JIT: Fold type checks with GDV

[EgorBo]

static A GetAObj() => new C();

[MethodImpl(MethodImplOptions.NoInlining)]
static int Test() => GetAObj().GetValue();


public class A
{
    public virtual int GetValue() => 1;
}

public sealed class C : A
{
    public override int GetValue() => 3;
}

Current codegen for Test (PGO, GDV):

G_M58954_IG01:    
       4883EC28             sub      rsp, 40
G_M58954_IG02:            
       48B9C084615BFE7F0000 mov      rcx, 0x7FFE5B6184C0
       E85DCB545F           call     CORINFO_HELP_NEWSFAST
       488BC8               mov      rcx, rax
       48B8C084615BFE7F0000 mov      rax, 0x7FFE5B6184C0
       483901               cmp      qword ptr [rcx], rax ;; this check should be folded and fallback is removed
       750A                 jne      SHORT G_M58954_IG04
       B803000000           mov      eax, 3
G_M58954_IG03:      
       4883C428             add      rsp, 40
       C3                   ret      
G_M58954_IG04:         
       FF157B301700         call     [C:GetValue():int:this]
       EBF3                 jmp      SHORT G_M58954_IG03

After inlining we know the exact type of the GetAObj, so we should fold this:

***** BB01
STMT00009 (IL 0x000...  ???)
N005 ( 16, 16) [000029] -AC-----R---              *  ASG       ref   
N004 (  1,  1) [000028] D------N----              +--*  LCL_VAR   ref    V04 tmp4         d:1
N003 ( 16, 16) [000027] --C---------              \--*  CALL help ref    HELPER.CORINFO_HELP_NEWSFAST
N002 (  2, 10) [000026] H----------- arg0 in rcx     \--*  CNS_INT(h) long   0x7ffe5b628668 method

***** BB01
STMT00001 (IL   ???...  ???)
N003 (  1,  3) [000004] -A------R---              *  ASG       ref   
N002 (  1,  1) [000003] D------N----              +--*  LCL_VAR   ref    V01 tmp1         d:1
N001 (  1,  1) [000032] ------------              \--*  LCL_VAR   ref    V04 tmp4         u:1

***** BB01
STMT00004 (IL   ???...  ???)
N005 (  8, 15) [000013] ---X--------              *  JTRUE     void  
N004 (  6, 13) [000012] J--X---N----              \--*  NE        int   
N002 (  3,  2) [000010] #--X--------                 +--*  IND       long  
N001 (  1,  1) [000009] ------------                 |  \--*  LCL_VAR   ref    V04 tmp4         u:1 (last use)
N003 (  2, 10) [000011] H-----------                 \--*  CNS_INT(h) long   0x7ffe5b628668 class

We assign the exact class (B) to tmp4 which is then copied to tmp1 and checked is B (which is always true).

/cc @AndyAyersMS

[EgorBo]

Ah, actually it should be pretty simple to implement in morph:

New codegen:

G_M58954_IG01:              ;; offset=0000H
       4883EC28             sub      rsp, 40
						;; bbWeight=1    PerfScore 0.25
G_M58954_IG02:              ;; offset=0004H
       B803000000           mov      eax, 3
						;; bbWeight=1    PerfScore 0.25
G_M58954_IG03:              ;; offset=0009H
       4883C428             add      rsp, 40
       C3                   ret      
						;; bbWeight=1    PerfScore 1.25

So I'm going to mark it as 6.0 and will file a PR later once I finish active PRs.

[AndyAyersMS]

I wonder if we can catch this in GDV itself, re-checking the type of the this.

Also curious why existing opts can't clean this up.

[AndyAyersMS]

I wonder if we can catch this in GDV itself, re-checking the type of the this.

Not in this case, as the GDV expansion happens before inlining.

Also curious why existing opts can't clean this up.

Assertion prop doesn't propagate exact typeness for newobj. Seems like it should...?

VN can't recover the vtable identity from an indir fed by a newobj. Seems like it could...? (also maybe non-nullness...?)

***** BB01, STMT00009(before)
N005 ( 16, 16) [000029] -AC-----R---              *  ASG       ref   
N004 (  1,  1) [000028] D------N----              +--*  LCL_VAR   ref    V04 tmp4         d:1
N003 ( 16, 16) [000027] --C---------              \--*  CALL help ref    HELPER.CORINFO_HELP_NEWSFAST
N002 (  2, 10) [000026] H----------- arg0 in rcx     \--*  CNS_INT(h) long   0x7ffba176b058 method

N001 [000042]   ARGPLACE  => $c0 {c0}
N002 [000026]   CNS_INT(h) 0x7ffba176b058 method => $100 {Hnd const: 0x00007FFBA176B058}
VN of ARGPLACE tree [000042] updated to $100 {Hnd const: 0x00007FFBA176B058}
N003 [000027]   CALL help => $180 {JitNew($100, $140)}
N004 [000028]   LCL_VAR   V04 tmp4         d:1 => $180 {JitNew($100, $140)}
N005 [000029]   ASG       => $180 {JitNew($100, $140)}

...

***** BB01, STMT00004(before)
N005 (  8, 15) [000013] ---X--------              *  JTRUE     void  
N004 (  6, 13) [000012] J--X---N----              \--*  NE        int   
N002 (  3,  2) [000010] #--X--------                 +--*  IND       long  
N001 (  1,  1) [000009] ------------                 |  \--*  LCL_VAR   ref    V04 tmp4         u:1 (last use)
N003 (  2, 10) [000011] H-----------                 \--*  CNS_INT(h) long   0x7ffba176b058 class

N001 [000009]   LCL_VAR   V04 tmp4         u:1 (last use) => $180 {JitNew($100, $140)}
    VNForMapSelect($2, $180):ref returns $181 {$VN.ReadOnlyHeap[$180]}
    VNForMapSelect($2, $180):ref returns $181 {$VN.ReadOnlyHeap[$180]}
N002 [000010]   IND       => $183 {norm=$181 {$VN.ReadOnlyHeap[$180]}, exc=$182 {NullPtrExc($180)}}
N003 [000011]   CNS_INT(h) 0x7ffba176b058 class => $101 {Hnd const: 0x00007FFBA176B058}
N004 [000012]   NE        => $1c1 {norm=$1c0 {NE($181, $101)}, exc=$182 {NullPtrExc($180)}}

[AndyAyersMS]

Have a fix for this in AP value numbering ... looking at diffs.

[AndyAyersMS]

As I noted over in #52476 you should go ahead with a fix in morph too. The earlier we can clean up control flow, the better.