JIT: Handle some "field offset computation" patterns

[jakobbotsch]

Both during local morph and during VN.

Fix#40021

Saves 3 KB on BasicMinimalApi after #84095 (there's 1111 __GetFieldHelper functions). About 0.04%. There's still a null check kept for each offset computation, which we cannot really get rid of, but NAOT could maybe emit the IL such that there is a dominating null check so that only one is emitted.

Example:
Base:

.managed:0000000140347CC0 loc_140347CC0: ; CODE XREF: S_P_CoreLib_System_Collections_Generic_KeyValuePair_2_System_Net_Security_System_Net_Security_SslSessionsCache_SslCredKey__System___Canon_____GetFieldHelper+23↑j.managed:0000000140347CC0 ; DATA XREF: .rdata:__readonlydata_S_P_CoreLib_System_Collections_Generic_KeyValuePair_2_System_Net_Security_System_Net_Security_SslSessionsCache_SslCredKey__System___Canon_____GetFieldHelper↓o.managed:0000000140347CC0 learax, ??_7Boxed_System_Net_Security_System_Net_Security_SslSessionsCache_SslCredKey@@6B@ ; jumptable 0000000140347CB3 case 0.managed:0000000140347CC7 mov[r9],rax.managed:0000000140347CCA cmp[rcx],cl.managed:0000000140347CCC learax,[rcx+8].managed:0000000140347CD0 subrax,rcx.managed:0000000140347CD3 addrsp,8.managed:0000000140347CD7 retn

Diff:

.managed:0000000140347AA0 loc_140347AA0: ; CODE XREF: S_P_CoreLib_System_Collections_Generic_KeyValuePair_2_System_Net_Security_System_Net_Security_SslSessionsCache_SslCredKey__System___Canon_____GetFieldHelper+23↑j.managed:0000000140347AA0 ; DATA XREF: .rdata:__readonlydata_S_P_CoreLib_System_Collections_Generic_KeyValuePair_2_System_Net_Security_System_Net_Security_SslSessionsCache_SslCredKey__System___Canon_____GetFieldHelper↓o.managed:0000000140347AA0 learax, ??_7Boxed_System_Net_Security_System_Net_Security_SslSessionsCache_SslCredKey@@6B@ ; jumptable 0000000140347A93 case 0.managed:0000000140347AA7 mov[r9],rax.managed:0000000140347AAA cmp[rcx],cl.managed:0000000140347AAC moveax,8.managed:0000000140347AB1 addrsp,8.managed:0000000140347AB5 retn

Local morph changes handle the pattern for local structs -- VN changes handle the pattern for classes (and more complicated struct cases, like storing them in locals, which there are a few examples of in #40021).

[ghost]

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Issue Details

---

Both during import and during VN. Just want to see TP impact, not sure if this is worth it.

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

[MichalStrehovsky]

Would this help in situation like #9791? NativeAOT generates similar code in support of ValueType.Equals/GetHashCode. The code RyuJIT currently generates for it is not particularly compact:

runtime/src/coreclr/nativeaot/System.Private.CoreLib/src/System/ValueType.cs

Lines 37 to 48 in 252018c

	// An override of this method will be injected by the compiler into all valuetypes that cannot be compared
	// using a simple memory comparison.
	// This API is a bit awkward because we want to avoid burning more than one vtable slot on this.
	// When index == GetNumFields, this method is expected to return the number of fields of this
	// valuetype. Otherwise, it returns the offset and type handle of the index-th field on this type.
	internalvirtualint__GetFieldHelper(intindex,outEETypePtreeType)
	{
	// Value types that don't override this method will use the fast path that looks at bytes, not fields.
	Debug.Assert(index==GetNumFields);
	eeType=default;
	returnUseFastHelper;
	}

[jakobbotsch]

Would this help in situation like #9791?

Yes, this should handle those kinds of patterns.

[ghost]

Draft Pull Request was automatically closed for 30 days of inactivity. Please let us know if you'd like to reopen it.

[jakobbotsch]

Reopening this given the additional motivation of #84095

[jakobbotsch]

Opened #84291 for this.

[jakobbotsch]

Diffs. This is primarily for NAOT so doesn't really reflect in the diffs. BasicMinimalApi has 1111 injected __GetFieldHelper methods that use this pattern.

TP cost is really from the local morph changes:

Base:99417650982, Diff:99454354685, +0.0369%?PostOrderVisit@LocalAddressVisitor@@QEAA?AW4fgWalkResult@Compiler@@PEAPEAUGenTree@@PEAU4@@Z:31130610:+6.61%:73.44%:+0.0313%?EvalUsingMathIdentity@ValueNumStore@@AEAAIW4var_types@@W4VNFunc@@II@Z:6168480:+6.05%:14.55%:+0.0062%
`ValueNumStore::EvalUsingMathIdentity'::`2'::<lambda_2>::operator() :860150:NA:2.03%:+0.0009%?fgPerBlockLocalVarLiveness@Compiler@@QEAAXXZ:174224:+0.04%:0.41%:+0.0002%?fgMorphSmpOp@Compiler@@AEAAPEAUGenTree@@PEAU2@PEAUMorphAddrContext@1@PEA_N@Z:141174:+0.01%:0.33%:+0.0001%?gtSetEvalOrder@Compiler@@QEAAIPEAUGenTree@@@Z:115047:+0.01%:0.27%:+0.0001%GenTreeVisitor<`Compiler::optCSE_GetMaskData'::`2'::MaskDataWalker>::WalkTree:59296:+0.14%:0.14%:+0.0001%GenTreeVisitor<`Compiler::fgSetTreeSeq'::`2'::SetTreeSeqVisitor>::WalkTree:57562:+0.01%:0.14%:+0.0001%?fgMorphTree@Compiler@@QEAAPEAUGenTree@@PEAU2@PEAUMorphAddrContext@1@@Z:51860:+0.01%:0.12%:+0.0001%
memset :-2570677:-0.34%:6.06%:-0.0026%

Haven't really checked more in detail.

cc @dotnet/jit-contrib PTAL @EgorBo