Auto merge of #121665 - erikdesjardins:ptradd, r=nikic

Always generate GEP i8 / ptradd for struct offsets

This implements #98615, and goes a bit further to remove `struct_gep` entirely.

Upstream LLVM is in the beginning stages of [migrating to `ptradd`](https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699). LLVM 19 will [canonicalize](https://github.com/llvm/llvm-project/pull/68882) all constant-offset GEPs to i8, which has roughly the same effect as this change.

Fixes #121719.

Split out from #121577.

r? `@nikic`

compiler/rustc_codegen_gcc/src/builder.rs

@@ -834,10 +834,13 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
             }
             else if let abi::Abi::ScalarPair(ref a, ref b) = place.layout.abi {
                 let b_offset = a.size(self).align_to(b.align(self).abi);
-                let pair_type = place.layout.gcc_type(self);
 
                 let mut load = |i, scalar: &abi::Scalar, align| {
-                    let llptr = self.struct_gep(pair_type, place.llval, i as u64);
+                    let llptr = if i == 0 {
+                        place.llval
+                    } else {
+                        self.inbounds_ptradd(place.llval, self.const_usize(b_offset.bytes()))
+                    };
                     let llty = place.layout.scalar_pair_element_gcc_type(self, i);
                     let load = self.load(llty, llptr, align);
                     scalar_load_metadata(self, load, scalar);
@@ -971,33 +974,6 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
         result.get_address(None)
     }
 
-    fn struct_gep(&mut self, value_type: Type<'gcc>, ptr: RValue<'gcc>, idx: u64) -> RValue<'gcc> {
-        // FIXME(antoyo): it would be better if the API only called this on struct, not on arrays.
-        assert_eq!(idx as usize as u64, idx);
-        let value = ptr.dereference(None).to_rvalue();
-
-        if value_type.dyncast_array().is_some() {
-            let index = self.context.new_rvalue_from_long(self.u64_type, i64::try_from(idx).expect("i64::try_from"));
-            let element = self.context.new_array_access(None, value, index);
-            element.get_address(None)
-        }
-        else if let Some(vector_type) = value_type.dyncast_vector() {
-            let array_type = vector_type.get_element_type().make_pointer();
-            let array = self.bitcast(ptr, array_type);
-            let index = self.context.new_rvalue_from_long(self.u64_type, i64::try_from(idx).expect("i64::try_from"));
-            let element = self.context.new_array_access(None, array, index);
-            element.get_address(None)
-        }
-        else if let Some(struct_type) = value_type.is_struct() {
-            // NOTE: due to opaque pointers now being used, we need to bitcast here.
-            let ptr = self.bitcast_if_needed(ptr, value_type.make_pointer());
-            ptr.dereference_field(None, struct_type.get_field(idx as i32)).get_address(None)
-        }
-        else {
-            panic!("Unexpected type {:?}", value_type);
-        }
-    }
-
     /* Casts */
     fn trunc(&mut self, value: RValue<'gcc>, dest_ty: Type<'gcc>) -> RValue<'gcc> {
         // TODO(antoyo): check that it indeed truncate the value.

compiler/rustc_codegen_gcc/src/type_of.rs

@@ -151,7 +151,6 @@ pub trait LayoutGccExt<'tcx> {
     fn immediate_gcc_type<'gcc>(&self, cx: &CodegenCx<'gcc, 'tcx>) -> Type<'gcc>;
     fn scalar_gcc_type_at<'gcc>(&self, cx: &CodegenCx<'gcc, 'tcx>, scalar: &abi::Scalar, offset: Size) -> Type<'gcc>;
     fn scalar_pair_element_gcc_type<'gcc>(&self, cx: &CodegenCx<'gcc, 'tcx>, index: usize) -> Type<'gcc>;
-    fn gcc_field_index(&self, index: usize) -> u64;
     fn pointee_info_at<'gcc>(&self, cx: &CodegenCx<'gcc, 'tcx>, offset: Size) -> Option<PointeeInfo>;
 }
 
@@ -306,24 +305,6 @@ impl<'tcx> LayoutGccExt<'tcx> for TyAndLayout<'tcx> {
         self.scalar_gcc_type_at(cx, scalar, offset)
     }
 
