[llvm] [InstCombine] Lower multi-dimensional GEP to ptradd (PR #150383)

[Usha Gupta via llvm-commits]

https://github.com/usha1830 created https://github.com/llvm/llvm-project/pull/150383


This change will help canonicalize multi-dimensional array geps  with exactly one variable index and all other indices constant into a flat, single-index gep over the element type. This would cover cases such as:

`%gep = getelementptr [9 x [9 x [9 x i32]]], ptr @arr, i64 0, i64 %i, i64 2, i64 3`




>From fbd143dc0ba7a90821de634a5787027f59a9babc Mon Sep 17 00:00:00 2001
From: Usha Gupta <usha.gupta at arm.com>
Date: Thu, 24 Jul 2025 07:26:14 +0000
Subject: [PATCH] [InstCombine] Lower multi-dimensional GEP to ptradd

---
 llvm/lib/Transforms/InstCombine/InstructionCombining.cpp | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index e2a9255ca9c6e..9b148e523b7a7 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2995,6 +2995,15 @@ static bool shouldCanonicalizeGEPToPtrAdd(GetElementPtrInst &GEP) {
                                  m_Shl(m_Value(), m_ConstantInt())))))
     return true;
 
+  // Flatten multidimensional GEPs with one variable index.
+  unsigned NumVarIndices = 0;
+  for (unsigned i = 1; i < GEP.getNumOperands(); ++i) {
+    if (!isa<ConstantInt>(GEP.getOperand(i)))
+      ++NumVarIndices;
+  }
+  if (NumVarIndices == 1)
+    return true;
+
   // gep (gep %p, C1), %x, C2 is expanded so the two constants can
   // possibly be merged together.
   auto PtrOpGep = dyn_cast<GEPOperator>(PtrOp);