Auto merge of rust-lang#111088 - nnethercote:fix-FileEncoder-buf-size, r=WaffleLapkin

Fix the `FileEncoder` buffer size.

It allows a variable size, but in practice we always use the default of 8192 bytes. This commit fixes it to that size, which makes things slightly faster because the size can be hard-wired in generated code.

The commit also:
- Rearranges some buffer capacity checks so they're all in the same form (`x > BUFSIZE`).
- Removes some buffer capacity assertions and comments about them. With an 8192 byte buffer, we're not in any danger of overflowing a `usize`.

r? `@WaffleLapkin`

Author: bors

compiler/rustc_serialize/src/opaque.rs

@@ -1,4 +1,4 @@
-use crate::leb128::{self, largest_max_leb128_len};
+use crate::leb128;
 use crate::serialize::{Decodable, Decoder, Encodable, Encoder};
 use std::fs::File;
 use std::io::{self, Write};
@@ -14,6 +14,9 @@ use std::ptr;
 
 pub type FileEncodeResult = Result<usize, io::Error>;
 
+/// The size of the buffer in `FileEncoder`.
+const BUF_SIZE: usize = 8192;
+
 /// `FileEncoder` encodes data to file via fixed-size buffer.
 ///
 /// There used to be a `MemEncoder` type that encoded all the data into a
@@ -35,26 +38,12 @@ pub struct FileEncoder {
 
 impl FileEncoder {
     pub fn new<P: AsRef<Path>>(path: P) -> io::Result<Self> {
-        const DEFAULT_BUF_SIZE: usize = 8192;
-        FileEncoder::with_capacity(path, DEFAULT_BUF_SIZE)
-    }
-
-    pub fn with_capacity<P: AsRef<Path>>(path: P, capacity: usize) -> io::Result<Self> {
-        // Require capacity at least as large as the largest LEB128 encoding
-        // here, so that we don't have to check or handle this on every write.
-        assert!(capacity >= largest_max_leb128_len());
-
-        // Require capacity small enough such that some capacity checks can be
-        // done using guaranteed non-overflowing add rather than sub, which
-        // shaves an instruction off those code paths (on x86 at least).
-        assert!(capacity <= usize::MAX - largest_max_leb128_len());
-
         // Create the file for reading and writing, because some encoders do both
         // (e.g. the metadata encoder when -Zmeta-stats is enabled)
         let file = File::options().read(true).write(true).create(true).truncate(true).open(path)?;
 
         Ok(FileEncoder {
-            buf: Box::new_uninit_slice(capacity),
+            buf: Box::new_uninit_slice(BUF_SIZE),
             buffered: 0,
             flushed: 0,
             file,
@@ -159,19 +148,11 @@ impl FileEncoder {
         &self.file
     }
 
-    #[inline]
-    fn capacity(&self) -> usize {
-        self.buf.len()
-    }
-
     #[inline]
     fn write_one(&mut self, value: u8) {
-        // We ensure this during `FileEncoder` construction.
-        debug_assert!(self.capacity() >= 1);
-
         let mut buffered = self.buffered;
 
-        if std::intrinsics::unlikely(buffered >= self.capacity()) {
+        if std::intrinsics::unlikely(buffered + 1 > BUF_SIZE) {
             self.flush();
             buffered = 0;
         }
@@ -187,13 +168,12 @@ impl FileEncoder {
 
     #[inline]
     fn write_all(&mut self, buf: &[u8]) {
-        let capacity = self.capacity();
         let buf_len = buf.len();
 
-        if std::intrinsics::likely(buf_len <= capacity) {
+        if std::intrinsics::likely(buf_len <= BUF_SIZE) {
             let mut buffered = self.buffered;
 
-            if std::intrinsics::unlikely(buf_len > capacity - buffered) {
+            if std::intrinsics::unlikely(buffered + buf_len > BUF_SIZE) {
                 self.flush();
                 buffered = 0;
             }
@@ -271,13 +251,11 @@ macro_rules! write_leb128 {
         fn $this_fn(&mut self, v: $int_ty) {
             const MAX_ENCODED_LEN: usize = $crate::leb128::max_leb128_len::<$int_ty>();
 
-            // We ensure this during `FileEncoder` construction.
-            debug_assert!(self.capacity() >= MAX_ENCODED_LEN);
-
             let mut buffered = self.buffered;
 
-            // This can't overflow. See assertion in `FileEncoder::with_capacity`.
-            if std::intrinsics::unlikely(buffered + MAX_ENCODED_LEN > self.capacity()) {
+            // This can't overflow because BUF_SIZE and MAX_ENCODED_LEN are both
+            // quite small.
+            if std::intrinsics::unlikely(buffered + MAX_ENCODED_LEN > BUF_SIZE) {
                 self.flush();
                 buffered = 0;
             }