JIT: Improve interference checks for thrown exceptions and enable CSE/hoisting of GVM resolution helpers

[jakobbotsch]

• Introduce ExceptionSetFlags::UnknownException. This special value marks that an operation may throw an exception that we cannot say anything about. Consumers of flags must be specially aware about this exception type.
• Make GenTree::OperExceptions callable on all trees by changing arbitrary user calls to return the new UnknownException type.
• Switch OperMayThrow to be literally OperExceptions() != None
• Enhance precision about exceptions thrown by helper calls. Instead of just modelling "may throw" and "may not throw", we model the exact exceptions a helper may throw via ExceptionSetFlags.
• Add helper call properties for CORINFO_HELP_VIRTUAL_FUNC_PTR, CORINFO_HELP_GVMLOOKUP_FOR_SLOT, CORINFO_HELP_READYTORUN_VIRTUAL_FUNC_PTR. These do not mutate the heap and we use the new mechanism above to indicate that the only exception they can throw is NRE. They are also pure, so add a VN function so VN can model them. This allows us to CSE and hoist these helpers.
• Update SideEffectSet to track and precisely handle interference for exceptions. We can now reorder two operations if we know they can only throw the exact same exception.
• Generalize OptimizeCallIndirectTargetEvaluation; manual interference handling is no longer necessary, and we can use it for all calls.

Fix #86173

Example:

public static void Test(Base b)
{
    b.Foo<string>();
    b.Foo<int>();
    b.Foo<string>();
    b.Foo<int>();

    for (int i = 0; i < 10; i++)
        b.Foo<double>();
}

public abstract class Base
{
    public abstract void Foo<T>();
}

public class Derived : Base
{
    public override void Foo<T>()
    {
    }
}

Before:

G_M7145_IG02:
       mov      rcx, rbx
       mov      rsi, 0xD1FFAB1E      ; Program+Base
       mov      rdx, rsi
       mov      r8, 0xD1FFAB1E      ; token handle
       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
       mov      rcx, rbx
       call     rax
       mov      rcx, rbx
       mov      rdx, rsi
       mov      r8, 0xD1FFAB1E      ; token handle
       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
       mov      rcx, rbx
       call     rax
       mov      rcx, rbx
       mov      rdx, rsi
       mov      r8, 0xD1FFAB1E      ; token handle
       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
       mov      rcx, rbx
       call     rax
       mov      rcx, rbx
       mov      rdx, rsi
       mov      r8, 0xD1FFAB1E      ; token handle
       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
       mov      rcx, rbx
       call     rax
       mov      edi, 10
						;; size=119 bbWeight=1 PerfScore 20.50
G_M7145_IG03:
       mov      rcx, rbx
       mov      rdx, rsi
       mov      r8, 0xD1FFAB1E      ; token handle
       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
       mov      rcx, rbx
       call     rax
       dec      edi
       jne      SHORT G_M7145_IG03
						;; size=30 bbWeight=8 PerfScore 50.00

After:

G_M7145_IG02:
       mov      rcx, rbx
       mov      rsi, 0xD1FFAB1E      ; Program+Base
       mov      rdx, rsi
       mov      r8, 0xD1FFAB1E      ; token handle
       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
       mov      rcx, rbx
       mov      rdi, rax
       call     rdi
       mov      rcx, rbx
       mov      rdx, rsi
       mov      r8, 0xD1FFAB1E      ; token handle
       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
       mov      rcx, rbx
       mov      rbp, rax
       call     rbp
       mov      rcx, rbx
       call     rdi
       mov      rcx, rbx
       call     rbp
       mov      rcx, rbx
       mov      rdx, rsi
       mov      r8, 0xD1FFAB1E      ; token handle
       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
       mov      rsi, rax
       mov      edi, 10
						;; size=107 bbWeight=1 PerfScore 19.50
G_M7145_IG03:
       mov      rcx, rbx
       call     rsi
       dec      edi
       jne      SHORT G_M7145_IG03
						;; size=9 bbWeight=8 PerfScore 36.00

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS @EgorBo

Diffs. On xarch this allows more containment of indirections, and on all platforms it allows us to CSE/hoist GVM resolution helpers (most of the diffs of that comes from tests).

[Copilot]

Pull request overview

This PR improves the JIT's ability to optimize GVM (Generic Virtual Method) resolution helpers by introducing more precise exception tracking and enabling CSE/hoisting optimizations.

Key changes:

• Introduces ExceptionSetFlags::UnknownException to model operations with unknown exception types
• Extends exception modeling to all helper calls with precise exception sets instead of just "may throw" vs "may not throw"
• Marks CORINFO_HELP_VIRTUAL_FUNC_PTR, CORINFO_HELP_GVMLOOKUP_FOR_SLOT, and CORINFO_HELP_READYTORUN_VIRTUAL_FUNC_PTR as pure with only NRE exceptions, enabling CSE/hoisting

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/coreclr/jit/valuenumfuncs.h	Adds VN function definitions for the three GVM resolution helpers
src/coreclr/jit/valuenum.cpp	Implements VN function handling and precise NRE exception modeling for GVM helpers; refactors fgValueNumberIndirNullCheckExceptions for reuse
src/coreclr/jit/utils.h	Moves ExceptionSetFlags enum and adds UnknownException value; updates HelperCallProperties to track precise exceptions
src/coreclr/jit/utils.cpp	Updates helper property initialization to use ExceptionSetFlags; marks GVM helpers as pure with NRE only
src/coreclr/jit/sideeffects.h	Adds m_preciseExceptions field to track precise exception sets
src/coreclr/jit/sideeffects.cpp	Implements precise exception interference checking - allows reordering when both sides throw the exact same single exception
src/coreclr/jit/morph.cpp	Swaps argument order to match new helper signature; updates exception interference checks for unknown exceptions
src/coreclr/jit/lower.cpp	Generalizes OptimizeCallIndirectTargetEvaluation to work with any call target; removes manual NRE handling now handled by SideEffectSet
src/coreclr/jit/gentree.h	Updates OperEffects signature to output precise exceptions; removes duplicate ExceptionSetFlags definition
src/coreclr/jit/gentree.cpp	Makes OperExceptions callable on all nodes including calls; simplifies OperMayThrow to use OperExceptions; updates OperEffects to populate precise exceptions
src/coreclr/jit/forwardsub.cpp	Updates exception accumulation to handle UnknownException and stop tracking once multiple/unknown exceptions are seen
src/coreclr/jit/compiler.h	Adds declaration for fgValueNumberIndirNullCheckExceptions