-    fn gcc_field_index(&self, index: usize) -> u64 {
-        match self.abi {
-            Abi::Scalar(_) | Abi::ScalarPair(..) => {
-                bug!("TyAndLayout::gcc_field_index({:?}): not applicable", self)
-            }
-            _ => {}
-        }
-        match self.fields {
-            FieldsShape::Primitive | FieldsShape::Union(_) => {
-                bug!("TyAndLayout::gcc_field_index({:?}): not applicable", self)
-            }
-
-            FieldsShape::Array { .. } => index as u64,
-
-            FieldsShape::Arbitrary { .. } => 1 + (self.fields.memory_index(index) as u64) * 2,
-        }
-    }
-
     fn pointee_info_at<'a>(&self, cx: &CodegenCx<'a, 'tcx>, offset: Size) -> Option<PointeeInfo> {
         if let Some(&pointee) = cx.pointee_infos.borrow().get(&(self.ty, offset)) {
             return pointee;
@@ -353,10 +334,6 @@ impl<'gcc, 'tcx> LayoutTypeMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         layout.is_gcc_scalar_pair()
     }
 
-    fn backend_field_index(&self, layout: TyAndLayout<'tcx>, index: usize) -> u64 {
-        layout.gcc_field_index(index)
-    }
-
     fn scalar_pair_element_backend_type(&self, layout: TyAndLayout<'tcx>, index: usize, _immediate: bool) -> Type<'gcc> {
         layout.scalar_pair_element_gcc_type(self, index)
     }

compiler/rustc_codegen_llvm/src/builder.rs

@@ -603,11 +603,7 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
                 let llptr = if i == 0 {
                     place.llval
                 } else {
-                    self.inbounds_gep(
+                    self.inbounds_ptradd(place.llval, self.const_usize(b_offset.bytes()))
-                        self.type_i8(),
-                        place.llval,
-                        &[self.const_usize(b_offset.bytes())],
-                    )
                 };
                 let llty = place.layout.scalar_pair_element_llvm_type(self, i, false);
                 let load = self.load(llty, llptr, align);
@@ -778,11 +774,6 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         }
     }
 
-    fn struct_gep(&mut self, ty: &'ll Type, ptr: &'ll Value, idx: u64) -> &'ll Value {
-        assert_eq!(idx as c_uint as u64, idx);
-        unsafe { llvm::LLVMBuildStructGEP2(self.llbuilder, ty, ptr, idx as c_uint, UNNAMED) }
-    }
-
     /* Casts */
     fn trunc(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMBuildTrunc(self.llbuilder, val, dest_ty, UNNAMED) }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1306,13 +1306,6 @@ extern "C" {
         NumIndices: c_uint,
         Name: *const c_char,
     ) -> &'a Value;
-    pub fn LLVMBuildStructGEP2<'a>(
-        B: &Builder<'a>,
-        Ty: &'a Type,
-        Pointer: &'a Value,
-        Idx: c_uint,
-        Name: *const c_char,
-    ) -> &'a Value;
 
     // Casts
     pub fn LLVMBuildTrunc<'a>(

compiler/rustc_codegen_llvm/src/type_.rs

@@ -261,9 +261,6 @@ impl<'ll, 'tcx> LayoutTypeMethods<'tcx> for CodegenCx<'ll, 'tcx> {
     fn is_backend_scalar_pair(&self, layout: TyAndLayout<'tcx>) -> bool {
         layout.is_llvm_scalar_pair()
     }
-    fn backend_field_index(&self, layout: TyAndLayout<'tcx>, index: usize) -> u64 {
-        layout.llvm_field_index(self, index)
-    }
     fn scalar_pair_element_backend_type(
         &self,
         layout: TyAndLayout<'tcx>,

compiler/rustc_codegen_llvm/src/type_of.rs

@@ -174,7 +174,6 @@ pub trait LayoutLlvmExt<'tcx> {
         index: usize,
         immediate: bool,
     ) -> &'a Type;
-    fn llvm_field_index<'a>(&self, cx: &CodegenCx<'a, 'tcx>, index: usize) -> u64;
     fn scalar_copy_llvm_type<'a>(&self, cx: &CodegenCx<'a, 'tcx>) -> Option<&'a Type>;
 }
 
@@ -326,42 +325,6 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
         self.scalar_llvm_type_at(cx, scalar)
     }
 
