JIT: Allow forwarding field accesses off of implicit byrefs #80852

jakobbotsch · 2023-01-19T15:08:03Z

The JIT currently allows forwarding implicit byrefs at their last uses to calls, but only if the full implicit byref is used. This change allows the JIT to forward any such access off of an implicit byref parameter.

For example:

using System.Runtime.CompilerServices;

class Program
{
    public static void Main()
    {
        Foo(default);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Foo(S1 s1)
    {
        Bar(s1.B);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Bar(S2 s)
    {
    }

    private struct S1
    {
        public int A;
        public S2 B;
    }

    private struct S2
    {
        public int C, D, E, F;
    }
}

Codegen before:

; Assembly listing for method Program:Foo(Program+S1)
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; Final local variable assignments
;
;  V00 arg0         [V00,T00] (  3,  6   )   byref  ->  rcx         single-def
;  V01 OutArgs      [V01    ] (  1,  1   )  lclBlk (32) [rsp+00H]   "OutgoingArgSpace"
;  V02 tmp1         [V02    ] (  2,  4   )  struct (16) [rsp+28H]   do-not-enreg[XS] addr-exposed "by-value struct argument"
;
; Lcl frame size = 56

G_M4574_IG01:              ;; offset=0000H
       4883EC38             sub      rsp, 56
       C5F877               vzeroupper
                                                ;; size=7 bbWeight=1 PerfScore 1.25
G_M4574_IG02:              ;; offset=0007H
       C5F8104104           vmovups  xmm0, xmmword ptr [rcx+04H]
       C5F811442428         vmovups  xmmword ptr [rsp+28H], xmm0
       488D4C2428           lea      rcx, [rsp+28H]
       FF1553A41C00         call     [Program:Bar(Program+S2)]
       90                   nop
                                                ;; size=23 bbWeight=1 PerfScore 8.75
G_M4574_IG03:              ;; offset=001EH
       4883C438             add      rsp, 56
       C3                   ret
                                                ;; size=5 bbWeight=1 PerfScore 1.25

; Total bytes of code 35, prolog size 7, PerfScore 14.75, instruction count 9, allocated bytes for code 35 (MethodHash=a8e6ee21) for method Program:Test(Program+S1)
; ============================================================

Codegen after:

; Assembly listing for method Program:Foo(Program+S1)
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; Final local variable assignments
;
;  V00 arg0         [V00,T00] (  3,  6   )   byref  ->  rcx         single-def
;  V01 OutArgs      [V01    ] (  1,  1   )  lclBlk (32) [rsp+00H]   "OutgoingArgSpace"
;
; Lcl frame size = 40

G_M4574_IG01:              ;; offset=0000H
       4883EC28             sub      rsp, 40
                                                ;; size=4 bbWeight=1 PerfScore 0.25
G_M4574_IG02:              ;; offset=0004H
       4883C104             add      rcx, 4
       FF1532A11C00         call     [Program:Bar(Program+S2)]
       90                   nop
                                                ;; size=11 bbWeight=1 PerfScore 3.50
G_M4574_IG03:              ;; offset=000FH
       4883C428             add      rsp, 40
       C3                   ret
                                                ;; size=5 bbWeight=1 PerfScore 1.25

; Total bytes of code 20, prolog size 4, PerfScore 7.00, instruction count 6, allocated bytes for code 20 (MethodHash=a8e6ee21) for method Program:Test(Program+S1)
; ============================================================

(The latter would also be tailcalled without NoInlining attribute)

The JIT currently allows forwarding implicit byrefs at their last uses to calls, but only if the full implicit byref is used. This change allows the JIT to forward any such access off of an implicit byref parameter.

ghost · 2023-01-19T15:08:21Z

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch, @kunalspathak
See info in area-owners.md if you want to be subscribed.

Issue Details

The JIT currently allows forwarding implicit byrefs at their last uses to calls, but only if the full implicit byref is used. This change allows the JIT to forward any such access off of an implicit byref parameter.

For example:

using System.Runtime.CompilerServices;

class Program
{
    public static void Main()
    {
        Foo(default);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Foo(S1 s1)
    {
        Bar(s1.B);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Bar(S2 s)
    {
    }

    private struct S1
    {
        public int A;
        public S2 B;
    }

    private struct S2
    {
        public int C, D, E, F;
    }
}

Codegen before:

; Assembly listing for method Program:Test(Program+S1)
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; Final local variable assignments
;
;  V00 arg0         [V00,T00] (  3,  6   )   byref  ->  rcx         single-def
;  V01 OutArgs      [V01    ] (  1,  1   )  lclBlk (32) [rsp+00H]   "OutgoingArgSpace"
;  V02 tmp1         [V02    ] (  2,  4   )  struct (16) [rsp+28H]   do-not-enreg[XS] addr-exposed "by-value struct argument"
;
; Lcl frame size = 56

