YJIT: Support splat calls to C methods with -1 arity

Usually we deal with splats by speculating that they're of a specific
size. In this case, the C method takes a pointer and a length, so
we can support changing sizes just fine.

bootstraptest/test_yjit.rb

@@ -4634,3 +4634,27 @@ def splat_nil_kw_splat(args) = identity(*args, **nil)
   empty = []
   [1.abs(*empty), 1.abs(**nil), 1.bit_length(*empty, **nil)]
 }
+
+# splat into C methods with -1 arity
+assert_equal '[[1, 2, 3], [0, 2, 3], [1, 2, 3], [2, 2, 3], [], [], [{}]]', %q{
+  class Foo < Array
+    def push(args) = super(1, *args)
+  end
+
+  def test_cfunc_vargs_splat(sub_instance, array_class, empty_kw_hash)
+    splat = [2, 3]
+    kw_splat = [empty_kw_hash]
+    [
+      sub_instance.push(splat),
+      array_class[0, *splat, **nil],
+      array_class[1, *splat, &nil],
+      array_class[2, *splat, **nil, &nil],
+      array_class.send(:[], *kw_splat),
+      # kw_splat disables keywords hash handling
+      array_class[*kw_splat],
+      array_class[*kw_splat, **nil],
+    ]
+  end
+
+  test_cfunc_vargs_splat(Foo.new, Array, Hash.ruby2_keywords_hash({}))
+}

yjit.c

@@ -890,6 +890,51 @@ rb_yjit_ruby2_keywords_splat_p(VALUE obj)
     return FL_TEST_RAW(last, RHASH_PASS_AS_KEYWORDS);
 }
 
+// Checks to establish preconditions for rb_yjit_splat_varg_cfunc()
+VALUE
+rb_yjit_splat_varg_checks(VALUE *sp, VALUE splat_array, rb_control_frame_t *cfp)
+{
+    if (!RB_TYPE_P(splat_array, T_ARRAY)) return Qfalse;
+    long len = RARRAY_LEN(splat_array);
+
+    // Large splat arrays need a separate allocation
+    if (len < 0 || len > VM_ARGC_STACK_MAX) return Qfalse;
+
+    // Would we overflow if we put the contents of the array onto the stack?
+    if (sp + len > (VALUE *)(cfp - 2)) return Qfalse;
+
+    return Qtrue;
+}
+
+// Push array elements to the stack for a C method that has a variable number
+// of parameters. Returns the number of arguments the splat array contributes.
+int
+rb_yjit_splat_varg_cfunc(VALUE *stack_splat_array, bool sole_splat)
+{
+    VALUE splat_array = *stack_splat_array;
+    int len;
+
+    // We already checked that length fits in `int`
+    RUBY_ASSERT(RB_TYPE_P(splat_array, T_ARRAY));
+    len = (int)RARRAY_LEN(splat_array);
+
+    // If this is a splat call without any keyword arguments, exclude the
+    // ruby2_keywords hash if it's empty
+    if (sole_splat && len > 0) {
+        VALUE last_hash = RARRAY_AREF(splat_array, len - 1);
+        if (RB_TYPE_P(last_hash, T_HASH) &&
+                FL_TEST_RAW(last_hash, RHASH_PASS_AS_KEYWORDS) &&
+                RHASH_EMPTY_P(last_hash)) {
+            len--;
+        }
+    }
+
+    // Push the contents of the array onto the stack
+    MEMCPY(stack_splat_array, RARRAY_CONST_PTR(splat_array), VALUE, len);
+
+    return len;
+}
+
 // Print the Ruby source location of some ISEQ for debugging purposes
 void
 rb_yjit_dump_iseq_loc(const rb_iseq_t *iseq, uint32_t insn_idx)

yjit/bindgen/src/main.rs

@@ -460,6 +460,8 @@ fn main() {
         .allowlist_function("rb_vm_base_ptr")
         .allowlist_function("rb_ec_stack_check")
         .allowlist_function("rb_vm_top_self")
+        .allowlist_function("rb_yjit_splat_varg_checks")
+        .allowlist_function("rb_yjit_splat_varg_cfunc")
 
