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Inline Assembly With Julia / LLVM

From: andrew cooke <andrew@...>

Date: Fri, 15 May 2015 11:38:45 -0300

Note that this is LLVM IR assembly, not x64.  But you can access even very
specific instructions, like PCLMULQDQ

First, you need to build Julia from trunk with LLVM 3.6 (instructions from which
really has everything you need to know - thanks Isiah)

  git clone git:// julia-llvm-3.6
  cd julia-llvm-3.6
  echo "LLVM_VER=3.6.0" > Make.user
  make -j 3

You can then check that:

  pushd usr/bin
  ./llc -version

Now, test assembly inside julia:

  f(x::Int64,y::Int64) = Base.llvmcall("""
    %3 = add i64 %0, %1
    ret i64 %3""",
    Tuple{Int64, Int64},
    x, y)
  f(1, 2)

which should give 3.


  f() = Base.llvmcall("""
    %1 = add i64 1, 2
    ret i64 %1""",

(I have no idea why that needs to be %1 rather than %3, but %3 gives an

Trying to get beyond this, I tried to print a string.  But the reference
manual (link below) uses "declare", which llvmcall doesn't seem to like.  And
I can't get literal strings to work (see "array constants" in the manual,
which says c"hello world" should be ok).

For example,

  g() = Base.llvmcall("""
    call i32 (i8*, ...)* @printf(i8* c"hello world\0")
    Void, Tuple{})

gives "error: expected string" right where the string is, while trying to
define it previously, with something like "@s = ..." gives "error: expected
insruction opcode".

So I tried to look at the output from code_llvm, but that's not terribly
helpful either:

julia> function h()
         print("hello world")
h (generic function with 1 method)

julia> code_llvm(h, ())

define %jl_value_t* @julia_h_44607() {
  %0 = alloca [4 x %jl_value_t*], align 8
  %.sub = getelementptr inbounds [4 x %jl_value_t*]* %0, i64 0, i64 0
  %1 = getelementptr [4 x %jl_value_t*]* %0, i64 0, i64 2
  %2 = bitcast [4 x %jl_value_t*]* %0 to i64*
  store i64 4, i64* %2, align 8
  %3 = getelementptr [4 x %jl_value_t*]* %0, i64 0, i64 1
  %4 = bitcast %jl_value_t** %3 to %jl_value_t***
  %5 = load %jl_value_t*** @jl_pgcstack, align 8
  store %jl_value_t** %5, %jl_value_t*** %4, align 8
  store %jl_value_t** %.sub, %jl_value_t*** @jl_pgcstack, align 8
  store %jl_value_t* null, %jl_value_t** %1, align 8
  %6 = getelementptr [4 x %jl_value_t*]* %0, i64 0, i64 3
  store %jl_value_t* null, %jl_value_t** %6, align 8
  %7 = load %jl_value_t** inttoptr (i64 139727723511144 to %jl_value_t**),
  align 8
  %8 = icmp eq %jl_value_t* %7, null
  br i1 %8, label %err, label %ok

err:                                              ; preds = %top
  call void @jl_undefined_var_error(%jl_value_t* inttoptr (i64 139727711444264
  to %jl_value_t*))

ok:                                               ; preds = %top
  store %jl_value_t* %7, %jl_value_t** %1, align 8
  store %jl_value_t* inttoptr (i64 139727767335888 to %jl_value_t*),
  %jl_value_t** %6, align 8
  %9 = call %jl_value_t* @jl_apply_generic(%jl_value_t* inttoptr (i64
  139727730966960 to %jl_value_t*), %jl_value_t** %1, i32 2)
  %10 = load %jl_value_t*** %4, align 8
  store %jl_value_t** %10, %jl_value_t*** @jl_pgcstack, align 8
  ret %jl_value_t* %9

Which doesn't help!

Finally, documentation:

  * There's an exmaple involving tuples in test/llvmcall.jl

  * The structure of the call is given in src/ccall.cpp as
    llvmcall(ir, (rettypes...), (argtypes...), args...)

  * The language ref for 3.6 is at

  * I can't find anything on x86 intrinsics, however.


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