-    fn llvm_field_index<'a>(&self, cx: &CodegenCx<'a, 'tcx>, index: usize) -> u64 {
-        match self.abi {
-            Abi::Scalar(_) | Abi::ScalarPair(..) => {
-                bug!("TyAndLayout::llvm_field_index({:?}): not applicable", self)
-            }
-            _ => {}
-        }
-        match self.fields {
-            FieldsShape::Primitive | FieldsShape::Union(_) => {
-                bug!("TyAndLayout::llvm_field_index({:?}): not applicable", self)
-            }
-
-            FieldsShape::Array { .. } => index as u64,
-
-            FieldsShape::Arbitrary { .. } => {
-                let variant_index = match self.variants {
-                    Variants::Single { index } => Some(index),
-                    _ => None,
-                };
-
-                // Look up llvm field if indexes do not match memory order due to padding. If
-                // `field_remapping` is `None` no padding was used and the llvm field index
-                // matches the memory index.
-                match cx.type_lowering.borrow().get(&(self.ty, variant_index)) {
-                    Some(TypeLowering { field_remapping: Some(ref remap), .. }) => {
-                        remap[index] as u64
-                    }
-                    Some(_) => self.fields.memory_index(index) as u64,
-                    None => {
-                        bug!("TyAndLayout::llvm_field_index({:?}): type info not found", self)
-                    }
-                }
-            }
-        }
-    }
-
     fn scalar_copy_llvm_type<'a>(&self, cx: &CodegenCx<'a, 'tcx>) -> Option<&'a Type> {
         debug_assert!(self.is_sized());
 

compiler/rustc_codegen_llvm/src/va_arg.rs

@@ -44,12 +44,12 @@ fn emit_direct_ptr_va_arg<'ll, 'tcx>(
 
     let aligned_size = size.align_to(slot_size).bytes() as i32;
     let full_direct_size = bx.cx().const_i32(aligned_size);
-    let next = bx.inbounds_gep(bx.type_i8(), addr, &[full_direct_size]);
+    let next = bx.inbounds_ptradd(addr, full_direct_size);
     bx.store(next, va_list_addr, bx.tcx().data_layout.pointer_align.abi);
 
     if size.bytes() < slot_size.bytes() && bx.tcx().sess.target.endian == Endian::Big {
         let adjusted_size = bx.cx().const_i32((slot_size.bytes() - size.bytes()) as i32);
-        let adjusted = bx.inbounds_gep(bx.type_i8(), addr, &[adjusted_size]);
+        let adjusted = bx.inbounds_ptradd(addr, adjusted_size);
         (adjusted, addr_align)
     } else {
         (addr, addr_align)
@@ -89,11 +89,31 @@ fn emit_aapcs_va_arg<'ll, 'tcx>(
     list: OperandRef<'tcx, &'ll Value>,
     target_ty: Ty<'tcx>,
 ) -> &'ll Value {
+    let dl = bx.cx.data_layout();
+
     // Implementation of the AAPCS64 calling convention for va_args see
     // https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst
+    //
+    // typedef struct  va_list {
+    //     void * stack; // next stack param
+    //     void * gr_top; // end of GP arg reg save area
+    //     void * vr_top; // end of FP/SIMD arg reg save area
+    //     int gr_offs; // offset from  gr_top to next GP register arg
+    //     int vr_offs; // offset from  vr_top to next FP/SIMD register arg
+    // } va_list;
     let va_list_addr = list.immediate();
-    let va_list_layout = list.deref(bx.cx).layout;
+
-    let va_list_ty = va_list_layout.llvm_type(bx);
+    // There is no padding between fields since `void*` is size=8 align=8, `int` is size=4 align=4.
+    // See https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst
+    // Table 1, Byte size and byte alignment of fundamental data types
+    // Table 3, Mapping of C & C++ built-in data types
+    let ptr_offset = 8;
+    let i32_offset = 4;
+    let gr_top = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(ptr_offset));
+    let vr_top = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(2 * ptr_offset));
+    let gr_offs = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(3 * ptr_offset));
+    let vr_offs = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(3 * ptr_offset + i32_offset));
+
     let layout = bx.cx.layout_of(target_ty);
 
     let maybe_reg = bx.append_sibling_block("va_arg.maybe_reg");
@@ -104,16 +124,12 @@ fn emit_aapcs_va_arg<'ll, 'tcx>(
     let offset_align = Align::from_bytes(4).unwrap();
 
     let gr_type = target_ty.is_any_ptr() || target_ty.is_integral();
-    let (reg_off, reg_top_index, slot_size) = if gr_type {
+    let (reg_off, reg_top, slot_size) = if gr_type {
-        let gr_offs =
-            bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 3));
         let nreg = (layout.size.bytes() + 7) / 8;
-        (gr_offs, va_list_layout.llvm_field_index(bx.cx, 1), nreg * 8)
+        (gr_offs, gr_top, nreg * 8)
     } else {
-        let vr_off =
-            bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 4));
         let nreg = (layout.size.bytes() + 15) / 16;
-        (vr_off, va_list_layout.llvm_field_index(bx.cx, 2), nreg * 16)
+        (vr_offs, vr_top, nreg * 16)
     };
 
