Illegal instruction with ccall to :memcpy

[timholy]

Here's a bug we're struggling with over at JuliaInterpreter. We've recently set up a test harness to run Julia's own tests under the interpreter (JuliaDebug/JuliaInterpreter.jl#13). Several of the tests are marked with ☠️, which we're using to indicate that running that test kills Julia. For one particular case we have a MWE in JuliaDebug/JuliaInterpreter.jl#28.

This particular bug can be triggered even without JuliaInterpreter:

julia> a, b = ['0'], ['a'];

julia> arr = Vector{Char}(undef, 2)
2-element Array{Char,1}:
 '\x74\xd8\x61\xf0'
 '\x00\x00\x7f\x60'

julia> ptr = pointer(arr)
Ptr{Char} @0x00007f6075562330

julia> elsz = sizeof(Char)
4

julia> na = length(a)
1

julia> nba = na * elsz
4

julia> ccall(:memcpy, Ptr{Cvoid}, (Ptr{Cvoid}, Ptr{Cvoid}, UInt), arr, a, nba)
Unreachable reached at 0x7f606c259cae

signal (4): Illegal instruction
in expression starting at REPL[7]:1
top-level scope at ./REPL[7]:1
jl_fptr_args at /home/tim/src/julia-master/src/gf.c:1906
jl_fptr_trampoline at /home/tim/src/julia-master/src/gf.c:1896
jl_toplevel_eval_flex at /home/tim/src/julia-master/src/toplevel.c:791
jl_toplevel_eval_flex at /home/tim/src/julia-master/src/toplevel.c:746
jl_toplevel_eval at /home/tim/src/julia-master/src/toplevel.c:806
jl_toplevel_eval_in at /home/tim/src/julia-master/src/toplevel.c:826
eval at ./boot.jl:328
jl_fptr_args at /home/tim/src/julia-master/src/gf.c:1906
jl_apply_generic at /home/tim/src/julia-master/src/gf.c:2251
eval_user_input at /home/tim/src/julia-master/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:86
run_backend at /home/tim/.julia/dev/Revise/src/Revise.jl:773
#61 at ./task.jl:261
jl_fptr_args at /home/tim/src/julia-master/src/gf.c:1906
jl_fptr_trampoline at /home/tim/src/julia-master/src/gf.c:1896
jl_apply_generic at /home/tim/src/julia-master/src/gf.c:2251
jl_apply at /home/tim/src/julia-master/src/julia.h:1578
start_task at /home/tim/src/julia-master/src/task.c:572
unknown function (ip: 0xffffffffffffffff)
Allocations: 9313301 (Pool: 9311285; Big: 2016); GC: 20
/home/tim/bin/julia-master: line 2: 14664 Illegal instruction     (core dumped) ~/src/julia-master/julia "$@"

[vtjnash]

Cute, I guess LLVM doesn't like that #28707 implemented the wrong return value:

julia> a, b = ['0'], ['a'];; arr = Vector{Char}(undef, 2); ptr = pointer(arr); elsz = sizeof(Char); na = length(a); nba = na * elsz;

julia> f() = ccall(:memcpy, Ptr{Cvoid}, (Ptr{Cvoid}, Ptr{Cvoid}, UInt), arr, a, nba)

julia> @code_llvm f()
...
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %57, i8* %54, i64 %value_phi, i32 1, i1 false)
  call void @llvm.trap()
...

[timholy]

This seems likely to be an easy fix. But where does one find doxygen docs for llvm-6? I can only find version 9 online.

[timholy]

I tried changing that to return ghostValue(jl_voidpointer_type); and got this:

...
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %44, i8* %42, i64 %value_phi, i32 1, i1 false)
  %45 = getelementptr %jl_value_t addrspace(10)*, %jl_value_t addrspace(10)** %gcframe, i32 1
  %46 = load %jl_value_t addrspace(10)*, %jl_value_t addrspace(10)** %45
  %47 = getelementptr %jl_value_t**, %jl_value_t*** %ptls, i32 0
  %48 = bitcast %jl_value_t*** %47 to %jl_value_t addrspace(10)**
  store %jl_value_t addrspace(10)* %46, %jl_value_t addrspace(10)** %48
  ret i64 undef
}