         // We define VALUE manually, don't import it
         .blocklist_type("VALUE")

yjit/src/codegen.rs

@@ -6140,18 +6140,16 @@ fn gen_send_cfunc(
     let cfunc_argc = unsafe { get_mct_argc(cfunc) };
     let mut argc = argc;
 
+    // Splat call to a C method that takes `VALUE *` and `len`
+    let variable_splat = flags & VM_CALL_ARGS_SPLAT != 0 && cfunc_argc == -1;
+    let block_arg = flags & VM_CALL_ARGS_BLOCKARG != 0;
+
     // If the function expects a Ruby array of arguments
     if cfunc_argc < 0 && cfunc_argc != -1 {
         gen_counter_incr(asm, Counter::send_cfunc_ruby_array_varg);
         return None;
     }
 
-    // We aren't handling a vararg cfuncs with splat currently.
-    if flags & VM_CALL_ARGS_SPLAT != 0 && cfunc_argc == -1 {
-        gen_counter_incr(asm, Counter::send_args_splat_cfunc_var_args);
-        return None;
-    }
-
     exit_if_kwsplat_non_nil(asm, flags, Counter::send_cfunc_kw_splat_non_nil)?;
     let kw_splat = flags & VM_CALL_KW_SPLAT != 0;
 
@@ -6217,6 +6215,16 @@ fn gen_send_cfunc(
     asm.cmp(CFP, stack_limit);
     asm.jbe(Target::side_exit(Counter::guard_send_se_cf_overflow));
 
+    // Guard for variable length splat call before any modifications to the stack
+    if variable_splat {
+        asm_comment!(asm, "guard variable length splat call servicable");
+        let sp = asm.ctx.sp_opnd(0);
+        let splat_array = asm.stack_opnd(i32::from(kw_splat) + i32::from(block_arg));
+        let proceed = asm.ccall(rb_yjit_splat_varg_checks as _, vec![sp, splat_array, CFP]);
+        asm.cmp(proceed, Qfalse.into());
+        asm.je(Target::side_exit(Counter::guard_send_cfunc_bad_splat_vargs));
+    }
+
     // Number of args which will be passed through to the callee
     // This is adjusted by the kwargs being combined into a hash.
     let mut passed_argc = if kw_arg.is_null() {
@@ -6242,7 +6250,6 @@ fn gen_send_cfunc(
         return None;
     }
 
-    let block_arg = flags & VM_CALL_ARGS_BLOCKARG != 0;
     let block_arg_type = if block_arg {
         Some(asm.ctx.get_opnd_type(StackOpnd(0)))
     } else {
@@ -6287,9 +6294,9 @@ fn gen_send_cfunc(
         argc -= 1;
     }
 
-    // push_splat_args does stack manipulation so we can no longer side exit
-    if flags & VM_CALL_ARGS_SPLAT != 0 {
-        assert!(cfunc_argc >= 0);
+    // Splat handling when C method takes a static number of arguments.
+    // push_splat_args() does stack manipulation so we can no longer side exit
+    if flags & VM_CALL_ARGS_SPLAT != 0 && cfunc_argc >= 0 {
         let required_args : u32 = (cfunc_argc as u32).saturating_sub(argc as u32 - 1);
         // + 1 because we pass self
         if required_args + 1 >= C_ARG_OPNDS.len() as u32 {
@@ -6312,15 +6319,34 @@ fn gen_send_cfunc(
         handle_opt_send_shift_stack(asm, argc);
     }
 
+    // Push a dynamic number of items from the splat array to the stack when calling a vargs method
+    let dynamic_splat_size = if variable_splat {
+        asm_comment!(asm, "variable length splat");
+        let just_splat = usize::from(!kw_splat && kw_arg.is_null()).into();
+        let stack_splat_array = asm.lea(asm.stack_opnd(0));
+        Some(asm.ccall(rb_yjit_splat_varg_cfunc as _, vec![stack_splat_array, just_splat]))
+    } else {
+        None
+    };
+
     // Points to the receiver operand on the stack
     let recv = asm.stack_opnd(argc);
 