     // if the offset >= 0 then the value will be on the stack
@@ -141,15 +157,14 @@ fn emit_aapcs_va_arg<'ll, 'tcx>(
 
     bx.switch_to_block(in_reg);
     let top_type = bx.type_ptr();
-    let top = bx.struct_gep(va_list_ty, va_list_addr, reg_top_index);
+    let top = bx.load(top_type, reg_top, dl.pointer_align.abi);
-    let top = bx.load(top_type, top, bx.tcx().data_layout.pointer_align.abi);
 
     // reg_value = *(@top + reg_off_v);
-    let mut reg_addr = bx.gep(bx.type_i8(), top, &[reg_off_v]);
+    let mut reg_addr = bx.ptradd(top, reg_off_v);
     if bx.tcx().sess.target.endian == Endian::Big && layout.size.bytes() != slot_size {
         // On big-endian systems the value is right-aligned in its slot.
         let offset = bx.const_i32((slot_size - layout.size.bytes()) as i32);
-        reg_addr = bx.gep(bx.type_i8(), reg_addr, &[offset]);
+        reg_addr = bx.ptradd(reg_addr, offset);
     }
     let reg_type = layout.llvm_type(bx);
     let reg_value = bx.load(reg_type, reg_addr, layout.align.abi);
@@ -173,11 +188,29 @@ fn emit_s390x_va_arg<'ll, 'tcx>(
     list: OperandRef<'tcx, &'ll Value>,
     target_ty: Ty<'tcx>,
 ) -> &'ll Value {
+    let dl = bx.cx.data_layout();
+
     // Implementation of the s390x ELF ABI calling convention for va_args see
     // https://github.com/IBM/s390x-abi (chapter 1.2.4)
+    //
+    // typedef struct __va_list_tag {
+    //     long __gpr;
+    //     long __fpr;
+    //     void *__overflow_arg_area;
+    //     void *__reg_save_area;
+    // } va_list[1];
     let va_list_addr = list.immediate();
-    let va_list_layout = list.deref(bx.cx).layout;
+
-    let va_list_ty = va_list_layout.llvm_type(bx);
+    // There is no padding between fields since `long` and `void*` both have size=8 align=8.
+    // https://github.com/IBM/s390x-abi (Table 1.1.: Scalar types)
+    let i64_offset = 8;
+    let ptr_offset = 8;
+    let gpr = va_list_addr;
+    let fpr = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(i64_offset));
+    let overflow_arg_area = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(2 * i64_offset));
+    let reg_save_area =
+        bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(2 * i64_offset + ptr_offset));
+
     let layout = bx.cx.layout_of(target_ty);
 
     let in_reg = bx.append_sibling_block("va_arg.in_reg");
@@ -192,15 +225,10 @@ fn emit_s390x_va_arg<'ll, 'tcx>(
     let padding = padded_size - unpadded_size;
 
     let gpr_type = indirect || !layout.is_single_fp_element(bx.cx);
-    let (max_regs, reg_count_field, reg_save_index, reg_padding) =
+    let (max_regs, reg_count, reg_save_index, reg_padding) =
-        if gpr_type { (5, 0, 2, padding) } else { (4, 1, 16, 0) };
+        if gpr_type { (5, gpr, 2, padding) } else { (4, fpr, 16, 0) };
 
     // Check whether the value was passed in a register or in memory.
-    let reg_count = bx.struct_gep(
-        va_list_ty,
-        va_list_addr,
-        va_list_layout.llvm_field_index(bx.cx, reg_count_field),
-    );
     let reg_count_v = bx.load(bx.type_i64(), reg_count, Align::from_bytes(8).unwrap());
     let use_regs = bx.icmp(IntPredicate::IntULT, reg_count_v, bx.const_u64(max_regs));
     bx.cond_br(use_regs, in_reg, in_mem);
@@ -209,12 +237,10 @@ fn emit_s390x_va_arg<'ll, 'tcx>(
     bx.switch_to_block(in_reg);
 
