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[mono] Optimize boxing in Pattern Matching idioms
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This PR optimizes boxing for pattern matching idioms. Basically, it copies existing optimizations from CoreCLR to mono, see [impBoxPatternMatch](https://github.com/dotnet/runtime/blob/084dd1a5aae0fbe6ed4f9164ac4ee1c5f0b2398f/src/coreclr/src/jit/importer.cpp#L5856-L6032) (one of them was recently added by me for coreclr, see dotnet/runtime#1817)
#### 1) `box + isinst + brtrue/brfalse`  -->  `ldc.i4.0/1 + brtrue/brfalse`
```csharp
public static int Case1<T>(T t)
{
    if (t is int)
        return 42;
    return 0;
}
/*
    IL_0000: ldarg.0
    IL_0001: box !!T
    IL_0006: isinst [System.Private.CoreLib]System.Int32
    IL_000b: brfalse.s IL_0010
    IL_000d: ldc.i4.s 42
    IL_000f: ret
    IL_0010: ldc.i4.0
    IL_0011: ret
*/
```
#### 2) `box + isinst + ldnull + ceq/cgt.un`  -->  `ldc.i4.0/1`
```csharp
public static bool Case2<T>(T t)
{
    return t is int;
}
/*
    IL_0000: ldarg.0
    IL_0001: box !!T
    IL_0006: isinst [System.Private.CoreLib]System.Int32
    IL_000b: ldnull
    IL_000c: cgt.un
    IL_000e: ret
*/
```
#### 3) `box + isinst + unbox.any`  -->  `nop`
```csharp
public static int Case3_1<T>(T o)
{
    if (o is int x)
        return x;
    return 0;
}

// or

public static int Case3_2<T>(T o) => o is int n ? n : 42;

// or

public static int Case3_3<T>(T o)
{
    return o switch
    {
        int n => n,
        string str => str.Length,
        _ => 0
    };
}
```
Tests can be found here: https://github.com/dotnet/runtime/tree/master/src/coreclr/tests/src/JIT/Generics/Conversions/Boxing

[Sharplab.io link](https://sharplab.io/#v2:EYLgtghgzgLgpgJwDQxASwDYB8CwAoAAQAYACAZQAsIEAHAGQmADoAlAVwDsY0w4BufPgIBmEgQCMANjEAmEgGF8Ab3wk1Y0ROlouC6HHEAeACoA+ABTGSMAJSr1KvOuck0AMxLmYrqK652nFyCCAHYSABYZAUDgsKJo5wBfezUUjTEpEmAAe2yMPSg4GRMLK1s0xyC1UOsfPxgE9WSYkjSRDO1deX1hAH0jM0sSbIDnSqr3T2y6nW8AD1Gq5xq5xtiSeLTmts1M2YK4PuLBqxGSAF5TYZndDhIAfhI7kAiowRb2rXqDvuESoZGFTSyzC0ygAHc0DAAMYUYFVcZLIL7O6XJ5IeFIjKkWAIC5XXFMOhwDgAcxgFAxLSxal6+I2mJciTWJGaznwiSAA===) to observe IL code for the samples above.

#### Codegen diff example:
```csharp
static bool Test(int n) => Validate(n);

static bool Validate<T>(T t) => t is int;
```
Before (for `Test`):
```asm
0000000000000000	pushq	%r14
0000000000000002	pushq	%rbx
0000000000000003	pushq	%rax
0000000000000004	movl	%edi, %ebx
0000000000000006	movabsq	$0x7fdf9c5090c0, %rax
0000000000000010	movabsq	$0x7fdfad0268e8, %r14
000000000000001a	leaq	0x1be800(%r14), %rdi
0000000000000021	movl	$0x14, %esi
0000000000000026	callq	*(%rax)
0000000000000028	movl	%ebx, 0x10(%rax)
000000000000002b	movq	(%rax), %rax
000000000000002e	cmpq	%r14, (%rax)
0000000000000031	sete	%al
0000000000000034	addq	$0x8, %rsp
0000000000000038	popq	%rbx
0000000000000039	popq	%r14
000000000000003b	retq
```
After:
```asm
0000000000000000	movb	$0x1, %al
0000000000000002	retq
```
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@EgorBo
EgorBo committed Mar 4, 2020
1 parent e1d3b7b commit 4abc037
Showing 1 changed file with 94 additions and 0 deletions.
94 changes: 94 additions & 0 deletions mono/mini/method-to-ir.c
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Original file line number Diff line number Diff line change
Expand Up @@ -9017,6 +9017,100 @@ mono_method_to_ir (MonoCompile *cfg, MonoMethod *method, MonoBasicBlock *start_b
break;
}
}

