Auto merge of #137152 - saethlin:bss-const-allocs, r=wesleywiser

Add a .bss-like scheme for encoded const allocs

This check if all bytes are zero feel like it should be too slow, and instead we should have a flag that we track, but that seems hard. Let's see how this perfs first.

Also we can probably stash the "it's all zero actually" flag inside one of the other struct members that's already not using an entire byte. This optimization doesn't fire all that often, so it's possible that by sticking it in the varint length field, this PR actually makes rmeta size worse.

Author: bors

compiler/rustc_abi/src/lib.rs

@@ -802,7 +802,7 @@ impl Align {
     }
 
     #[inline]
-    pub fn bytes(self) -> u64 {
+    pub const fn bytes(self) -> u64 {
         1 << self.pow2
     }
 
@@ -812,7 +812,7 @@ impl Align {
     }
 
     #[inline]
-    pub fn bits(self) -> u64 {
+    pub const fn bits(self) -> u64 {
         self.bytes() * 8
     }
 

compiler/rustc_middle/src/mir/interpret/allocation.rs

@@ -15,7 +15,9 @@ use provenance_map::*;
 use rustc_abi::{Align, HasDataLayout, Size};
 use rustc_ast::Mutability;
 use rustc_data_structures::intern::Interned;
-use rustc_macros::{HashStable, TyDecodable, TyEncodable};
+use rustc_macros::HashStable;
+use rustc_serialize::{Decodable, Encodable};
+use rustc_type_ir::{TyDecoder, TyEncoder};
 
 use super::{
     AllocId, BadBytesAccess, CtfeProvenance, InterpErrorKind, InterpResult, Pointer,
@@ -77,7 +79,7 @@ impl AllocBytes for Box<[u8]> {
 /// module provides higher-level access.
 // Note: for performance reasons when interning, some of the `Allocation` fields can be partially
 // hashed. (see the `Hash` impl below for more details), so the impl is not derived.
-#[derive(Clone, Eq, PartialEq, TyEncodable, TyDecodable)]
+#[derive(Clone, Eq, PartialEq)]
 #[derive(HashStable)]
 pub struct Allocation<Prov: Provenance = CtfeProvenance, Extra = (), Bytes = Box<[u8]>> {
     /// The actual bytes of the allocation.
@@ -101,6 +103,117 @@ pub struct Allocation<Prov: Provenance = CtfeProvenance, Extra = (), Bytes = Box
     pub extra: Extra,
 }
 
+/// Helper struct that packs an alignment, mutability, and "all bytes are zero" flag together.
+///
+/// Alignment values always have 2 free high bits, and we check for this in our [`Encodable`] impl.
+struct AllocFlags {
+    align: Align,
+    mutability: Mutability,
+    all_zero: bool,
+}
+
+impl<E: TyEncoder> Encodable<E> for AllocFlags {
+    fn encode(&self, encoder: &mut E) {
+        // Make sure Align::MAX can be stored with the high 2 bits unset.
+        const {
+            let max_supported_align_repr = u8::MAX >> 2;
+            let max_supported_align = 1 << max_supported_align_repr;
+            assert!(Align::MAX.bytes() <= max_supported_align)
+        }
+
+        let mut flags = self.align.bytes().trailing_zeros() as u8;
+        flags |= match self.mutability {
+            Mutability::Not => 0,
+            Mutability::Mut => 1 << 6,
+        };
+        flags |= (self.all_zero as u8) << 7;
+        flags.encode(encoder);
+    }
+}
+
+impl<D: TyDecoder> Decodable<D> for AllocFlags {
+    fn decode(decoder: &mut D) -> Self {
+        let flags: u8 = Decodable::decode(decoder);
+        let align = flags & 0b0011_1111;
+        let mutability = flags & 0b0100_0000;
+        let all_zero = flags & 0b1000_0000;
+
+        let align = Align::from_bytes(1 << align).unwrap();
+        let mutability = match mutability {
+            0 => Mutability::Not,
+            _ => Mutability::Mut,
+        };
+        let all_zero = all_zero > 0;
+
+        AllocFlags { align, mutability, all_zero }
+    }
+}
+
+/// Efficiently detect whether a slice of `u8` is all zero.
+///
+/// This is used in encoding of [`Allocation`] to special-case all-zero allocations. It is only
+/// optimized a little, because for many allocations the encoding of the actual bytes does not
+/// dominate runtime.
+#[inline]
+fn all_zero(buf: &[u8]) -> bool {
+    // In the empty case we wouldn't encode any contents even without this system where we
+    // special-case allocations whose contents are all 0. We can return anything in the empty case.
+    if buf.is_empty() {
+        return true;
+    }
+    // Just fast-rejecting based on the first element significantly reduces the amount that we end
+    // up walking the whole array.
+    if buf[0] != 0 {
+        return false;
+    }
+
+    // This strategy of combining all slice elements with & or | is unbeatable for the large
+    // all-zero case because it is so well-understood by autovectorization.
+    buf.iter().fold(true, |acc, b| acc & (*b == 0))
+}
+
+/// Custom encoder for [`Allocation`] to more efficiently represent the case where all bytes are 0.
+impl<Prov: Provenance, Extra, Bytes, E: TyEncoder> Encodable<E> for Allocation<Prov, Extra, Bytes>
+where
+    Bytes: AllocBytes,
+    ProvenanceMap<Prov>: Encodable<E>,
+    Extra: Encodable<E>,
+{
+    fn encode(&self, encoder: &mut E) {
+        let all_zero = all_zero(&self.bytes);
+        AllocFlags { align: self.align, mutability: self.mutability, all_zero }.encode(encoder);
+
+        encoder.emit_usize(self.bytes.len());
+        if !all_zero {
+            encoder.emit_raw_bytes(&self.bytes);
+        }
+        self.provenance.encode(encoder);
+        self.init_mask.encode(encoder);
+        self.extra.encode(encoder);
+    }
+}
+
+impl<Prov: Provenance, Extra, Bytes, D: TyDecoder> Decodable<D> for Allocation<Prov, Extra, Bytes>
+where
+    Bytes: AllocBytes,
+    ProvenanceMap<Prov>: Decodable<D>,
+    Extra: Decodable<D>,
+{
+    fn decode(decoder: &mut D) -> Self {
+        let AllocFlags { align, mutability, all_zero } = Decodable::decode(decoder);
+
+        let len = decoder.read_usize();
+        let bytes = if all_zero { vec![0u8; len] } else { decoder.read_raw_bytes(len).to_vec() };
+        let bytes = Bytes::from_bytes(bytes, align);
+
+        let provenance = Decodable::decode(decoder);
+        let init_mask = Decodable::decode(decoder);
+        let extra = Decodable::decode(decoder);
+
+        Self { bytes, provenance, init_mask, align, mutability, extra }
+    }
+}
+
 /// This is the maximum size we will hash at a time, when interning an `Allocation` and its
 /// `InitMask`. Note, we hash that amount of bytes twice: at the start, and at the end of a buffer.
 /// Used when these two structures are large: we only partially hash the larger fields in that