     // Work out the address of the value in the register save area.
-    let reg_ptr =
+    let reg_ptr_v = bx.load(bx.type_ptr(), reg_save_area, dl.pointer_align.abi);
-        bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 3));
-    let reg_ptr_v = bx.load(bx.type_ptr(), reg_ptr, bx.tcx().data_layout.pointer_align.abi);
     let scaled_reg_count = bx.mul(reg_count_v, bx.const_u64(8));
     let reg_off = bx.add(scaled_reg_count, bx.const_u64(reg_save_index * 8 + reg_padding));
-    let reg_addr = bx.gep(bx.type_i8(), reg_ptr_v, &[reg_off]);
+    let reg_addr = bx.ptradd(reg_ptr_v, reg_off);
 
     // Update the register count.
     let new_reg_count_v = bx.add(reg_count_v, bx.const_u64(1));
@@ -225,27 +251,23 @@ fn emit_s390x_va_arg<'ll, 'tcx>(
     bx.switch_to_block(in_mem);
 
     // Work out the address of the value in the argument overflow area.
-    let arg_ptr =
+    let arg_ptr_v =
-        bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 2));
+        bx.load(bx.type_ptr(), overflow_arg_area, bx.tcx().data_layout.pointer_align.abi);
-    let arg_ptr_v = bx.load(bx.type_ptr(), arg_ptr, bx.tcx().data_layout.pointer_align.abi);
     let arg_off = bx.const_u64(padding);
-    let mem_addr = bx.gep(bx.type_i8(), arg_ptr_v, &[arg_off]);
+    let mem_addr = bx.ptradd(arg_ptr_v, arg_off);
 
     // Update the argument overflow area pointer.
     let arg_size = bx.cx().const_u64(padded_size);
-    let new_arg_ptr_v = bx.inbounds_gep(bx.type_i8(), arg_ptr_v, &[arg_size]);
+    let new_arg_ptr_v = bx.inbounds_ptradd(arg_ptr_v, arg_size);
-    bx.store(new_arg_ptr_v, arg_ptr, bx.tcx().data_layout.pointer_align.abi);
+    bx.store(new_arg_ptr_v, overflow_arg_area, dl.pointer_align.abi);
     bx.br(end);
 
     // Return the appropriate result.
     bx.switch_to_block(end);
     let val_addr = bx.phi(bx.type_ptr(), &[reg_addr, mem_addr], &[in_reg, in_mem]);
     let val_type = layout.llvm_type(bx);
-    let val_addr = if indirect {
+    let val_addr =
-        bx.load(bx.cx.type_ptr(), val_addr, bx.tcx().data_layout.pointer_align.abi)
+        if indirect { bx.load(bx.cx.type_ptr(), val_addr, dl.pointer_align.abi) } else { val_addr };
-    } else {
-        val_addr
-    };
     bx.load(val_type, val_addr, layout.align.abi)
 }
 

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -437,8 +437,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
                 let align = dest.align;
                 bx.store_with_flags(val, dest.llval, align, flags);
 
-                let llptr =
+                let llptr = bx.inbounds_ptradd(dest.llval, bx.const_usize(b_offset.bytes()));
-                    bx.inbounds_gep(bx.type_i8(), dest.llval, &[bx.const_usize(b_offset.bytes())]);
                 let val = bx.from_immediate(b);
                 let align = dest.align.restrict_for_offset(b_offset);
                 bx.store_with_flags(val, llptr, align, flags);
@@ -476,7 +475,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
         let address = bx.ptrtoint(alloca, bx.type_isize());
         let neg_address = bx.neg(address);
         let offset = bx.and(neg_address, align_minus_1);
-        let dst = bx.inbounds_gep(bx.type_i8(), alloca, &[offset]);
+        let dst = bx.inbounds_ptradd(alloca, offset);
         bx.memcpy(dst, min_align, llptr, min_align, size, MemFlags::empty());
 
         // Store the allocated region and the extra to the indirect place.

