Don't compute optimized PointerKind for unoptimized builds

This saves us both the Freeze/Unpin queries, and avoids placing
noalias attributes, which have a compile-time impact on LLVM
even in optnone builds (due to always_inline functions).

Author: nikic

compiler/rustc_middle/src/ty/layout.rs

@@ -11,7 +11,7 @@ use rustc_hir as hir;
 use rustc_hir::lang_items::LangItem;
 use rustc_index::bit_set::BitSet;
 use rustc_index::vec::{Idx, IndexVec};
-use rustc_session::{DataTypeKind, FieldInfo, SizeKind, VariantInfo};
+use rustc_session::{config::OptLevel, DataTypeKind, FieldInfo, SizeKind, VariantInfo};
 use rustc_span::symbol::{Ident, Symbol};
 use rustc_span::DUMMY_SP;
 use rustc_target::abi::call::{
@@ -2318,23 +2318,30 @@ where
             ty::Ref(_, ty, mt) if offset.bytes() == 0 => {
                 let address_space = addr_space_of_ty(ty);
                 let tcx = cx.tcx();
-                let kind = match mt {
-                    hir::Mutability::Not => {
-                        if ty.is_freeze(tcx.at(DUMMY_SP), cx.param_env()) {
-                            PointerKind::Frozen
-                        } else {
-                            PointerKind::Shared
+                let kind = if tcx.sess.opts.optimize == OptLevel::No {
+                    // Use conservative pointer kind if not optimizing. This saves us the
+                    // Freeze/Unpin queries, and can save time in the codegen backend (noalias
+                    // attributes in LLVM have compile-time cost even in unoptimized builds).
+                    PointerKind::Shared
+                } else {
+                    match mt {
+                        hir::Mutability::Not => {
+                            if ty.is_freeze(tcx.at(DUMMY_SP), cx.param_env()) {
+                                PointerKind::Frozen
+                            } else {
+                                PointerKind::Shared
+                            }
                         }
-                    }
-                    hir::Mutability::Mut => {
-                        // References to self-referential structures should not be considered
-                        // noalias, as another pointer to the structure can be obtained, that
-                        // is not based-on the original reference. We consider all !Unpin
-                        // types to be potentially self-referential here.
-                        if ty.is_unpin(tcx.at(DUMMY_SP), cx.param_env()) {
-                            PointerKind::UniqueBorrowed
-                        } else {
-                            PointerKind::Shared
+                        hir::Mutability::Mut => {
+                            // References to self-referential structures should not be considered
+                            // noalias, as another pointer to the structure can be obtained, that
+                            // is not based-on the original reference. We consider all !Unpin
+                            // types to be potentially self-referential here.
+                            if ty.is_unpin(tcx.at(DUMMY_SP), cx.param_env()) {
+                                PointerKind::UniqueBorrowed
+                            } else {
+                                PointerKind::Shared
+                            }
                         }
                     }
                 };

src/test/codegen/function-arguments.rs

@@ -1,4 +1,4 @@
-// compile-flags: -C no-prepopulate-passes
+// compile-flags: -O -C no-prepopulate-passes
 // ignore-tidy-linelength
 // min-system-llvm-version: 12.0
 

src/test/codegen/noalias-unpin.rs

@@ -1,4 +1,4 @@
-// compile-flags: -Z mutable-noalias=yes
+// compile-flags: -O -Z mutable-noalias=yes
 
 #![crate_type = "lib"]
 

src/test/codegen/packed.rs

@@ -1,5 +1,5 @@
 // ignore-tidy-linelength
-// compile-flags: -C no-prepopulate-passes
+// compile-flags: -O -C no-prepopulate-passes
 
 #![crate_type = "lib"]