     // Store incremented PC into current control frame in case callee raises.
     jit_save_pc(jit, asm);
 
-    // Increment the stack pointer by 3 (in the callee)
-    // sp += 3
-    let sp = asm.lea(asm.ctx.sp_opnd(SIZEOF_VALUE_I32 * 3));
+    // Find callee's SP with space for metadata.
+    // Usually sp+3.
+    let sp = if let Some(splat_size) = dynamic_splat_size {
+        // Compute the callee's SP at runtime in case we accept a variable size for the splat array
+        const _: () = assert!(SIZEOF_VALUE == 8, "opting for a shift since mul on A64 takes no immediates");
+        let splat_size_bytes = asm.lshift(splat_size, 3usize.into());
+        // 3 items for method metadata, minus one to remove the splat array
+        let static_stack_top = asm.lea(asm.ctx.sp_opnd(SIZEOF_VALUE_I32 * 2));
+        asm.add(static_stack_top, splat_size_bytes)
+    } else {
+        asm.lea(asm.ctx.sp_opnd(SIZEOF_VALUE_I32 * 3))
+    };
 
     let specval = if block_arg_type == Some(Type::BlockParamProxy) {
         SpecVal::BlockHandler(Some(BlockHandler::BlockParamProxy))
@@ -6382,8 +6408,17 @@ fn gen_send_cfunc(
     else if cfunc_argc == -1 {
         // The method gets a pointer to the first argument
         // rb_f_puts(int argc, VALUE *argv, VALUE recv)
+
+        let passed_argc_opnd = if let Some(splat_size) = dynamic_splat_size {
+            // The final argc is the size of the splat, minus one for the splat array itself
+            asm.add(splat_size, (passed_argc - 1).into())
+        } else {
+            // Without a splat, passed_argc is static
+            Opnd::Imm(passed_argc.into())
+        };
+
         vec![
-            Opnd::Imm(passed_argc.into()),
+            passed_argc_opnd,
             asm.lea(asm.ctx.sp_opnd(-argc * SIZEOF_VALUE_I32)),
             asm.stack_opnd(argc),
         ]

yjit/src/cruby_bindings.inc.rs

@@ -1194,6 +1194,15 @@ extern "C" {
     pub fn rb_yjit_fix_div_fix(recv: VALUE, obj: VALUE) -> VALUE;
     pub fn rb_yjit_fix_mod_fix(recv: VALUE, obj: VALUE) -> VALUE;
     pub fn rb_yjit_ruby2_keywords_splat_p(obj: VALUE) -> usize;
+    pub fn rb_yjit_splat_varg_checks(
+        sp: *mut VALUE,
+        splat_array: VALUE,
+        cfp: *mut rb_control_frame_t,
+    ) -> VALUE;
+    pub fn rb_yjit_splat_varg_cfunc(
+        stack_splat_array: *mut VALUE,
+        sole_splat: bool,
+    ) -> ::std::os::raw::c_int;
     pub fn rb_yjit_dump_iseq_loc(iseq: *const rb_iseq_t, insn_idx: u32);
     pub fn rb_yjit_iseq_inspect(iseq: *const rb_iseq_t) -> *mut ::std::os::raw::c_char;
     pub fn rb_FL_TEST(obj: VALUE, flags: VALUE) -> VALUE;

yjit/src/stats.rs

@@ -386,7 +386,6 @@ make_counters! {
     send_args_splat_aref,
     send_args_splat_aset,
     send_args_splat_opt_call,
-    send_args_splat_cfunc_var_args,
     send_iseq_splat_arity_error,
     send_splat_too_long,
     send_send_wrong_args,
@@ -444,6 +443,8 @@ make_counters! {
     guard_send_not_string,
     guard_send_respond_to_mid_mismatch,
 
+    guard_send_cfunc_bad_splat_vargs,
+
     guard_invokesuper_me_changed,
 
     guard_invokeblock_tag_changed,