Remove support for dynamic allocas

Author: mejrs

compiler/rustc_codegen_gcc/src/builder.rs

@@ -931,10 +931,6 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
             .get_address(self.location)
     }
 
-    fn dynamic_alloca(&mut self, _len: RValue<'gcc>, _align: Align) -> RValue<'gcc> {
-        unimplemented!();
-    }
-
     fn load(&mut self, pointee_ty: Type<'gcc>, ptr: RValue<'gcc>, align: Align) -> RValue<'gcc> {
         let block = self.llbb();
         let function = block.get_function();

compiler/rustc_codegen_llvm/src/builder.rs

@@ -538,16 +538,6 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         }
     }
 
-    fn dynamic_alloca(&mut self, size: &'ll Value, align: Align) -> &'ll Value {
-        unsafe {
-            let alloca =
-                llvm::LLVMBuildArrayAlloca(self.llbuilder, self.cx().type_i8(), size, UNNAMED);
-            llvm::LLVMSetAlignment(alloca, align.bytes() as c_uint);
-            // Cast to default addrspace if necessary
-            llvm::LLVMBuildPointerCast(self.llbuilder, alloca, self.cx().type_ptr(), UNNAMED)
-        }
-    }
-
     fn load(&mut self, ty: &'ll Type, ptr: &'ll Value, align: Align) -> &'ll Value {
         unsafe {
             let load = llvm::LLVMBuildLoad2(self.llbuilder, ty, ptr, UNNAMED);

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1492,12 +1492,6 @@ unsafe extern "C" {
         Ty: &'a Type,
         Name: *const c_char,
     ) -> &'a Value;
-    pub(crate) fn LLVMBuildArrayAlloca<'a>(
-        B: &Builder<'a>,
-        Ty: &'a Type,
-        Val: &'a Value,
-        Name: *const c_char,
-    ) -> &'a Value;
     pub(crate) fn LLVMBuildLoad2<'a>(
         B: &Builder<'a>,
         Ty: &'a Type,

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -140,8 +140,8 @@ enum LocalRef<'tcx, V> {
     Place(PlaceRef<'tcx, V>),
     /// `UnsizedPlace(p)`: `p` itself is a thin pointer (indirect place).
     /// `*p` is the wide pointer that references the actual unsized place.
-    /// Every time it is initialized, we have to reallocate the place
-    /// and update the wide pointer. That's the reason why it is indirect.
+    ///
+    /// Rust has no alloca and thus no ability to move the unsized place.
     UnsizedPlace(PlaceRef<'tcx, V>),
     /// The backend [`OperandValue`] has already been generated.
     Operand(OperandRef<'tcx, V>),
@@ -498,7 +498,7 @@ fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
                         LocalRef::Place(PlaceRef::new_sized(llarg, arg.layout))
                     }
                 }
-                // Unsized indirect qrguments
+                // Unsized indirect arguments
                 PassMode::Indirect { attrs: _, meta_attrs: Some(_), on_stack: _ } => {
                     // As the storage for the indirect argument lives during
                     // the whole function call, we just copy the wide pointer.

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -15,9 +15,9 @@ use tracing::{debug, instrument};
 use super::place::{PlaceRef, PlaceValue};
 use super::rvalue::transmute_immediate;
 use super::{FunctionCx, LocalRef};
+use crate::MemFlags;
 use crate::common::IntPredicate;
 use crate::traits::*;
-use crate::{MemFlags, size_of_val};
 
