dotnet/roslyn

Remove unneccessary instructions from variable-less filter

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#4,388 opened on Aug 6, 2015

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Area-CompilersConcept-Continuous ImprovementFeature Requesthelp wanted

Description

When an exception filter doesn’t specify the exception variable, for example:

            try   {   …   }  catch when (F())  {  …   }

we emit

filter
  {
    IL_0006:  isinst     [mscorlib]System.Object
    IL_000b:  dup
    IL_000c:  brtrue.s   IL_0012
    IL_000e:  pop
    IL_000f:  ldc.i4.0
    IL_0010:  br.s       IL_001b
    IL_0012:  pop
    IL_0013:  call       bool ConsoleApplication1.Program::F()
    IL_0018:  ldc.i4.0
    IL_0019:  cgt.un
    IL_001b:  endfilter
  }  // end filter

The Object type check is unnecessary. The following IL sequence is verifiable and works equally well imo:

filter
  {
    IL_0012:  pop
    IL_0013:  call       bool ConsoleApplication1.Program::F()
    IL_0018:  ldc.i4.0
    IL_0019:  cgt.un
    IL_001b:  endfilter
  }  // end filter

Avoiding implicit reference to System.Object would simplify some low-level .NET Native scenarios that are layered below CoreFX and thus can't have a dependency on System.Object.

Contributor guide

Tech stack
csharp
Domain
tooling
Issue type
refactor
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of C# compiler ILFamiliarity with Roslyn's code generation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Look into the IL emission code for catch clauses, likely in the CodeGen or ConversionCodeGen files. Find where the 'isinst' and 'dup' instructions are emitted for filter clauses without a variable. Remove the unnecessary Object type check and ensure the IL remains verifiable. Check if there are any existing tests related to filter clauses.