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -9,7 +9,7 @@ use rustc_middle::mir;
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, TyAndLayout};
 use rustc_middle::ty::{self, Ty};
-use rustc_target::abi::{Abi, Align, FieldsShape, Int, Pointer, TagEncoding};
+use rustc_target::abi::{Align, FieldsShape, Int, Pointer, TagEncoding};
 use rustc_target::abi::{VariantIdx, Variants};
 
 #[derive(Copy, Clone, Debug)]
@@ -102,34 +102,14 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
         // `simple` is called when we don't need to adjust the offset to
         // the dynamic alignment of the field.
         let mut simple = || {
-            let llval = match self.layout.abi {
+            let llval = if offset.bytes() == 0 {
-                _ if offset.bytes() == 0 => {
-                    // Unions and newtypes only use an offset of 0.
-                    // Also handles the first field of Scalar, ScalarPair, and Vector layouts.
                 self.llval
-                }
+            } else if field.is_zst() {
-                Abi::ScalarPair(..) => {
+                // FIXME(erikdesjardins): it should be fine to use inbounds for ZSTs too;
-                    // FIXME(nikic): Generate this for all ABIs.
+                // keeping this logic for now to preserve previous behavior.
-                    bx.inbounds_gep(bx.type_i8(), self.llval, &[bx.const_usize(offset.bytes())])
+                bx.ptradd(self.llval, bx.const_usize(offset.bytes()))
-                }
+            } else {
-                Abi::Scalar(_) | Abi::Vector { .. } if field.is_zst() => {
+                bx.inbounds_ptradd(self.llval, bx.const_usize(offset.bytes()))
-                    // ZST fields (even some that require alignment) are not included in Scalar,
-                    // ScalarPair, and Vector layouts, so manually offset the pointer.
-                    bx.gep(bx.cx().type_i8(), self.llval, &[bx.const_usize(offset.bytes())])
-                }
-                Abi::Scalar(_) => {
-                    // All fields of Scalar layouts must have been handled by this point.
-                    // Vector layouts have additional fields for each element of the vector, so don't panic in that case.
-                    bug!(
-                        "offset of non-ZST field `{:?}` does not match layout `{:#?}`",
-                        field,
-                        self.layout
-                    );
-                }
-                _ => {
-                    let ty = bx.backend_type(self.layout);
-                    bx.struct_gep(ty, self.llval, bx.cx().backend_field_index(self.layout, ix))
-                }
             };
             PlaceRef {
                 llval,
@@ -188,7 +168,8 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
         debug!("struct_field_ptr: DST field offset: {:?}", offset);
 
         // Adjust pointer.
-        let ptr = bx.gep(bx.cx().type_i8(), self.llval, &[offset]);
+        // FIXME(erikdesjardins): should be able to use inbounds here too.
+        let ptr = bx.ptradd(self.llval, offset);
 
         PlaceRef { llval: ptr, llextra: self.llextra, layout: field, align: effective_field_align }
     }

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -190,7 +190,12 @@ pub trait BuilderMethods<'a, 'tcx>:
         ptr: Self::Value,
         indices: &[Self::Value],
     ) -> Self::Value;
-    fn struct_gep(&mut self, ty: Self::Type, ptr: Self::Value, idx: u64) -> Self::Value;
+    fn ptradd(&mut self, ptr: Self::Value, offset: Self::Value) -> Self::Value {
+        self.gep(self.cx().type_i8(), ptr, &[offset])
+    }
+    fn inbounds_ptradd(&mut self, ptr: Self::Value, offset: Self::Value) -> Self::Value {
+        self.inbounds_gep(self.cx().type_i8(), ptr, &[offset])
+    }
 
     fn trunc(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
     fn sext(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;

compiler/rustc_codegen_ssa/src/traits/type_.rs

@@ -113,7 +113,6 @@ pub trait LayoutTypeMethods<'tcx>: Backend<'tcx> {
     fn immediate_backend_type(&self, layout: TyAndLayout<'tcx>) -> Self::Type;
     fn is_backend_immediate(&self, layout: TyAndLayout<'tcx>) -> bool;
     fn is_backend_scalar_pair(&self, layout: TyAndLayout<'tcx>) -> bool;
-    fn backend_field_index(&self, layout: TyAndLayout<'tcx>, index: usize) -> u64;
     fn scalar_pair_element_backend_type(
         &self,
         layout: TyAndLayout<'tcx>,

tests/codegen/align-struct.rs

@@ -26,7 +26,6 @@ pub enum Enum64 {
     B(i32),
 }
 // CHECK: %Enum64 = type { i32, [31 x i32] }
-// CHECK: %"Enum64::A" = type { [8 x i64], %Align64 }
 
