[SPIR-V] Add spv.gep support for ptrcast legal

Adds support the the spv.gep intrinsic to the spv ptrcast legalization
step. Those intrinsics are generated by the backend thus not
directly visible in the tests.
This is a pre-requisite to implement addrspacecast legalization for
logical SPIR-V.

Author: Keenuts

llvm/lib/Target/SPIRV/SPIRVLegalizePointerCast.cpp

@@ -170,6 +170,12 @@ class SPIRVLegalizePointerCast : public FunctionPass {
           DeadInstructions.push_back(Intrin);
           continue;
         }
+
+        if (Intrin->getIntrinsicID() == Intrinsic::spv_gep) {
+          GR->replaceAllUsesWith(CastedOperand, OriginalOperand,
+                                 /* DeleteOld= */ false);
+          continue;
+        }
       }
 
       llvm_unreachable("Unsupported ptrcast user. Please fix.");

llvm/test/CodeGen/SPIRV/pointers/getelementptr-downcast-struct.ll

@@ -0,0 +1,47 @@
+; RUN: llc -verify-machineinstrs -O0 -mtriple=spirv-unknown-vulkan-compute %s -o - | FileCheck %s --match-full-lines
+; RUN: %if spirv-tools %{ llc -O0 -mtriple=spirv-unknown-vulkan %s -o - -filetype=obj | spirv-val %}
+
+; CHECK-DAG:       %[[#uint:]] = OpTypeInt 32 0
+; CHECK-DAG:     %[[#uint64:]] = OpTypeInt 64 0
+; CHECK-DAG:    %[[#uint_pp:]] = OpTypePointer Private %[[#uint]]
+; CHECK-DAG:     %[[#uint_0:]] = OpConstant %[[#uint]] 0
+; CHECK-DAG:     %[[#uint_1:]] = OpConstant %[[#uint]] 1
+; CHECK-DAG:    %[[#uint_10:]] = OpConstant %[[#uint]] 10
+; CHECK-DAG:    %[[#t_array:]] = OpTypeArray %[[#uint]] %[[#uint_10]]
+; CHECK-DAG:       %[[#t_s1:]] = OpTypeStruct %[[#t_array]]
+; CHECK-DAG:       %[[#t_s2_s_a_s:]] = OpTypeStruct %[[#uint]] %[[#uint]]
+; CHECK-DAG:       %[[#t_s2_s_a:]] = OpTypeArray %[[#t_s2_s_a_s]] %[[#uint_10]]
+; CHECK-DAG:       %[[#t_s2_s:]] = OpTypeStruct %[[#t_s2_s_a]]
+; CHECK-DAG:       %[[#t_s2:]] = OpTypeStruct %[[#t_s2_s]] %[[#uint]]
+; CHECK-DAG:     %[[#null_s1:]] = OpConstantNull %[[#t_s1]]
+; CHECK-DAG:     %[[#null_s2:]] = OpConstantNull %[[#t_s2]]
+; CHECK-DAG:  %[[#ptr_s1:]] = OpTypePointer Private %[[#t_s1]]
+; CHECK-DAG:  %[[#ptr_s2:]] = OpTypePointer Private %[[#t_s2]]
+
+%S1 = type { [10 x i32] }
+%S2 = type { { [10 x { i32, i32 } ] }, i32 }
+
+; CHECK-DAG: %[[#global1:]] = OpVariable %[[#ptr_s1]] Private %[[#null_s1]]
+@global1 = internal addrspace(10) global %S1 zeroinitializer
+; CHECK-DAG: %[[#global2:]] = OpVariable %[[#ptr_s2]] Private %[[#null_s2]]
+@global2 = internal addrspace(10) global %S2 zeroinitializer
+
+define spir_func noundef i32 @foo(i64 noundef %index) local_unnamed_addr {
+; CHECK: %[[#index:]] = OpFunctionParameter %[[#uint64]]
+entry:
+; CHECK: %[[#ptr:]] = OpInBoundsAccessChain %[[#uint_pp]] %[[#global1]] %[[#uint_0]] %[[#index]]
+  %ptr = getelementptr inbounds %S1, ptr addrspace(10) @global1, i64 0, i32 0, i64 %index
+; CHECK: %[[#val:]] = OpLoad %[[#uint]] %[[#ptr]] Aligned 4
+  %val = load i32, ptr addrspace(10) %ptr
+  ret i32 %val
+}
+
+define spir_func noundef i32 @bar(i64 noundef %index) local_unnamed_addr {
+; CHECK: %[[#index:]] = OpFunctionParameter %[[#uint64]]
+entry:
+; CHECK: %[[#ptr:]] = OpInBoundsAccessChain %[[#uint_pp]] %[[#global2]] %[[#uint_0]] %[[#uint_0]] %[[#index]] %[[#uint_1]]
+  %ptr = getelementptr inbounds %S2, ptr addrspace(10) @global2, i64 0, i32 0, i32 0, i64 %index, i32 1
+; CHECK: %[[#val:]] = OpLoad %[[#uint]] %[[#ptr]] Aligned 4
+  %val = load i32, ptr addrspace(10) %ptr
+  ret i32 %val
+}