[mono] Support LLVM inline assembly. (#62558)

This change adds a convenience function, named `mono_llvm_inline_asm`,
that wraps `LLVMGetInlineAsm` and `LLVMBuildCall2`.

Inline assembly isn't used right now for normal code generation. But
this can be used to ease debugging and development by inserting
easy-to-find markers in the generated code. Towards that end, this
change also adds a convenience `LLVMTypeRef`, named `void_func_t`, that
denotes a function type that takes no parameters and does not return a
value.

Example:

    mono_llvm_inline_asm (builder, void_func_t, "int $$0x3", "", LLVM_ASM_SIDE_EFFECT, NULL, 0, "");

src/mono/mono/mini/mini-llvm-cpp.cpp

@@ -755,3 +755,26 @@ unsigned int
 mono_llvm_get_prim_size_bits (LLVMTypeRef type) {
 	return unwrap (type)->getPrimitiveSizeInBits ();
 }
+
+/*
+ * Inserts a call to a fragment of inline assembly.
+ *
+ * Return values correspond to output constraints. Parameter values correspond
+ * to input constraints. Example of use:
+ * mono_llvm_inline_asm (builder, void_func_t, "int $$0x3", "", LLVM_ASM_SIDE_EFFECT, NULL, 0, "");
+ */
+LLVMValueRef
+mono_llvm_inline_asm (LLVMBuilderRef builder, LLVMTypeRef type,
+	const char *asmstr, const char *constraints,
+	MonoLLVMAsmFlags flags, LLVMValueRef *args, unsigned num_args,
+	const char *name)
+{
+	const auto asmstr_len = strlen (asmstr);
+	const auto constraints_len = strlen (constraints);
+	const auto asmval = LLVMGetInlineAsm (type,
+		const_cast<char *>(asmstr), asmstr_len,
+		const_cast<char *>(constraints), constraints_len,
+		(flags & LLVM_ASM_SIDE_EFFECT) != 0, (flags & LLVM_ASM_ALIGN_STACK) != 0,
+		LLVMInlineAsmDialectATT);
+	return LLVMBuildCall2 (builder, type, asmval, args, num_args, name);
+}

src/mono/mono/mini/mini-llvm-cpp.h

@@ -220,6 +220,17 @@ mono_llvm_register_overloaded_intrinsic (LLVMModuleRef module, IntrinsicId id, L
 unsigned int
 mono_llvm_get_prim_size_bits (LLVMTypeRef type);
 
+typedef enum {
+	LLVM_ASM_SIDE_EFFECT = 1 << 0,
+	LLVM_ASM_ALIGN_STACK = 1 << 1,
+} MonoLLVMAsmFlags;
+
+LLVMValueRef
+mono_llvm_inline_asm (LLVMBuilderRef builder, LLVMTypeRef type,
+	const char *asmstr, const char *constraints,
+	MonoLLVMAsmFlags flags, LLVMValueRef *args, unsigned num_args,
+	const char *name);
+
 G_END_DECLS
 
 #endif /* __MONO_MINI_LLVM_CPP_H__ */  

src/mono/mono/mini/mini-llvm.c

@@ -318,6 +318,8 @@ static LLVMTypeRef sse_i1_t, sse_i2_t, sse_i4_t, sse_i8_t, sse_r4_t, sse_r8_t;
 static LLVMTypeRef v64_i1_t, v64_i2_t, v64_i4_t, v64_i8_t, v64_r4_t, v64_r8_t;
 static LLVMTypeRef v128_i1_t, v128_i2_t, v128_i4_t, v128_i8_t, v128_r4_t, v128_r8_t;
 
+static LLVMTypeRef void_func_t;
+
 static MonoLLVMModule *init_jit_module (void);
 
 static void emit_dbg_loc (EmitContext *ctx, LLVMBuilderRef builder, const unsigned char *cil_code);
@@ -12404,6 +12406,8 @@ mono_llvm_init (gboolean enable_jit)
 
 	intrins_id_to_intrins = g_hash_table_new (NULL, NULL);
 
+	void_func_t = LLVMFunctionType0 (LLVMVoidType (), FALSE);
+
 	if (enable_jit)
 		mono_llvm_jit_init ();
 }