Rollup merge of #143477 - folkertdev:use-is-multiple-of, r=joshtriplett

use `is_multiple_of` and `div_ceil`

In tricky logic, these functions are much more informative than the manual implementations. They also catch subtle bugs:

- the manual `is_multiple_of` often does not handle division by zero
- manual `div_ceil` often does not consider overflow

The transformation is free for `is_multiple_of` if the divisor is compile-time known to be non-zero. For `div_ceil` there is a small cost to considering overflow. Here is some assembly https://godbolt.org/z/5zP8KaE1d.

Author: matthiaskrgr

compiler/rustc_abi/src/lib.rs

@@ -527,8 +527,7 @@ impl Size {
     /// not a multiple of 8.
     pub fn from_bits(bits: impl TryInto<u64>) -> Size {
         let bits = bits.try_into().ok().unwrap();
-        // Avoid potential overflow from `bits + 7`.
-        Size { raw: bits / 8 + ((bits % 8) + 7) / 8 }
+        Size { raw: bits.div_ceil(8) }
     }
 
     #[inline]

compiler/rustc_borrowck/src/polonius/legacy/location.rs

@@ -109,6 +109,6 @@ impl PoloniusLocationTable {
 impl LocationIndex {
     fn is_start(self) -> bool {
         // even indices are start points; odd indices are mid points
-        (self.index() % 2) == 0
+        self.index().is_multiple_of(2)
     }
 }

compiler/rustc_codegen_llvm/src/abi.rs

@@ -146,7 +146,7 @@ impl LlvmType for CastTarget {
                 "total size {:?} cannot be divided into units of zero size",
                 self.rest.total
             );
-            if self.rest.total.bytes() % self.rest.unit.size.bytes() != 0 {
+            if !self.rest.total.bytes().is_multiple_of(self.rest.unit.size.bytes()) {
                 assert_eq!(self.rest.unit.kind, RegKind::Integer, "only int regs can be split");
             }
             self.rest.total.bytes().div_ceil(self.rest.unit.size.bytes())

compiler/rustc_codegen_llvm/src/va_arg.rs

@@ -172,10 +172,10 @@ fn emit_aapcs_va_arg<'ll, 'tcx>(
 
     let gr_type = target_ty.is_any_ptr() || target_ty.is_integral();
     let (reg_off, reg_top, slot_size) = if gr_type {
-        let nreg = (layout.size.bytes() + 7) / 8;
+        let nreg = layout.size.bytes().div_ceil(8);
         (gr_offs, gr_top, nreg * 8)
     } else {
-        let nreg = (layout.size.bytes() + 15) / 16;
+        let nreg = layout.size.bytes().div_ceil(16);
         (vr_offs, vr_top, nreg * 16)
     };
 

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -537,7 +537,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
         #[inline]
         fn is_offset_misaligned(offset: u64, align: Align) -> Option<Misalignment> {
-            if offset % align.bytes() == 0 {
+            if offset.is_multiple_of(align.bytes()) {
                 None
             } else {
                 // The biggest power of two through which `offset` is divisible.
@@ -1554,7 +1554,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                         // If the allocation is N-aligned, and the offset is not divisible by N,
                         // then `base + offset` has a non-zero remainder after division by `N`,
                         // which means `base + offset` cannot be null.
-                        if offset.bytes() % info.align.bytes() != 0 {
+                        if !offset.bytes().is_multiple_of(info.align.bytes()) {
                             return interp_ok(false);
                         }
                         // We don't know enough, this might be null.

compiler/rustc_index/src/bit_set.rs

@@ -1744,13 +1744,13 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
 
 #[inline]
 fn num_words<T: Idx>(domain_size: T) -> usize {
-    (domain_size.index() + WORD_BITS - 1) / WORD_BITS
+    domain_size.index().div_ceil(WORD_BITS)
 }
 
 #[inline]
 fn num_chunks<T: Idx>(domain_size: T) -> usize {
     assert!(domain_size.index() > 0);
-    (domain_size.index() + CHUNK_BITS - 1) / CHUNK_BITS
+    domain_size.index().div_ceil(CHUNK_BITS)
 }
 
 #[inline]

compiler/rustc_passes/src/liveness/rwu_table.rs

@@ -44,7 +44,7 @@ impl RWUTable {
     const WORD_RWU_COUNT: usize = Self::WORD_BITS / Self::RWU_BITS;
 
     pub(super) fn new(live_nodes: usize, vars: usize) -> RWUTable {
-        let live_node_words = (vars + Self::WORD_RWU_COUNT - 1) / Self::WORD_RWU_COUNT;
+        let live_node_words = vars.div_ceil(Self::WORD_RWU_COUNT);
         Self { live_nodes, vars, live_node_words, words: vec![0u8; live_node_words * live_nodes] }
     }
 

compiler/rustc_query_system/src/query/plumbing.rs

@@ -597,7 +597,7 @@ where
         // from disk. Re-hashing results is fairly expensive, so we can't
         // currently afford to verify every hash. This subset should still
         // give us some coverage of potential bugs though.
-        let try_verify = prev_fingerprint.split().1.as_u64() % 32 == 0;
+        let try_verify = prev_fingerprint.split().1.as_u64().is_multiple_of(32);
         if std::intrinsics::unlikely(
             try_verify || qcx.dep_context().sess().opts.unstable_opts.incremental_verify_ich,
         ) {

compiler/rustc_serialize/src/leb128.rs

@@ -7,7 +7,7 @@ use crate::serialize::Decoder;
 /// Returns the length of the longest LEB128 encoding for `T`, assuming `T` is an integer type
 pub const fn max_leb128_len<T>() -> usize {
     // The longest LEB128 encoding for an integer uses 7 bits per byte.
-    (size_of::<T>() * 8 + 6) / 7
+    (size_of::<T>() * 8).div_ceil(7)
 }
 
 /// Returns the length of the longest LEB128 encoding of all supported integer types.

compiler/rustc_span/src/edit_distance.rs

@@ -130,7 +130,7 @@ pub fn edit_distance_with_substrings(a: &str, b: &str, limit: usize) -> Option<u
         1 // Exact substring match, but not a total word match so return non-zero
     } else if !big_len_diff {
         // Not a big difference in length, discount cost of length difference
-        score + (len_diff + 1) / 2
+        score + len_diff.div_ceil(2)
     } else {
         // A big difference in length, add back the difference in length to the score
         score + len_diff