[llvm] Fix assert in SlotIndexes::getInstructionIndex in debug builds (PR #189370)

[via llvm-commits]

https://github.com/MarcoCastorina created https://github.com/llvm/llvm-project/pull/189370

In some cases, when debug info is enabled, the assertion in SlotIndexes::getInstructionIndex fires as the interval contains only debug instructions. This change ensures only valid instructions are taken into account.

>From 14a3f05692e51257b2f4b0a464aeb4c5e9464d60 Mon Sep 17 00:00:00 2001
From: "Castorina, Marco" <marco.castorina at amd.com>
Date: Mon, 30 Mar 2026 12:32:13 +0100
Subject: [PATCH] Fix assert in SlotIndexes::getInstructionIndex in debug
 builds

In some cases, when debug info is enabled, the assertion in
SlotIndexes::getInstructionIndex fires as the interval contains only debug
instructions. This change ensures only valid instructions are taken into
account.
---
 llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp b/llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp
index 5fd0c1e1064cb..f67b5cd65ec92 100644
--- a/llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp
+++ b/llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp
@@ -1174,12 +1174,15 @@ void SIWholeQuadMode::toExact(MachineBasicBlock &MBB,
                               Register SaveWQM) {
   assert(LiveMaskReg.isVirtual());
 
-  bool IsTerminator = Before == MBB.end();
+  auto MBBE = MBB.end();
+  bool IsTerminator = Before == MBBE;
   if (!IsTerminator) {
     auto FirstTerm = MBB.getFirstTerminator();
-    if (FirstTerm != MBB.end()) {
-      SlotIndex FirstTermIdx = LIS->getInstructionIndex(*FirstTerm);
-      SlotIndex BeforeIdx = LIS->getInstructionIndex(*Before);
+    auto FirstNonDbg = skipDebugInstructionsForward(FirstTerm, MBBE);
+    if (FirstNonDbg != MBBE) {
+      auto BeforeNonDbg = skipDebugInstructionsForward(Before, MBBE);
+      SlotIndex FirstTermIdx = LIS->getInstructionIndex(*FirstNonDbg);
+      SlotIndex BeforeIdx = LIS->getInstructionIndex(*BeforeNonDbg);
       IsTerminator = BeforeIdx > FirstTermIdx;
     }
   }