 /// The representation of a Rust value. The enum variant is in fact
 /// uniquely determined by the value's type, but is kept as a
@@ -771,44 +771,6 @@ impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
             }
         }
     }
-
-    pub fn store_unsized<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
-        self,
-        bx: &mut Bx,
-        indirect_dest: PlaceRef<'tcx, V>,
-    ) {
-        debug!("OperandRef::store_unsized: operand={:?}, indirect_dest={:?}", self, indirect_dest);
-        // `indirect_dest` must have `*mut T` type. We extract `T` out of it.
-        let unsized_ty = indirect_dest
-            .layout
-            .ty
-            .builtin_deref(true)
-            .unwrap_or_else(|| bug!("indirect_dest has non-pointer type: {:?}", indirect_dest));
-
-        let OperandValue::Ref(PlaceValue { llval: llptr, llextra: Some(llextra), .. }) = self
-        else {
-            bug!("store_unsized called with a sized value (or with an extern type)")
-        };
-
-        // Allocate an appropriate region on the stack, and copy the value into it. Since alloca
-        // doesn't support dynamic alignment, we allocate an extra align - 1 bytes, and align the
-        // pointer manually.
-        let (size, align) = size_of_val::size_and_align_of_dst(bx, unsized_ty, Some(llextra));
-        let one = bx.const_usize(1);
-        let align_minus_1 = bx.sub(align, one);
-        let size_extra = bx.add(size, align_minus_1);
-        let min_align = Align::ONE;
-        let alloca = bx.dynamic_alloca(size_extra, min_align);
-        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_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.
-        let indirect_operand = OperandValue::Pair(dst, llextra);
-        indirect_operand.store(bx, indirect_dest);
-    }
 }
 
 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -364,27 +364,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         Some(imm)
     }
 
-    pub(crate) fn codegen_rvalue_unsized(
-        &mut self,
-        bx: &mut Bx,
-        indirect_dest: PlaceRef<'tcx, Bx::Value>,
-        rvalue: &mir::Rvalue<'tcx>,
-    ) {
-        debug!(
-            "codegen_rvalue_unsized(indirect_dest.llval={:?}, rvalue={:?})",
-            indirect_dest.val.llval, rvalue
-        );
-
-        match *rvalue {
-            mir::Rvalue::Use(ref operand) => {
-                let cg_operand = self.codegen_operand(bx, operand);
-                cg_operand.val.store_unsized(bx, indirect_dest);
-            }
-
-            _ => bug!("unsized assignment other than `Rvalue::Use`"),
-        }
-    }
-
     pub(crate) fn codegen_rvalue_operand(
         &mut self,
         bx: &mut Bx,

compiler/rustc_codegen_ssa/src/mir/statement.rs

@@ -15,7 +15,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     match self.locals[index] {
                         LocalRef::Place(cg_dest) => self.codegen_rvalue(bx, cg_dest, rvalue),
                         LocalRef::UnsizedPlace(cg_indirect_dest) => {
-                            self.codegen_rvalue_unsized(bx, cg_indirect_dest, rvalue)
+                            let ty = cg_indirect_dest.layout.ty;
+                            span_bug!(
+                                statement.source_info.span,
+                                "cannot reallocate from `UnsizedPlace({ty})` \
+                                into `{rvalue:?}`; dynamic alloca is not supported",
+                            );
                         }
                         LocalRef::PendingOperand => {
                             let operand = self.codegen_rvalue_operand(bx, rvalue);

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -224,7 +224,6 @@ pub trait BuilderMethods<'a, 'tcx>:
     fn to_immediate_scalar(&mut self, val: Self::Value, scalar: Scalar) -> Self::Value;
 
     fn alloca(&mut self, size: Size, align: Align) -> Self::Value;
-    fn dynamic_alloca(&mut self, size: Self::Value, align: Align) -> Self::Value;
 
     fn load(&mut self, ty: Self::Type, ptr: Self::Value, align: Align) -> Self::Value;
     fn volatile_load(&mut self, ty: Self::Type, ptr: Self::Value) -> Self::Value;

library/core/src/mem/mod.rs

@@ -151,7 +151,7 @@ pub const fn forget<T>(t: T) {
 ///
 /// While Rust does not permit unsized locals since its removal in [#111942] it is
 /// still possible to call functions with unsized values from a function argument
-/// or in-place construction.
+/// or place expression.
 ///
 /// ```rust
 /// #![feature(unsized_fn_params, forget_unsized)]