Auto merge of #111768 - oli-obk:pair_const_llvm, r=cjgillot

Optimize scalar and scalar pair representations loaded from ByRef in llvm

in https://github.com/rust-lang/rust/pull/105653 I noticed that we were generating suboptimal LLVM IR if we had a `ConstValue::ByRef` that could be represented by a `ScalarPair`. Before https://github.com/rust-lang/rust/pull/105653 this is probably rare, but after it, every slice will go down this suboptimal code path that requires LLVM to untangle a bunch of indirections and translate static allocations that are only used once to read a scalar pair from.

1 files changed, 17 insertions, 32 deletions

diff --git a/compiler/rustc_codegen_llvm/src/common.rs b/compiler/rustc_codegen_llvm/src/common.rs
index 9127fba388b..a3910fef954 100644
--- a/compiler/rustc_codegen_llvm/src/common.rs
+++ b/compiler/rustc_codegen_llvm/src/common.rs
@@ -8,16 +8,15 @@ use crate::type_of::LayoutLlvmExt;
 use crate::value::Value;
 
 use rustc_ast::Mutability;
-use rustc_codegen_ssa::mir::place::PlaceRef;
 use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::stable_hasher::{Hash128, HashStable, StableHasher};
 use rustc_hir::def_id::DefId;
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::{ConstAllocation, GlobalAlloc, Scalar};
-use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
+use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::cstore::{DllCallingConvention, DllImport, PeImportNameType};
-use rustc_target::abi::{self, AddressSpace, HasDataLayout, Pointer, Size};
+use rustc_target::abi::{self, AddressSpace, HasDataLayout, Pointer};
 use rustc_target::spec::Target;
 
 use libc::{c_char, c_uint};
@@ -307,38 +306,24 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         const_alloc_to_llvm(self, alloc)
     }
 
-    fn from_const_alloc(
-        &self,
-        layout: TyAndLayout<'tcx>,
-        alloc: ConstAllocation<'tcx>,
-        offset: Size,
-    ) -> PlaceRef<'tcx, &'ll Value> {
-        let alloc_align = alloc.inner().align;
-        assert_eq!(alloc_align, layout.align.abi);
-        let llty = self.type_ptr_to(layout.llvm_type(self));
-        let llval = if layout.size == Size::ZERO {
-            let llval = self.const_usize(alloc_align.bytes());
-            unsafe { llvm::LLVMConstIntToPtr(llval, llty) }
-        } else {
-            let init = const_alloc_to_llvm(self, alloc);
-            let base_addr = self.static_addr_of(init, alloc_align, None);
-
-            let llval = unsafe {
-                llvm::LLVMRustConstInBoundsGEP2(
-                    self.type_i8(),
-                    self.const_bitcast(base_addr, self.type_i8p()),
-                    &self.const_usize(offset.bytes()),
-                    1,
-                )
-            };
-            self.const_bitcast(llval, llty)
-        };
-        PlaceRef::new_sized(llval, layout)
-    }
-
     fn const_ptrcast(&self, val: &'ll Value, ty: &'ll Type) -> &'ll Value {
         consts::ptrcast(val, ty)
     }
+
+    fn const_bitcast(&self, val: &'ll Value, ty: &'ll Type) -> &'ll Value {
+        self.const_bitcast(val, ty)
+    }
+
+    fn const_ptr_byte_offset(&self, base_addr: Self::Value, offset: abi::Size) -> Self::Value {
+        unsafe {
+            llvm::LLVMRustConstInBoundsGEP2(
+                self.type_i8(),
+                self.const_bitcast(base_addr, self.type_i8p()),
+                &self.const_usize(offset.bytes()),
+                1,
+            )
+        }
+    }
 }
 
 /// Get the [LLVM type][Type] of a [`Value`].