Rollup merge of #114382 - scottmcm:compare-bytes-intrinsic, r=cjgillot

Add a new `compare_bytes` intrinsic instead of calling `memcmp` directly

As discussed in #113435, this lets the backends be the place that can have the "don't call the function if n == 0" logic, if it's needed for the target.  (I didn't actually *add* those checks, though, since as I understood it we didn't actually need them on known targets?)

Doing this also let me make it `const` (unstable), which I don't think `extern "C" fn memcmp` can be.

cc `@RalfJung` `@Amanieu`

Author: matthiaskrgr

compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

@@ -1155,6 +1155,20 @@ fn codegen_regular_intrinsic_call<'tcx>(
             ret.write_cvalue(fx, CValue::by_val(is_eq_value, ret.layout()));
         }
 
+        sym::compare_bytes => {
+            intrinsic_args!(fx, args => (lhs_ptr, rhs_ptr, bytes_val); intrinsic);
+            let lhs_ptr = lhs_ptr.load_scalar(fx);
+            let rhs_ptr = rhs_ptr.load_scalar(fx);
+            let bytes_val = bytes_val.load_scalar(fx);
+
+            let params = vec![AbiParam::new(fx.pointer_type); 3];
+            let returns = vec![AbiParam::new(types::I32)];
+            let args = &[lhs_ptr, rhs_ptr, bytes_val];
+            // Here we assume that the `memcmp` provided by the target is a NOP for size 0.
+            let cmp = fx.lib_call("memcmp", params, returns, args)[0];
+            ret.write_cvalue(fx, CValue::by_val(cmp, ret.layout()));
+        }
+
         sym::const_allocate => {
             intrinsic_args!(fx, args => (_size, _align); intrinsic);
 

compiler/rustc_codegen_gcc/src/intrinsic/mod.rs

@@ -302,6 +302,21 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                     }
                 }
 
+                sym::compare_bytes => {
+                    let a = args[0].immediate();
+                    let b = args[1].immediate();
+                    let n = args[2].immediate();
+
+                    let void_ptr_type = self.context.new_type::<*const ()>();
+                    let a_ptr = self.bitcast(a, void_ptr_type);
+                    let b_ptr = self.bitcast(b, void_ptr_type);
+
+                    // Here we assume that the `memcmp` provided by the target is a NOP for size 0.
+                    let builtin = self.context.get_builtin_function("memcmp");
+                    let cmp = self.context.new_call(None, builtin, &[a_ptr, b_ptr, n]);
+                    self.sext(cmp, self.type_ix(32))
+                }
+
                 sym::black_box => {
                     args[0].val.store(self, result);
 

compiler/rustc_codegen_llvm/src/context.rs

@@ -891,7 +891,8 @@ impl<'ll> CodegenCx<'ll, '_> {
         ifn!("llvm.prefetch", fn(ptr, t_i32, t_i32, t_i32) -> void);
 
         // This isn't an "LLVM intrinsic", but LLVM's optimization passes
-        // recognize it like one and we assume it exists in `core::slice::cmp`
+        // recognize it like one (including turning it into `bcmp` sometimes)
+        // and we use it to implement intrinsics like `raw_eq` and `compare_bytes`
         match self.sess().target.arch.as_ref() {
             "avr" | "msp430" => ifn!("memcmp", fn(ptr, ptr, t_isize) -> t_i16),
             _ => ifn!("memcmp", fn(ptr, ptr, t_isize) -> t_i32),

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -329,6 +329,16 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 }
             }
 
+            sym::compare_bytes => {
+                // Here we assume that the `memcmp` provided by the target is a NOP for size 0.
+                let cmp = self.call_intrinsic(
+                    "memcmp",
+                    &[args[0].immediate(), args[1].immediate(), args[2].immediate()],
+                );
+                // Some targets have `memcmp` returning `i16`, but the intrinsic is always `i32`.
+                self.sext(cmp, self.type_ix(32))
+            }
+
             sym::black_box => {
                 args[0].val.store(self, result);
                 let result_val_span = [result.llval];

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -261,6 +261,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             sym::write_bytes => {
                 self.write_bytes_intrinsic(&args[0], &args[1], &args[2])?;
             }
+            sym::compare_bytes => {
+                let result = self.compare_bytes_intrinsic(&args[0], &args[1], &args[2])?;
+                self.write_scalar(result, dest)?;
+            }
             sym::arith_offset => {
                 let ptr = self.read_pointer(&args[0])?;
                 let offset_count = self.read_target_isize(&args[1])?;
@@ -643,6 +647,24 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         self.write_bytes_ptr(dst, bytes)
     }
 
+    pub(crate) fn compare_bytes_intrinsic(
+        &mut self,
+        left: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
+        right: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
+        byte_count: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
+    ) -> InterpResult<'tcx, Scalar<M::Provenance>> {
+        let left = self.read_pointer(left)?;
+        let right = self.read_pointer(right)?;
+        let n = Size::from_bytes(self.read_target_usize(byte_count)?);
+
+        let left_bytes = self.read_bytes_ptr_strip_provenance(left, n)?;
+        let right_bytes = self.read_bytes_ptr_strip_provenance(right, n)?;
+
+        // `Ordering`'s discriminants are -1/0/+1, so casting does the right thing.
+        let result = Ord::cmp(left_bytes, right_bytes) as i32;
+        Ok(Scalar::from_i32(result))
+    }
+
     pub(crate) fn raw_eq_intrinsic(
         &mut self,
         lhs: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -273,6 +273,10 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
                 ],
                 Ty::new_unit(tcx),
             ),
+            sym::compare_bytes => {
+                let byte_ptr = Ty::new_imm_ptr(tcx, tcx.types.u8);
+                (0, vec![byte_ptr, byte_ptr, tcx.types.usize], tcx.types.i32)
+            }
             sym::write_bytes | sym::volatile_set_memory => (
                 1,
                 vec![

compiler/rustc_span/src/symbol.rs

@@ -498,6 +498,7 @@ symbols! {
         cold,
         collapse_debuginfo,
         column,
+        compare_bytes,
         compare_exchange,
         compare_exchange_weak,
         compile_error,

library/core/src/intrinsics.rs

@@ -2385,6 +2385,25 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn raw_eq<T>(a: &T, b: &T) -> bool;
 
