Optimize access of array member in a structure.

Author: shwqf

src/codegen/llvm.zig

@@ -5843,8 +5843,8 @@ pub const FuncGen = struct {
         const elem_ty = array_ty.childType();
         if (isByRef(array_ty)) {
             const indices: [2]*llvm.Value = .{ self.context.intType(32).constNull(), rhs };
-            const elem_ptr = self.builder.buildInBoundsGEP(array_llvm_ty, array_llvm_val, &indices, indices.len, "");
             if (isByRef(elem_ty)) {
+                const elem_ptr = self.builder.buildInBoundsGEP(array_llvm_ty, array_llvm_val, &indices, indices.len, "");
                 if (canElideLoad(self, body_tail))
                     return elem_ptr;
 
@@ -5865,11 +5865,13 @@ pub const FuncGen = struct {
                         .struct_field_ptr_index_3 => {
                             const load_ptr_inst = try self.resolveInst(load_ptr);
                             const gep = self.builder.buildInBoundsGEP(array_llvm_ty, load_ptr_inst, &indices, indices.len, "");
+                            array_llvm_val.removeUnusedLoadArray();
                             return self.builder.buildLoad(elem_llvm_ty, gep, "");
                         },
                         else => {}
                     }
                 }
+                const elem_ptr = self.builder.buildInBoundsGEP(array_llvm_ty, array_llvm_val, &indices, indices.len, "");
                 return self.builder.buildLoad(elem_llvm_ty, elem_ptr, "");
             }
         }

src/codegen/llvm/bindings.zig

@@ -256,6 +256,9 @@ pub const Value = opaque {
 
     pub const addByValAttr = ZigLLVMAddByValAttr;
     extern fn ZigLLVMAddByValAttr(Fn: *Value, ArgNo: c_uint, type: *Type) void;
+
+    pub const removeUnusedLoadArray = ZigLLVMRemoveUnusedLoadArray;
+    extern fn ZigLLVMRemoveUnusedLoadArray(Val: *Value) void;
 };
 
 pub const Type = opaque {

src/zig_llvm.cpp

@@ -186,6 +186,20 @@ unsigned ZigLLVMDataLayoutGetProgramAddressSpace(LLVMTargetDataRef TD) {
     return unwrap(TD)->getProgramAddressSpace();
 }
 
+void ZigLLVMRemoveUnusedLoadArray(LLVMValueRef Val) {
+    auto *Ptr = unwrap(Val);
+    if (auto *AI = dyn_cast<AllocaInst>(Ptr)) {
+        if (AI->hasOneUse()) {
+            if (auto *CI = dyn_cast<CallInst>(AI->user_back())) {
+                if (CI->getIntrinsicID() == Intrinsic::memcpy) {
+                    CI->eraseFromParent();
+                    AI->eraseFromParent();
+                }
+            }
+        }
+    }
+}
+
 namespace {
 // LLVM's time profiler can provide a hierarchy view of the time spent
 // in each component. It generates JSON report in Chrome's "Trace Event"

src/zig_llvm.h

@@ -599,4 +599,6 @@ ZIG_EXTERN_C void ZigLLVMGetNativeTarget(enum ZigLLVM_ArchType *arch_type,
 ZIG_EXTERN_C unsigned ZigLLVMDataLayoutGetStackAlignment(LLVMTargetDataRef TD);
 ZIG_EXTERN_C unsigned ZigLLVMDataLayoutGetProgramAddressSpace(LLVMTargetDataRef TD);
 
+ZIG_EXTERN_C void ZigLLVMRemoveUnusedLoadArray(LLVMValueRef Val);
+
 #endif