dotnet/roslyn

Conditionnally skip over Add/Sub/Or when used with a ternary and constant zero

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#68,050 opened on May 2, 2023

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Area-CompilersCode Gen Qualityhelp wanted

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Description

When operations + - | (Add, Subtract, Bitwise-Or) are used with a ternary operator, in which one of the expression evaluates to a constant 0, I would expect the IL to just skip over that operation

(suggestion could be expanded to Multiply/Divide by ternary with constant 1, or Bitwise-And with ~0)

Version Used: sdk\7.0.203

Steps to Reproduce:

Here is my test code, with ChatInviteFlags being a [Flags] enum:

var ci = (ChatInviteFlags)int.Parse(Console.ReadLine());

int flags = 1024 |
	(ci.HasFlag(ChatInviteFlags.broadcast) ? 32 : 0) |
	(ci.HasFlag(ChatInviteFlags.pub) ? 64 : 0) |
	(ci.HasFlag(ChatInviteFlags.megagroup) ? 128 : 0);

Console.WriteLine(flags);

Expected Behavior: I would expect the OR operations to be optimized by skipping over them entirely when the ternary conditions are false:

	IL_0021: brfalse.s IL_0026
	IL_0023: ldc.i4.s 32
	IL_0025: or

Actual Behavior: Here is the IL currently generated:

	IL_0021: brtrue.s IL_0026
	IL_0023: ldc.i4.0
	IL_0024: br.s IL_0028
	IL_0026: ldc.i4.s 32
	IL_0028: or

Contributor guide

Research direction
Locate the IL emission code for conditional expressions in the Roslyn compiler, specifically when ternary operators produce constant zero, and modify it to skip the Add, Sub, Or operations.
Tech stack
csharp
Domain
compilers
Issue type
Performance
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.
1-2 days
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Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C#Roslyn codebaseIL basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50