+    /// Lexicographically compare `[left, left + bytes)` and `[right, right + bytes)`
+    /// as unsigned bytes, returning negative if `left` is less, zero if all the
+    /// bytes match, or positive if `right` is greater.
+    ///
+    /// This underlies things like `<[u8]>::cmp`, and will usually lower to `memcmp`.
+    ///
+    /// # Safety
+    ///
+    /// `left` and `right` must each be [valid] for reads of `bytes` bytes.
+    ///
+    /// Note that this applies to the whole range, not just until the first byte
+    /// that differs.  That allows optimizations that can read in large chunks.
+    ///
+    /// [valid]: crate::ptr#safety
+    #[cfg(not(bootstrap))]
+    #[rustc_const_unstable(feature = "const_intrinsic_compare_bytes", issue = "none")]
+    #[rustc_nounwind]
+    pub fn compare_bytes(left: *const u8, right: *const u8, bytes: usize) -> i32;
+
     /// See documentation of [`std::hint::black_box`] for details.
     ///
     /// [`std::hint::black_box`]: crate::hint::black_box
@@ -2825,3 +2844,18 @@ pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
         write_bytes(dst, val, count)
     }
 }
+
+/// Backfill for bootstrap
+#[cfg(bootstrap)]
+pub unsafe fn compare_bytes(left: *const u8, right: *const u8, bytes: usize) -> i32 {
+    extern "C" {
+        fn memcmp(s1: *const u8, s2: *const u8, n: usize) -> crate::ffi::c_int;
+    }
+
+    if bytes != 0 {
+        // SAFETY: Since bytes is non-zero, the caller has met `memcmp`'s requirements.
+        unsafe { memcmp(left, right, bytes).into() }
+    } else {
+        0
+    }
+}

library/core/src/slice/cmp.rs

@@ -1,22 +1,12 @@
 //! Comparison traits for `[T]`.
 
 use crate::cmp::{self, BytewiseEq, Ordering};
-use crate::ffi;
+use crate::intrinsics::compare_bytes;
 use crate::mem;
 
 use super::from_raw_parts;
 use super::memchr;
 
-extern "C" {
-    /// Calls implementation provided memcmp.
-    ///
-    /// Interprets the data as u8.
-    ///
-    /// Returns 0 for equal, < 0 for less than and > 0 for greater
-    /// than.
-    fn memcmp(s1: *const u8, s2: *const u8, n: usize) -> ffi::c_int;
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A, B> PartialEq<[B]> for [A]
 where
@@ -74,7 +64,8 @@ where
     }
 }
 
-// Use memcmp for bytewise equality when the types allow
+// When each element can be compared byte-wise, we can compare all the bytes
+// from the whole size in one call to the intrinsics.
 impl<A, B> SlicePartialEq<B> for [A]
 where
     A: BytewiseEq<B>,
@@ -88,7 +79,7 @@ where
         // The two slices have been checked to have the same size above.
         unsafe {
             let size = mem::size_of_val(self);
-            memcmp(self.as_ptr() as *const u8, other.as_ptr() as *const u8, size) == 0
+            compare_bytes(self.as_ptr() as *const u8, other.as_ptr() as *const u8, size) == 0
         }
     }
 }
@@ -183,7 +174,7 @@ impl<A: Ord> SliceOrd for A {
     }
 }
 
-// memcmp compares a sequence of unsigned bytes lexicographically.
+// `compare_bytes` compares a sequence of unsigned bytes lexicographically.
 // this matches the order we want for [u8], but no others (not even [i8]).
 impl SliceOrd for u8 {
     #[inline]
@@ -195,7 +186,7 @@ impl SliceOrd for u8 {
         // SAFETY: `left` and `right` are references and are thus guaranteed to be valid.
         // We use the minimum of both lengths which guarantees that both regions are
         // valid for reads in that interval.
-        let mut order = unsafe { memcmp(left.as_ptr(), right.as_ptr(), len) as isize };
+        let mut order = unsafe { compare_bytes(left.as_ptr(), right.as_ptr(), len) as isize };
         if order == 0 {
             order = diff;
         }

src/tools/miri/tests/fail/uninit_buffer.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: reading memory at ALLOC[0x0..0x10], but memory is uninitialized at [0x4..0x10], and this operation requires initialized memory
   --> RUSTLIB/core/src/slice/cmp.rs:LL:CC
    |
-LL |         let mut order = unsafe { memcmp(left.as_ptr(), right.as_ptr(), len) as isize };
-   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ reading memory at ALLOC[0x0..0x10], but memory is uninitialized at [0x4..0x10], and this operation requires initialized memory
+LL |         let mut order = unsafe { compare_bytes(left.as_ptr(), right.as_ptr(), len) as isize };
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ reading memory at ALLOC[0x0..0x10], but memory is uninitialized at [0x4..0x10], and this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information