However,

julia> f()
Ptr{Nothing} @0x00007f2a1724d2b0

julia> ccall(:memcpy, Ptr{Cvoid}, (Ptr{Cvoid}, Ptr{Cvoid}, UInt), arr, a, nba)

signal (11): Segmentation fault
in expression starting at REPL[5]:1
init_bits_value at /home/tim/src/julia-master/src/cgutils.cpp:1905
init_bits_cgval at /home/tim/src/julia-master/src/cgutils.cpp:1918 [inlined]
boxed at /home/tim/src/julia-master/src/cgutils.cpp:2242
emit_function at /home/tim/src/julia-master/src/codegen.cpp:6156
jl_compile_linfo at /home/tim/src/julia-master/src/codegen.cpp:1145
jl_fptr_trampoline at /home/tim/src/julia-master/src/gf.c:1861
jl_toplevel_eval_flex at /home/tim/src/julia-master/src/toplevel.c:791
jl_toplevel_eval_flex at /home/tim/src/julia-master/src/toplevel.c:746
jl_toplevel_eval_in at /home/tim/src/julia-master/src/toplevel.c:826
eval at ./boot.jl:327
jl_apply_generic at /home/tim/src/julia-master/src/gf.c:2251
eval_user_input at /home/tim/src/julia-master/usr/share/julia/stdlib/v1.2/REPL/src/REPL.jl:86
run_backend at /home/tim/.julia/dev/Revise/src/Revise.jl:773
#61 at ./task.jl:261
jl_fptr_trampoline at /home/tim/src/julia-master/src/gf.c:1896
jl_apply_generic at /home/tim/src/julia-master/src/gf.c:2251
jl_apply at /home/tim/src/julia-master/src/julia.h:1593 [inlined]
start_task at /home/tim/src/julia-master/src/task.c:572
unknown function (ip: 0xffffffffffffffff)
Allocations: 9084779 (Pool: 9082932; Big: 1847); GC: 20
Segmentation fault (core dumped)

[vtjnash]

jl_voidpointer_type isn't a ghost object (such as jl_void_type), so the compiler's going to be rather confused there. it should return the first argument

[timholy]

My flailing has gotten me here:

diff --git a/src/ccall.cpp b/src/ccall.cpp
index 657d5ee1b7..0340705416 100644
--- a/src/ccall.cpp
+++ b/src/ccall.cpp
@@ -1781,15 +1781,15 @@ static jl_cgval_t emit_ccall(jl_codectx_t &ctx, jl_value_t **args, size_t nargs)
         const jl_cgval_t &dst = argv[0];
         const jl_cgval_t &src = argv[1];
         const jl_cgval_t &n = argv[2];
+        Value *destp = emit_unbox(ctx, T_size, dst, (jl_value_t*)jl_voidpointer_type);
         ctx.builder.CreateMemCpy(
-                ctx.builder.CreateIntToPtr(
-                    emit_unbox(ctx, T_size, dst, (jl_value_t*)jl_voidpointer_type), T_pint8),
+                ctx.builder.CreateIntToPtr(destp, T_pint8),
                 ctx.builder.CreateIntToPtr(
                     emit_unbox(ctx, T_size, src, (jl_value_t*)jl_voidpointer_type), T_pint8),
                 emit_unbox(ctx, T_size, n, (jl_value_t*)jl_ulong_type), 1,
                 false);
         JL_GC_POP();
-        return ghostValue(jl_void_type);
+        return mark_or_box_ccall_result(ctx, destp, retboxed, rt, unionall, static_rt);
     }
 
     jl_cgval_t retval = sig.emit_a_ccall(

or here

$ git diff
diff --git a/src/ccall.cpp b/src/ccall.cpp
index 657d5ee1b7..3a2a5da548 100644
--- a/src/ccall.cpp
+++ b/src/ccall.cpp
@@ -1781,7 +1781,7 @@ static jl_cgval_t emit_ccall(jl_codectx_t &ctx, jl_value_t **args, size_t nargs)
         const jl_cgval_t &dst = argv[0];
         const jl_cgval_t &src = argv[1];
         const jl_cgval_t &n = argv[2];
-        ctx.builder.CreateMemCpy(
+        CallInst *ret = ctx.builder.CreateMemCpy(
                 ctx.builder.CreateIntToPtr(
                     emit_unbox(ctx, T_size, dst, (jl_value_t*)jl_voidpointer_type), T_pint8),
                 ctx.builder.CreateIntToPtr(
@@ -1789,7 +1789,7 @@ static jl_cgval_t emit_ccall(jl_codectx_t &ctx, jl_value_t **args, size_t nargs)
                 emit_unbox(ctx, T_size, n, (jl_value_t*)jl_ulong_type), 1,
                 false);
         JL_GC_POP();
-        return ghostValue(jl_void_type);
+        return mark_or_box_ccall_result(ctx, ret, retboxed, rt, unionall, static_rt);
     }
 
     jl_cgval_t retval = sig.emit_a_ccall(

but neither works. Any more concrete tips? As should be apparent, I don't know what I'm doing.

[timholy]

I'm sufficiently swamped (and behind on other tasks due to prioritizing the debugger) that I don't think I'll get to this soon. @Keno, can I assign this to you? You obviously have the relevant expertise, whereas I do not.