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Rollup merge of #132423 - RalfJung:const-eval-align-offset, r=dtolnay

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remove const-support for align_offset and is_aligned

As part of the recent discussion to stabilize `ptr.is_null()` in const context, the general vibe was that it's okay for a const function to panic when the same operation would work at runtime (that's just a case of "dynamically detecting that something is not supported as a const operation"), but it is *not* okay for a const function to just return a different result.

Following that, `is_aligned` and `is_aligned_to` have their const status revoked in this PR, since they do return actively wrong results at const time. In the future we can consider having a new intrinsic or so that can check whether a pointer is "guaranteed to be aligned", but the current implementation based on `align_offset` does not have the behavior we want.

In fact `align_offset` itself behaves quite strangely in const, and that support needs a bunch of special hacks. That doesn't seem worth it. Instead, the users that can fall back to a different implementation should just use const_eval_select directly, and everything else should not be made const-callable. So this PR does exactly that, and entirely removes const support for align_offset.

Closes some tracking issues by removing the associated features:
Closes https://github.com/rust-lang/rust/issues/90962
Closes https://github.com/rust-lang/rust/issues/104203

Cc `@rust-lang/wg-const-eval` `@rust-lang/libs-api`
This commit is contained in:
Jubilee 2024-11-03 20:08:13 -08:00 • committed by GitHub
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16 changed files with 185 additions and 1022 deletions
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88
compiler/rustc_const_eval/src/const_eval/machine.rs
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@ -1,7 +1,6 @@
use std::borrow::{Borrow, Cow};
use std::fmt;
use std::hash::Hash;
use std::ops::ControlFlow;
use rustc_abi::{Align, ExternAbi, Size};
use rustc_ast::Mutability;
@ -10,7 +9,7 @@ use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::{self as hir, CRATE_HIR_ID, LangItem};
use rustc_middle::mir::AssertMessage;
use rustc_middle::query::TyCtxtAt;
use rustc_middle::ty::layout::{FnAbiOf, TyAndLayout};
use rustc_middle::ty::layout::TyAndLayout;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::{bug, mir};
use rustc_span::Span;
@ -22,9 +21,9 @@ use crate::errors::{LongRunning, LongRunningWarn};
use crate::fluent_generated as fluent;
use crate::interpret::{
self, AllocId, AllocRange, ConstAllocation, CtfeProvenance, FnArg, Frame, GlobalAlloc, ImmTy,
InterpCx, InterpResult, MPlaceTy, OpTy, Pointer, PointerArithmetic, RangeSet, Scalar,
StackPopCleanup, compile_time_machine, interp_ok, throw_exhaust, throw_inval, throw_ub,
throw_ub_custom, throw_unsup, throw_unsup_format,
InterpCx, InterpResult, MPlaceTy, OpTy, Pointer, RangeSet, Scalar, compile_time_machine,
interp_ok, throw_exhaust, throw_inval, throw_ub, throw_ub_custom, throw_unsup,
throw_unsup_format,
};
/// When hitting this many interpreted terminators we emit a deny by default lint
@ -226,8 +225,8 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
&mut self,
instance: ty::Instance<'tcx>,
args: &[FnArg<'tcx>],
dest: &MPlaceTy<'tcx>,
ret: Option<mir::BasicBlock>,
_dest: &MPlaceTy<'tcx>,
_ret: Option<mir::BasicBlock>,
) -> InterpResult<'tcx, Option<ty::Instance<'tcx>>> {
let def_id = instance.def_id();
@ -259,85 +258,10 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
);
return interp_ok(Some(new_instance));
} else if self.tcx.is_lang_item(def_id, LangItem::AlignOffset) {
let args = self.copy_fn_args(args);
// For align_offset, we replace the function call if the pointer has no address.
match self.align_offset(instance, &args, dest, ret)? {
ControlFlow::Continue(()) => return interp_ok(Some(instance)),
ControlFlow::Break(()) => return interp_ok(None),
}
}
interp_ok(Some(instance))
}
/// `align_offset(ptr, target_align)` needs special handling in const eval, because the pointer
/// may not have an address.
///
/// If `ptr` does have a known address, then we return `Continue(())` and the function call should
/// proceed as normal.
///
/// If `ptr` doesn't have an address, but its underlying allocation's alignment is at most
/// `target_align`, then we call the function again with an dummy address relative to the
/// allocation.
///
/// If `ptr` doesn't have an address and `target_align` is stricter than the underlying
/// allocation's alignment, then we return `usize::MAX` immediately.
fn align_offset(
&mut self,
instance: ty::Instance<'tcx>,
args: &[OpTy<'tcx>],
dest: &MPlaceTy<'tcx>,
ret: Option<mir::BasicBlock>,
) -> InterpResult<'tcx, ControlFlow<()>> {
assert_eq!(args.len(), 2);
let ptr = self.read_pointer(&args[0])?;
let target_align = self.read_scalar(&args[1])?.to_target_usize(self)?;
if !target_align.is_power_of_two() {
throw_ub_custom!(
fluent::const_eval_align_offset_invalid_align,
target_align = target_align,
);
}
match self.ptr_try_get_alloc_id(ptr, 0) {
Ok((alloc_id, offset, _extra)) => {
let (_size, alloc_align, _kind) = self.get_alloc_info(alloc_id);
if target_align <= alloc_align.bytes() {
// Extract the address relative to the allocation base that is definitely
// sufficiently aligned and call `align_offset` again.
let addr = ImmTy::from_uint(offset.bytes(), args[0].layout).into();
let align = ImmTy::from_uint(target_align, args[1].layout).into();
let fn_abi = self.fn_abi_of_instance(instance, ty::List::empty())?;
// Push the stack frame with our own adjusted arguments.
self.init_stack_frame(
instance,
self.load_mir(instance.def, None)?,
fn_abi,
&[FnArg::Copy(addr), FnArg::Copy(align)],
/* with_caller_location = */ false,
dest,
StackPopCleanup::Goto { ret, unwind: mir::UnwindAction::Unreachable },
)?;
interp_ok(ControlFlow::Break(()))
} else {
// Not alignable in const, return `usize::MAX`.
let usize_max = Scalar::from_target_usize(self.target_usize_max(), self);
self.write_scalar(usize_max, dest)?;
self.return_to_block(ret)?;
interp_ok(ControlFlow::Break(()))
}
}
Err(_addr) => {
// The pointer has an address, continue with function call.
interp_ok(ControlFlow::Continue(()))
}
}
}
/// See documentation on the `ptr_guaranteed_cmp` intrinsic.
fn guaranteed_cmp(&mut self, a: Scalar, b: Scalar) -> InterpResult<'tcx, u8> {
interp_ok(match (a, b) {