFunctionPass::runOnFunction(llvm::Function&) [clone .part.43] ()                                                                              
#8  0x00007ffff6411029 in llvm::FPPassManager::runOnFunction(llvm::Function&) ()
#9  0x00007ffff6411081 in llvm::FPPassManager::runOnModule(llvm::Module&) ()
#10 0x00007ffff64103a2 in llvm::legacy::PassManagerImpl::run(llvm::Module&) ()
#11 0x00007ffff26cd787 in ZigLLVMTargetMachineEmitToFile ()
#12 0x00007ffff25ff98c in codegen_build_and_link(CodeGen*) ()
#13 0x00007ffff251c415 in main ()

The zig compiler survives if you emit LLVM, and lli doesn't like that asm block when it tries to run it:

$ lli asm.ll                                                                                      
lli: asm.ll:30372:13: error: invalid type for inline asm constraint string                                                                                                            
  call void asm sideeffect " mov $$12, $0", "=m"(i64 undef), !dbg !13268                                                                                                              
            ^

I don't currently have the ability to run with a LLVM build with asserts enabled, but I can try that later tonight if that will help.

[Sahnvour]

I am not sure what you are trying to do, shouldn't your code be:

const std = @import("std");

pub fn main() void {
    var foo: i64 = undefined;

    asm volatile (
        \\ movq $12, %[foo]
        : [foo] "=m" (foo)
    );

    std.debug.warn("{}\n", foo);
}

?

Changes:

• foo is const in your example
• =m is an output but you listed it in the inputs
• with the above change, llvm suggested to use movq instead

Granted, a lot of these could be caught by Zig.

[nektro]

@Sahnvour's fixed version prints 12

@tyler569's original code after fixing the last line to std.debug.print("{}\n", .{foo}); still hits an llvm assertion

Failure value returned from cantFail wrapped call
number of input constraints does not match number of parameters
UNREACHABLE executed at /llvm/include/llvm/Support/Error.h:755!

Thread 1 "zig2" received signal SIGABRT, Aborted.
0x00007fffeb37cc1f in __pthread_kill_implementation () from /nix/store/scd5n7xsn0hh0lvhhnycr9gx0h8xfzsl-glibc-2.34-210/lib/libc.so.6
(gdb) bt
#0  0x00007fffeb37cc1f in __pthread_kill_implementation () from /nix/store/scd5n7xsn0hh0lvhhnycr9gx0h8xfzsl-glibc-2.34-210/lib/libc.so.6
#1  0x00007fffeb332042 in raise () from /nix/store/scd5n7xsn0hh0lvhhnycr9gx0h8xfzsl-glibc-2.34-210/lib/libc.so.6
#2  0x00007fffeb31d49c in abort () from /nix/store/scd5n7xsn0hh0lvhhnycr9gx0h8xfzsl-glibc-2.34-210/lib/libc.so.6
#3  0x00007fffec58e83a in llvm::llvm_unreachable_internal (msg=<optimized out>, file=file@entry=0x7ffff02a81a8 "/llvm/include/llvm/Support/Error.h", line=line@entry=755) at /llvm/lib/Support/ErrorHandling.cpp:212
#4  0x00007fffec7bb52c in llvm::cantFail (Msg=<optimized out>, Err=...) at /llvm/include/llvm/Support/Error.h:755
#5  llvm::InlineAsm::InlineAsm (this=0x2573df0, FTy=0x248e6a0, asmString=" mov $$12, $0", constraints=..., hasSideEffects=<optimized out>, isAlignStack=<optimized out>, asmDialect=llvm::InlineAsm::AD_ATT, canThrow=false) at /llvm/lib/IR/InlineAsm.cpp:39
#6  0x00007fffec7bced4 in llvm::InlineAsmKeyType::create (Ty=<optimized out>, this=0x7ffffffeebb0) at /llvm/lib/IR/ConstantsContext.h:429
#7  llvm::ConstantUniqueMap<llvm::InlineAsm>::create (this=0x23d2620, Ty=0x248d780, V=..., HashKey={...}) at /llvm/lib/IR/ConstantsContext.h:623
#8  0x00007fffec7bd21b in llvm::ConstantUniqueMap<llvm::InlineAsm>::getOrCreate (this=0x23d2620, Ty=0x248d780, V=...) at /llvm/lib/IR/ConstantsContext.h:642
#9  0x00007fffec7bb619 in llvm::InlineAsm::get (FTy=0x248e6a0, AsmString=..., Constraints=..., hasSideEffects=true, isAlignStack=false, asmDialect=llvm::InlineAsm::AD_ATT, canThrow=false) at /llvm/lib/IR/InlineAsm.cpp:50
#10 0x0000000000e2dbb6 in codegen.llvm.FuncGen.airAssembly (self=0x7fffffff1218, inst=5) at /zig/src/codegen/llvm.zig:6211
#11 0x0000000000e0a141 in codegen.llvm.FuncGen.genBody (self=0x7fffffff1218, body=...) at /zig/src/codegen/llvm.zig:4512
#12 0x0000000000e046cf in codegen.llvm.Object.updateFunc (o=0x23d1d40, module=0x23d0970, func=0x24700e0, air=..., liveness=...) at /zig/src/codegen/llvm.zig:1207
#13 0x0000000000b8f338 in link.Elf.updateFunc (self=0x23d1610, module=0x23d0970, func=0x24700e0, air=..., liveness=...) at /zig/src/link/Elf.zig:2411
#14 0x0000000000990d0a in link.File.updateFunc (base=0x23d1610, module=0x23d0970, func=0x24700e0, air=..., liveness=...) at /zig/src/link.zig:524
#15 0x000000000096ea41 in Module.ensureFuncBodyAnalyzed (mod=0x23d0970, func=0x24700e0) at /zig/src/Module.zig:4335
#16 0x000000000065a64d in Compilation.processOneJob (comp=0x23cf8c8, job=...) at /zig/src/Compilation.zig:3047
#17 0x00000000006483a0 in Compilation.performAllTheWork (comp=0x23cf8c8, main_progress_node=0x7fffffff31a8) at /zig/src/Compilation.zig:2985
#18 0x0000000000640a4e in Compilation.update (comp=0x23cf8c8) at /zig/src/Compilation.zig:2325
#19 0x00000000005cbfd0 in main.updateModule (gpa=..., comp=0x23cf8c8, hook=...) at /zig/src/main.zig:3318
#20 0x0000000000515d86 in main.buildOutputType (gpa=..., arena=..., all_args=..., arg_mode=...) at /zig/src/main.zig:3003
#21 0x00000000004aed60 in main.mainArgs (gpa=..., arena=..., args=...) at /zig/src/main.zig:238
#22 0x00000000004ae3d6 in main (argc=3, argv=0x7fffffffb788) at /zig/src/stage1.zig:56