 // CHECK-LABEL: @align64
 #[no_mangle]

tests/codegen/i128-x86-align.rs

@@ -94,9 +94,9 @@ pub fn store_struct(x: &mut Struct) {
     // CHECK-SAME: align 16 dereferenceable(32) %x
     // CHECK:      [[TMP:%.*]] = alloca %Struct, align 16
     // CHECK:      store i32 1, ptr [[TMP]], align 16
-    // CHECK-NEXT: [[GEP1:%.*]] = getelementptr inbounds %Struct, ptr [[TMP]], i32 0, i32 1
+    // CHECK-NEXT: [[GEP1:%.*]] = getelementptr inbounds i8, ptr [[TMP]], i64 4
     // CHECK-NEXT: store i32 2, ptr [[GEP1]], align 4
-    // CHECK-NEXT: [[GEP2:%.*]] = getelementptr inbounds %Struct, ptr [[TMP]], i32 0, i32 3
+    // CHECK-NEXT: [[GEP2:%.*]] = getelementptr inbounds i8, ptr [[TMP]], i64 16
     // CHECK-NEXT: store i128 3, ptr [[GEP2]], align 16
     // CHECK-NEXT: call void @llvm.memcpy.p0.p0.i64(ptr align 16 %x, ptr align 16 [[TMP]], i64 32, i1 false)
     *x = Struct { a: 1, b: 2, c: 3 };

tests/codegen/issues/issue-105386-ub-in-debuginfo.rs

@@ -16,8 +16,8 @@ pub fn outer_function(x: S, y: S) -> usize {
 // when generating debuginfo.
 // CHECK-LABEL: @outer_function
 // CHECK: [[spill:%.*]] = alloca %"{closure@{{.*.rs}}:9:23: 9:25}"
-// CHECK-NOT: [[ptr_tmp:%.*]] = getelementptr inbounds %"{closure@{{.*.rs}}:9:23: 9:25}", ptr [[spill]]
+// CHECK-NOT: [[ptr_tmp:%.*]] = getelementptr inbounds i8, ptr [[spill]]
 // CHECK-NOT: [[load:%.*]] = load ptr, ptr
 // CHECK: call void @llvm.lifetime.start{{.*}}({{.*}}, ptr [[spill]])
-// CHECK: [[inner:%.*]] = getelementptr inbounds %"{{.*}}", ptr [[spill]]
+// CHECK: [[inner:%.*]] = getelementptr inbounds i8, ptr [[spill]]
 // CHECK: call void @llvm.memcpy{{.*}}(ptr {{align .*}} [[inner]], ptr {{align .*}} %x

tests/codegen/issues/issue-121719-common-field-offset.rs

@@ -0,0 +1,44 @@
+//! This test checks that match branches which all access a field
+//! at the same offset are merged together.
+//!
+//@ compile-flags: -O
+#![crate_type = "lib"]
+
+#[repr(C)]
+pub struct A {
+    x: f64,
+    y: u64,
+}
+#[repr(C)]
+pub struct B {
+    x: f64,
+    y: u32,
+}
+#[repr(C)]
+pub struct C {
+    x: f64,
+    y: u16,
+}
+#[repr(C)]
+pub struct D {
+    x: f64,
+    y: u8,
+}
+
+pub enum E {
+    A(A),
+    B(B),
+    C(C),
+    D(D),
+}
+
+// CHECK-LABEL: @match_on_e
+#[no_mangle]
+pub fn match_on_e(e: &E) -> &f64 {
+    // CHECK: start:
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: ret
+    match e {
+        E::A(A { x, .. }) | E::B(B { x, .. }) | E::C(C { x, .. }) | E::D(D { x, .. }) => x,
+    }
+}

tests/codegen/zst-offset.rs

@@ -22,7 +22,7 @@ pub fn scalar_layout(s: &(u64, ())) {
 // CHECK-LABEL: @scalarpair_layout
 #[no_mangle]
 pub fn scalarpair_layout(s: &(u64, u32, ())) {
-// CHECK: getelementptr inbounds i8, {{.+}}, [[USIZE]] 12
+// CHECK: getelementptr i8, {{.+}}, [[USIZE]] 12
     let x = &s.2;
     witness(&x); // keep variable in an alloca
 }