Introduce a special case in IntRange::from_const.

The `if let Some(val) = value.try_eval_bits(...)` branch in `from_const()` is
very hot for the `unicode_normalization` benchmark.

This commit introduces a special-case alternative for scalars that avoids
`try_eval_bits()` and all the functions it calls (`Const::eval()`,
`ConstValue::try_to_bits()`, `ConstValue::try_to_scalar()`, and
`Scalar::to_bits()`), instead extracting the result immediately.

The type and value checking done by `Scalar::to_bits()` is replicated by moving
it into a new function `Scalar::check_raw()` and using that new function in the
special case.

PR #64673 introduced some special-case handling of scalar types in
`Const::try_eval_bits()`. This handling is now moved out of that function into
the new `IntRange::integral_size_and_signed_bias` function.

This commit reduces the instruction count for
`unicode_normalization-check-clean` by about 10%.

Author: nnethercote

src/librustc/mir/interpret/value.rs

@@ -343,14 +343,19 @@ impl<'tcx, Tag> Scalar<Tag> {
         }
     }
 
+    #[inline(always)]
+    pub fn check_raw(data: u128, size: u8, target_size: Size) {
+        assert_eq!(target_size.bytes(), size as u64);
+        assert_ne!(size, 0, "you should never look at the bits of a ZST");
+        Scalar::check_data(data, size);
+    }
+
     /// Do not call this method!  Use either `assert_bits` or `force_bits`.
     #[inline]
     pub fn to_bits(self, target_size: Size) -> InterpResult<'tcx, u128> {
         match self {
             Scalar::Raw { data, size } => {
-                assert_eq!(target_size.bytes(), size as u64);
-                assert_ne!(size, 0, "you should never look at the bits of a ZST");
-                Scalar::check_data(data, size);
+                Self::check_raw(data, size, target_size);
                 Ok(data)
             }
             Scalar::Ptr(_) => throw_unsup!(ReadPointerAsBytes),

src/librustc/ty/sty.rs

@@ -13,7 +13,7 @@ use rustc_macros::HashStable;
 use crate::ty::subst::{InternalSubsts, Subst, SubstsRef, GenericArg, GenericArgKind};
 use crate::ty::{self, AdtDef, Discr, DefIdTree, TypeFlags, Ty, TyCtxt, TypeFoldable};
 use crate::ty::{List, TyS, ParamEnvAnd, ParamEnv};
-use crate::ty::layout::{Size, Integer, IntegerExt, VariantIdx};
+use crate::ty::layout::VariantIdx;
 use crate::util::captures::Captures;
 use crate::mir::interpret::{Scalar, GlobalId};
 
@@ -24,7 +24,6 @@ use std::marker::PhantomData;
 use std::ops::Range;
 use rustc_target::spec::abi;
 use syntax::ast::{self, Ident};
-use syntax::attr::{SignedInt, UnsignedInt};
 use syntax::symbol::{kw, InternedString};
 
 use self::InferTy::*;
@@ -2299,20 +2298,7 @@ impl<'tcx> Const<'tcx> {
         ty: Ty<'tcx>,
     ) -> Option<u128> {
         assert_eq!(self.ty, ty);
-        // This is purely an optimization -- layout_of is a pretty expensive operation,
-        // but if we can determine the size without calling it, we don't need all that complexity
-        // (hashing, caching, etc.). As such, try to skip it.
-        let size = match ty.kind {
-            ty::Bool => Size::from_bytes(1),
-            ty::Char => Size::from_bytes(4),
-            ty::Int(ity) => {
-                Integer::from_attr(&tcx, SignedInt(ity)).size()
-            }
-            ty::Uint(uty) => {
-                Integer::from_attr(&tcx, UnsignedInt(uty)).size()
-            }
-            _ => tcx.layout_of(param_env.with_reveal_all().and(ty)).ok()?.size,
-        };
+        let size = tcx.layout_of(param_env.with_reveal_all().and(ty)).ok()?.size;
         // if `ty` does not depend on generic parameters, use an empty param_env
         self.eval(tcx, param_env).val.try_to_bits(size)
     }

src/librustc_mir/hair/pattern/_match.rs

@@ -842,21 +842,42 @@ impl<'tcx> IntRange<'tcx> {
         }
     }
 
+    #[inline]
+    fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
+        match ty.kind {
+            ty::Char => Some((Size::from_bytes(4), 0)),
+            ty::Int(ity) => {
+                let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
+                Some((size, 1u128 << (size.bits() as u128 - 1)))
+            }
+            ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
+            _ => None,
+        }
+    }
+
     #[inline]
     fn from_const(
         tcx: TyCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         value: &Const<'tcx>,
     ) -> Option<IntRange<'tcx>> {
-        if Self::is_integral(value.ty) {
+        if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
             let ty = value.ty;
-            if let Some(val) = value.try_eval_bits(tcx, param_env, ty) {
-                let bias = IntRange::signed_bias(tcx, ty);
-                let val = val ^ bias;
-                Some(IntRange { range: val..=val, ty })
+            let val = if let ConstValue::Scalar(Scalar::Raw { data, size }) = value.val {
+                // For this specific pattern we can skip a lot of effort and go
+                // straight to the result, after doing a bit of checking. (We
+                // could remove this branch and just use the next branch, which
+                // is more general but much slower.)
+                Scalar::<()>::check_raw(data, size, target_size);
+                data
+            } else if let Some(val) = value.try_eval_bits(tcx, param_env, ty) {
+                // This is a more general form of the previous branch.
+                val
             } else {
-                None
-            }
+                return None
+            };
+            let val = val ^ bias;
+            Some(IntRange { range: val..=val, ty })
         } else {
             None
         }