[llvm] [X86] Record the enclosed register in X86DomainReassignment::buildClosure (PR #202534)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: mbhade-amd

<details>
<summary>Changes</summary>


buildClosure recorded the seed register Reg in the function-wide EnclosedEdges map on every worklist iteration instead of CurReg, the register actually being added to the closure. EnclosedEdges therefore only ever contained the seed of each closure.

The driver loop in runOnMachineFunction skips registers already present in EnclosedEdges before starting a new closure. Because only seeds were recorded, every non-seed member of an already-built closure looked like a fresh seed, so a redundant closure was built for it and then immediately discarded by the EnclosedInstrs cross-closure check. The emitted code is unchanged; the pass just performed redundant work proportional to closure size.

Key EnclosedEdges by CurReg so each enclosed register is recorded once.

This was found as part of @<!-- -->jlebar's X86 LLVM bug hunt / FuzzX effort:
https://github.com/SemiAnalysisAI/FuzzX/tree/master/x86/bugs/007-domain-reassignment-wrong-enclosed-key

cc @<!-- -->jlebar

---
Full diff: https://github.com/llvm/llvm-project/pull/202534.diff


1 Files Affected:

- (modified) llvm/lib/Target/X86/X86DomainReassignment.cpp (+1-1) 


``````````diff
diff --git a/llvm/lib/Target/X86/X86DomainReassignment.cpp b/llvm/lib/Target/X86/X86DomainReassignment.cpp
index a9f68393aa3e2..dae1cd2640378 100644
--- a/llvm/lib/Target/X86/X86DomainReassignment.cpp
+++ b/llvm/lib/Target/X86/X86DomainReassignment.cpp
@@ -553,7 +553,7 @@ void X86DomainReassignmentImpl::buildClosure(Closure &C, Register Reg) {
     // Register already in this closure.
     if (!C.insertEdge(CurReg))
       continue;
-    EnclosedEdges[Reg] = C.getID();
+    EnclosedEdges[CurReg] = C.getID();
 
     MachineInstr *DefMI = MRI->getVRegDef(CurReg);
     if (!encloseInstr(C, DefMI))

``````````

</details>


https://github.com/llvm/llvm-project/pull/202534