[llvm] [MachinePipeliner] Fix use-after-free coping values of the same DenseMap (PR #130311)

[Vitaly Buka via llvm-commits]

https://github.com/vitalybuka created https://github.com/llvm/llvm-project/pull/130311

After #130165.

In the code: `VRMap[CurStageNum][Def] = VRMap[CurStageNum][LoopVal]`
`VRMap[CurStageNum][LoopVal]` calculates a reference before
`VRMap[CurStageNum][Def]` which may rehash the DenseMap.
Then the reference can be dead.


>From 6a2437677db8e104a26e2f4dfaf79ecac391ae4d Mon Sep 17 00:00:00 2001
From: Vitaly Buka <vitalybuka at google.com>
Date: Fri, 7 Mar 2025 09:33:27 -0800
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.4
---
 llvm/lib/CodeGen/ModuloSchedule.cpp | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/ModuloSchedule.cpp b/llvm/lib/CodeGen/ModuloSchedule.cpp
index c6d1a416407f2..04d07f4ef5428 100644
--- a/llvm/lib/CodeGen/ModuloSchedule.cpp
+++ b/llvm/lib/CodeGen/ModuloSchedule.cpp
@@ -410,8 +410,11 @@ void ModuloScheduleExpander::generateExistingPhis(
       Register NewReg = VRMap[PrevStage][LoopVal];
       rewriteScheduledInstr(NewBB, InstrMap, CurStageNum, 0, &*BBI, Def,
                             InitVal, NewReg);
-      if (VRMap[CurStageNum].count(LoopVal))
-        VRMap[CurStageNum][Def] = VRMap[CurStageNum][LoopVal];
+      auto It = VRMap[CurStageNum].find(LoopVal);
+      if (It != VRMap[CurStageNum].end()) {
+        llvm::Register Reg = It->second;
+        VRMap[CurStageNum][Def] = Reg;
+      }
     }
     // Adjust the number of Phis needed depending on the number of prologs left,
     // and the distance from where the Phi is first scheduled. The number of