Delay exceptions when copy-propagating an address into a StObj target - #3964
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siegfriedpammer wants to merge 1 commit into
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Delay exceptions when copy-propagating an address into a StObj target#3964siegfriedpammer wants to merge 1 commit into
siegfriedpammer wants to merge 1 commit into
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Below C# 7 ref locals are unavailable, so CopyPropagation is allowed to copy LdFlda/LdElema. When such a copy lands in a StObj target slot whose value is impure, it violates the invariant checked by StObj.CheckTargetSlot: C# computes the value to be stored before dereferencing the target, so the exception moves. ILInlining resolves the same conflict by marking the address as delayed rather than falling back to a ref local; copy propagation now does the same, which keeps the generated code unchanged and only repairs the IL. Refusing the propagation instead would emit a ref local at a language version that cannot express one. The relaxation is bound to the setting that gates InliningOptions.AllowChangingOrderOfEvaluationForExceptions, and the check sits in DoPropagate rather than CanPerformCopyPropagation because the public Propagate() entry point bypasses the latter -- AsyncAwaitDecompiler copies an ldflda of the builder field through it irrespective of the setting. Assisted-by: Claude:claude-opus-5:Claude Code
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Decompiling with a language version below C# 7 can trip a debug assertion in
StObj.CheckTargetSlot:Reproduces on the existing test assemblies, e.g.
CompoundAssignmentTest.ShortUnsignedRightShiftTestat C# 6 and below, andDynamicTests.StructMemberAccessat C# 3 and below. Minimal case:CanPerformCopyPropagationpermits copyingLdFlda/LdElemawhenUseRefLocalsForAccurateOrderOfEvaluationis off, butDoPropagateputs the clone into aStObjtarget slot without consultingStObj.SatisfiesSlotRestrictionForInlining, leavingstobj(ldflda F(ldloc C_0), <impure value>).ILInlining.FindLoadInNextalready handles the same conflict by settingDelayExceptionson the address instead of falling back to a ref local. Copy propagation now does the same, gated on the same setting that drivesInliningOptions.AllowChangingOrderOfEvaluationForExceptions. The check sits inDoPropagaterather thanCanPerformCopyPropagationbecause the publicPropagate()entry point bypasses the latter --AsyncAwaitDecompilercopies anldfldaof the builder field through it regardless of the setting.Decompiled output for
DynamicTestsandCompoundAssignmentTestat C# 3 and C# 6 is byte-identical before and after; only the IL invariant changes.New Ugly test
NoUnsignedRightShift(decompiled atLanguageVersion.CSharp6) fails on master and passes with the fix.Prepared by an AI agent (Claude) on Siegfried's behalf.