G_M4574_IG01:              ;; offset=0000H
       4883EC38             sub      rsp, 56
       C5F877               vzeroupper
                                                ;; size=7 bbWeight=1 PerfScore 1.25
G_M4574_IG02:              ;; offset=0007H
       C5F8104104           vmovups  xmm0, xmmword ptr [rcx+04H]
       C5F811442428         vmovups  xmmword ptr [rsp+28H], xmm0
       488D4C2428           lea      rcx, [rsp+28H]
       FF1553A41C00         call     [Program:Bar(Program+S2)]
       90                   nop
                                                ;; size=23 bbWeight=1 PerfScore 8.75
G_M4574_IG03:              ;; offset=001EH
       4883C438             add      rsp, 56
       C3                   ret
                                                ;; size=5 bbWeight=1 PerfScore 1.25

; Total bytes of code 35, prolog size 7, PerfScore 14.75, instruction count 9, allocated bytes for code 35 (MethodHash=a8e6ee21) for method Program:Test(Program+S1)
; ============================================================

Codegen after:

; Assembly listing for method Program:Test(Program+S1)
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; Final local variable assignments
;
;  V00 arg0         [V00,T00] (  3,  6   )   byref  ->  rcx         single-def
;  V01 OutArgs      [V01    ] (  1,  1   )  lclBlk (32) [rsp+00H]   "OutgoingArgSpace"
;
; Lcl frame size = 40

G_M4574_IG01:              ;; offset=0000H
       4883EC28             sub      rsp, 40
                                                ;; size=4 bbWeight=1 PerfScore 0.25
G_M4574_IG02:              ;; offset=0004H
       4883C104             add      rcx, 4
       FF1532A11C00         call     [Program:Bar(Program+S2)]
       90                   nop
                                                ;; size=11 bbWeight=1 PerfScore 3.50
G_M4574_IG03:              ;; offset=000FH
       4883C428             add      rsp, 40
       C3                   ret
                                                ;; size=5 bbWeight=1 PerfScore 1.25

; Total bytes of code 20, prolog size 4, PerfScore 7.00, instruction count 6, allocated bytes for code 20 (MethodHash=a8e6ee21) for method Program:Test(Program+S1)
; ============================================================

(The latter would also be tailcalled without NoInlining attribute)

Author:	jakobbotsch
Assignees:	jakobbotsch
Labels:	`area-CodeGen-coreclr`
Milestone:	-

jakobbotsch · 2023-01-19T17:58:05Z

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Small set of diffs. The main benefit is that it makes the copy elision work consistently for both implicit byrefs and normal locals.

jakobbotsch · 2023-01-23T16:33:00Z

Ping @AndyAyersMS

…0852) The JIT currently allows forwarding implicit byrefs at their last uses to calls, but only if the full implicit byref is used. This change allows the JIT to forward any such access off of an implicit byref parameter.

JIT: Allow forwarding field accesses off of implicit byrefs

a5a8186

The JIT currently allows forwarding implicit byrefs at their last uses to calls, but only if the full implicit byref is used. This change allows the JIT to forward any such access off of an implicit byref parameter.

ghost assigned jakobbotsch Jan 19, 2023

dotnet-issue-labeler bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jan 19, 2023

Nit

21168b5

runfoapp bot mentioned this pull request Jan 19, 2023

Infra improvements for Helix #68176

Closed

jakobbotsch requested a review from AndyAyersMS January 19, 2023 17:58

This was referenced Jan 19, 2023

Tracking issue for CI build timeouts #76454

Closed

tracing/eventpipe/eventsourceerror/eventsourceerror/eventsourceerror failure #80666

Closed

jakobbotsch closed this Jan 23, 2023

jakobbotsch reopened this Jan 23, 2023

AndyAyersMS approved these changes Jan 23, 2023

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jakobbotsch merged commit 5fda6fd into dotnet:main Jan 23, 2023

jakobbotsch deleted the forward-more-implicit-byrefs branch January 23, 2023 16:43

ghost locked as resolved and limited conversation to collaborators Feb 22, 2023

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JIT: Allow forwarding field accesses off of implicit byrefs #80852

JIT: Allow forwarding field accesses off of implicit byrefs #80852

jakobbotsch commented Jan 19, 2023 •

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ghost commented Jan 19, 2023

jakobbotsch commented Jan 19, 2023 •

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jakobbotsch commented Jan 23, 2023

JIT: Allow forwarding field accesses off of implicit byrefs #80852

JIT: Allow forwarding field accesses off of implicit byrefs #80852

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