[InstSimplify] Remove zero-index opaque pointer GEP

With opaque pointers, a zero-index GEP is a no-op. It does not
need to be retained for the pointer element type change it may
perform.

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4473,6 +4473,12 @@ static Value *SimplifyGEPInst(Type *SrcTy, Value *Ptr,
     }
   }
 
+  // For opaque pointers an all-zero GEP is a no-op. For typed pointers,
+  // it may be equivalent to a bitcast.
+  if (Ptr->getType()->isOpaquePointerTy() &&
+      all_of(Indices, [](const auto *V) { return match(V, m_Zero()); }))
+    return Ptr;
+
   // getelementptr poison, idx -> poison
   // getelementptr baseptr, poison -> poison
   if (isa<PoisonValue>(Ptr) ||

llvm/test/Transforms/InstSimplify/opaque-ptr.ll

@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -instsimplify -opaque-pointers < %s | FileCheck %s
+
+define ptr @gep_zero_indices(ptr %p) {
+; CHECK-LABEL: @gep_zero_indices(
+; CHECK-NEXT:    ret ptr [[P:%.*]]
+;
+  %p2 = getelementptr { i64, i64 }, ptr %p, i64 0, i32 0
+  ret ptr %p2
+}
+
+define ptr @gep_non_zero_indices1(ptr %p) {
+; CHECK-LABEL: @gep_non_zero_indices1(
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr { i64, i64 }, ptr [[P:%.*]], i64 0, i32 1
+; CHECK-NEXT:    ret ptr [[P2]]
+;
+  %p2 = getelementptr { i64, i64 }, ptr %p, i64 0, i32 1
+  ret ptr %p2
+}
+
+define ptr @gep_non_zero_indices2(ptr %p) {
+; CHECK-LABEL: @gep_non_zero_indices2(
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr { i64, i64 }, ptr [[P:%.*]], i64 1, i32 0
+; CHECK-NEXT:    ret ptr [[P2]]
+;
+  %p2 = getelementptr { i64, i64 }, ptr %p, i64 1, i32 0
+  ret ptr %p2
+}