guint32 isinst_tk = 0;
if ((ip = il_read_op_and_token (next_ip, end, CEE_ISINST, MONO_CEE_ISINST, &isinst_tk)) &&
ip_in_bb (cfg, cfg->cbb, ip)) {
MonoClass *isinst_class = mini_get_class (method, isinst_tk, generic_context);
if (!mono_class_is_nullable (klass) && !mono_class_is_nullable (isinst_class) &&
!mini_is_gsharedvt_variable_klass (klass) && !mini_is_gsharedvt_variable_klass (isinst_class) &&
!mono_class_is_open_constructed_type (m_class_get_byval_arg (klass)) &&
!mono_class_is_open_constructed_type (m_class_get_byval_arg (isinst_class))) {

// Optimize
//
// box
// isinst [Type]
// brfalse/brtrue
//
// to
//
// ldc.i4.0 (or 1)
// brfalse/brtrue
//
guchar* br_ip = NULL;
if ((br_ip = il_read_brtrue (ip, end, &target)) || (br_ip = il_read_brtrue_s (ip, end, &target)) ||
(br_ip = il_read_brfalse (ip, end, &target)) || (br_ip = il_read_brfalse_s (ip, end, &target))) {

gboolean isinst = mono_class_is_assignable_from_internal (isinst_class, klass);
next_ip = ip;
il_op = (MonoOpcodeEnum) (isinst ? CEE_LDC_I4_1 : CEE_LDC_I4_0);
EMIT_NEW_ICONST (cfg, ins, isinst ? 1 : 0);
ins->type = STACK_I4;
*sp++ = ins;
break;
}

// Optimize
//
// box
// isinst [Type]
// ldnull
// ceq/cgt.un
//
// to
//
// ldc.i4.0 (or 1)
//
guchar* ldnull_ip = NULL;
if ((ldnull_ip = il_read_op (ip, end, CEE_LDNULL, MONO_CEE_LDNULL)) && ip_in_bb (cfg, cfg->cbb, ldnull_ip)) {
gboolean is_eq = FALSE, is_neq = FALSE;
if ((ip = il_read_op (ldnull_ip, end, CEE_PREFIX1, MONO_CEE_CEQ)))
is_eq = TRUE;
else if ((ip = il_read_op (ldnull_ip, end, CEE_PREFIX1, MONO_CEE_CGT_UN)))
is_neq = TRUE;

if ((is_eq || is_neq) && ip_in_bb (cfg, cfg->cbb, ip) &&
!mono_class_is_nullable (klass) && !mini_is_gsharedvt_klass (klass)) {
gboolean isinst = mono_class_is_assignable_from_internal (isinst_class, klass);
next_ip = ip;
if (is_eq)
isinst = !isinst;
il_op = (MonoOpcodeEnum) (isinst ? CEE_LDC_I4_1 : CEE_LDC_I4_0);
EMIT_NEW_ICONST (cfg, ins, isinst ? 1 : 0);
ins->type = STACK_I4;
*sp++ = ins;
break;
}
}

// Optimize
//
// box
// isinst [Type]
// unbox.any
//
// to
//
// nop
//
guchar* unbox_ip = NULL;
guint32 unbox_token = 0;
if ((unbox_ip = il_read_unbox_any (ip, end, &unbox_token)) && ip_in_bb (cfg, cfg->cbb, unbox_ip)) {
MonoClass *unbox_klass = mini_get_class (method, unbox_token, generic_context);
CHECK_TYPELOAD (unbox_klass);
if (!mono_class_is_nullable (unbox_klass) &&
!mini_is_gsharedvt_klass (unbox_klass) &&
klass == isinst_class &&
klass == unbox_klass)
{
*sp++ = val;
next_ip = unbox_ip;
break;
}
}
}
}
#endif

gboolean is_true;
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