dotnet/runtime

JIT: Arm `IsContainableImmed` doesnt handle shifts

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#125578 opened on Mar 15, 2026

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area-CodeGen-coreclrhelp wantedoptimization

Description

GenTree* shiftConst = m_compiler->gtNewIconNode(31, TYP_INT);
GenTree* sar        = m_compiler->gtNewOperNode(GT_RSH, select->TypeGet(), relopOp1, shiftConst);
CheckImmedAndMakeContained(sar, shiftConst); // false

CheckImmedAndMakeContained calls into IsContainableImmed which on arm doesn't handle shifts and returns false so that shiftConst doesn't get contained even though it could.

The weird thing is that the higher level function ContainCheckNode handles it. That's because it doesn't actually call into CheckImmedAndMakeContained. Meanwhile for other nodes like add the higher level function does call into lower level CheckImmedAndMakeContained:

Add: ContainCheckNode -> ContainCheckBinary -> CheckImmedAndMakeContained -> IsContainableImmed -> MakeSrcContained

Rsh: ContainCheckNode -> ContainCheckShiftRotate -> shiftBy->IsCnsIntOrI -> MakeSrcContained

Maybe ContainCheckShiftRotate should call CheckImmedAndMakeContained which should handle shifts?

Contributor guide

Tech stack
cpp
Domain
backendperformance
Issue type
bug
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.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++JIT compilerARM assembly
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
The issue identifies that `IsContainableImmed` in `lowerarmarch.cpp` does not handle shift immediate operands, causing missed containment optimizations. The fix should extend `CheckImmedAndMakeContained` to support shift operations, ensuring that `ContainCheckShiftRotate` calls the appropriate containment logic. Review the existing `ContainCheckShiftRotate` and `CheckImmedAndMakeContained` functions, and modify the containment check to include shifts.